ViewArticleOnlineR)CheckforupdatesView JournalJournalofMaterialsChemistryCAcceptedManuscriptThisarticlecanbecitedbeforepaqenumbershavebeenissued,todothispleaseuse:FTang,ZC.SuH.Ye,S.J.Xu,G.Wang.Y.Cao,W.GaoandX.Pan,J.Mater.Chem.C,2017,DOl:10.1039/C7TC04695BThisisanAcceptedManuscript,whichhasbeenthroughtheRoyalSocietyofChemistrypeerreviewprocessandhasbeenacceptedforpublication.JournalofMaterialsChemistryCAcceptedManuscriptsarepublishedonlineshortlyafteracceptance,beforetechnicalediting.formattingandproofreadingUsingthisfreeservice,authors canmaketheirresultsavailabletothe community.in citableform,beforewepublish theeditedarticle.Wewillreplacethis AcceptedManuscriptwiththeeditedandformattedAdvanceArticleas soonas itisavailableYoucanfindmoreinformationaboutAcceptedManuscriptsintheauthorguidelinesPleasenotethattechnical editingmayintroduceminorchangestothetextand/orgraphics,which.mayaltercontent.Thejournal'sstandardTerms&Conditionsandtheethicalguidelines,outlinedinourauthorandreviewerresourcecentrestill apply.Inno175eventshalltheRoyalSocietyofChemistrybeheldresponsibleforanyerrors oromissions in this AcceptedManuscriptoranyconsequencesarisingfromtheuseofanyinformationitcontainsROYALSOCIETYrsc.li/materials-cOFCHEMISTRY
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the author guidelines. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the ethical guidelines, outlined in our author and reviewer resource centre, still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. Accepted Manuscript rsc.li/materials-c Journal of Materials Chemistry C Materials for optical, magnetic and electronic devices www.rsc.org/MaterialsC ISSN 2050-7526 PAPER Nguyên T. K. Thanh, Xiaodi Su et al. Fine-tuning of gold nanorod dimensions and plasmonic properties using the Hofmeister eff ects ~ Volume 4 Number 1 7 January 2016 Pages 1–224 Journal of Materials Chemistry C Materials for optical, magnetic and electronic devices View Article Online View Journal This article can be cited before page numbers have been issued, to do this please use: F. Tang, Z.C. Su, H. Ye, S. J. Xu, G. Wang, Y. Cao, W. Gao and X. Pan, J. Mater. Chem. C, 2017, DOI: 10.1039/C7TC04695B
Page1of 8Journal of Materials Chemistry CCHEMISTTC04695BJournal NameARTICLEBoostingupphonon-induced luminescenceinredfluoridephosphors via composition variation driven structuralReceived 00th January 20xx,transformationsAccepted 00th January 20xxFei Tang,"Zhicheng Su,"Honggang Ye,"Shijie Xu,aWang Guo,Yongge Cao,Wenpei GaoandDOI:10.1039/×0xx00000xXiaoqing Pan dpawww.rsc.org/In this study, a series of (KNa-)2siFs:Mn red phosphors with systematic composition variations of alkali metals were兰synthesized via a low-temperature full-solution approach. Driven by the composition variation, a sequence of continuousstructural phase transformations, ie., from trigonal to mixed, then to orthorhombic, and eventually to cubic phase, is8evidently observed in this series of red phosphors. More excitingly, phonon-induced luminescence is promoted as theOmost efficient anddominant light emission mechanism in cubic phosphorof K,SiFs:Mn*tat room temperature.As a result,Uthe overall emission intensity of cubic KzsiFs:Mn*t is increased by more than 5 folds with respect to that of trigonalNazSiF:Mn*+,High-resolution x-ray diffraction, electron paramagnetic resonance and micro-Raman scattering experimentsAconsistently reveal a decisive relationship between fluorescence property and crystallinestructure.OKNaSiFa:Mn4+ is intermediate of those of homo-dialkalineIntroductionK2SiF6:Mn4+andNazSiF6:Mn4+redphosphors.7OtherEadvantages of transition-metal Mn4+-activated phosphorstIn recent years, the demand for high-efficiency red phosphorsinclude abundant resources of involved elements and mildShas rapidlyraised up notonlybecause of the critical roleofproduction conditions.Furthermore, thermal stability andsuch phosphors in the improvement of color rendering indexwtemperature robustness of Mn4+-activated fluoride phosphors(CRl)forphosphor-convertedwhitelight-emittingdiodes(pc-are the last but more technologically critical properties forWLEDs),butalsoduetothe significancethat the additionoftheir practical applications in pc-WLEDs.18-20theredcomponentcanefficientlyenlargethecolorgamutofback lighting in the LED-based display devices.1-8 Therefore,Ontheotherhand,pioneertheoretical studieshavefirmlyhuntingforsuchhigh-efficiencyredphosphorshasemergedasshownthattheforbiddenelectronictransitions inafreeatoman interesting but challenging subject in materials science andSmaybecome allowedand efficient when theatomissolid state lighting. Very recently, manganese ion (Mn4+)-incorporated into a crystal due to the mixing of impurityactivated fluorides and oxides have attracted an increasingelectronicstateswithhostphononsvialocalelectron-phononattention due to their high-efficiency yield and narrow-bandV coupling.21-26 Such transitions, usually termed as phonon-red light emissions.9-17 For example, Adachi et al. reported the0assisted or phonon-induced optical transitions or simplysynthesisof thehetero-dialkalinehexafluorosilicate redvibronic transitions,27 have been theoretically predicted tophosphorKNasiFg:Mn4+byetchingSiwafersinaaexhibitoutstandingtemperaturebehaviorssuchasHF/KMnOa/NaMnO4-HzOmixedsolution.7Theyemployedthetemperature induced great increase in emission probabilityX-ray diffraction analysis to reveal that the synthesizeddue to the deep participation of thermal phonons.26 Thesephosphorhasanorthorhombicstructurewiththespacegroupfoutstanding theoretical studies provide a solid base andOD26 - pnma .Moreover,they further investigated theguideline for developing high-efficiency and temperature-luminescenceproperties of thephosphor,and found that therobust impurity-activated phosphorsforpc-WLEDs.Inturn,theadevelopmentofsuchphosphorsalsooffersagreatchangefor=testing these theoretical predictions and pushing the furtherDepartment of Physics, and Shenzhen Institute of Research and Innovation (HKUISIRI), The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. Chinaprogress in relevant theoretical studies.Therefore, conducting5FujanInstituteofResearchontheStructureofMatter,ChineseAcademyfa systematicinvestigation ofMn4+-activatedfluorideSciences,Fuzhou350002, P.R.China0phosphorsof(K,Na1-x)2SiFwithavaryingalkalimetal.Department of Physics, Renmin University of China, Beijing100872, P.R.China.Department of Chemical Engineering andMaterials Science,Universityofcomposition is of both technical and scientific significance.California-rvine, Irvine,CA,USACorresponding author: Email:sjxu@hku.hkElectronicSupplementary Information (ESl)available:[Rietveld structuralrefinement on powder XRD patternsJ.See DOI: 10.1039/x0xx00000xThis journal is@TheRoyal Society of Chemistry2OxxJ.Name.,2013,00,1-3|1Please do not adjust margins
Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1 Please do not adjust margins Please do not adjust margins a.Department of Physics, and Shenzhen Institute of Research and Innovation (HKUSIRI), The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China. b.Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China. c.Department of Physics, Renmin University of China, Beijing 100872, P. R. China. d.Department of Chemical Engineering and Materials Science, University of California–Irvine, Irvine, CA, USA. * Corresponding author: Email: sjxu@hku.hk Electronic Supplementary Information (ESI) available: [Rietveld structural refinement on powder XRD patterns]. See DOI: 10.1039/x0xx00000x Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/ Boosting up phonon-induced luminescence in red fluoride phosphors via composition variation driven structural transformations Fei Tang, a Zhicheng Su, a Honggang Ye, a Shijie Xu, a,* Wang Guo, b Yongge Cao, c Wenpei Gao d and Xiaoqing Pan d In this study, a series of (KxNa1-x)2SiF6:Mn4+ red phosphors with systematic composition variations of alkali metals were synthesized via a low-temperature full-solution approach. Driven by the composition variation, a sequence of continuous structural phase transformations, i.e., from trigonal to mixed, then to orthorhombic, and eventually to cubic phase, is evidently observed in this series of red phosphors. More excitingly, phonon-induced luminescence is promoted as the most efficient and dominant light emission mechanism in cubic phosphor of K2SiF6:Mn4+ at room temperature. As a result, the overall emission intensity of cubic K2SiF6:Mn4+ is increased by more than 5 folds with respect to that of trigonal Na2SiF6:Mn4+ . High-resolution x-ray diffraction, electron paramagnetic resonance and micro-Raman scattering experiments consistently reveal a decisive relationship between fluorescence property and crystalline structure. Introduction In recent years, the demand for high-efficiency red phosphors has rapidly raised up not only because of the critical role of such phosphors in the improvement of color rendering index (CRI) for phosphor-converted white light-emitting diodes (pcWLEDs), but also due to the significance that the addition of the red component can efficiently enlarge the color gamut of back lighting in the LED-based display devices.1-8 Therefore, hunting for such high-efficiency red phosphors has emerged as an interesting but challenging subject in materials science and solid state lighting. Very recently, manganese ion (Mn4+)- activated fluorides and oxides have attracted an increasing attention due to their high-efficiency yield and narrow-band red light emissions.9-17 For example, Adachi et al. reported the synthesis of the hetero-dialkaline hexafluorosilicate red phosphor KNaSiF6:Mn4+ by etching Si wafers in a HF/KMnO4/NaMnO4·H2O mixed solution.7 They employed the X-ray diffraction analysis to reveal that the synthesized phosphor has an orthorhombic structure with the space group D2h 16 − pnma . Moreover, they further investigated the luminescence properties of the phosphor, and found that the KNaSiF6:Mn4+ is intermediate of those of homo-dialkaline K2SiF6:Mn4+ and Na2SiF6:Mn4+ red phosphors.7 Other advantages of transition-metal Mn4+-activated phosphors include abundant resources of involved elements and mild production conditions. Furthermore, thermal stability and temperature robustness of Mn4+-activated fluoride phosphors are the last but more technologically critical properties for their practical applications in pc-WLEDs.18-20 On the other hand, pioneer theoretical studies have firmly shown that the forbidden electronic transitions in a free atom may become allowed and efficient when the atom is incorporated into a crystal due to the mixing of impurity electronic states with host phonons via local electron-phonon coupling.21-26 Such transitions, usually termed as phononassisted or phonon-induced optical transitions or simply vibronic transitions,27 have been theoretically predicted to exhibit outstanding temperature behaviors such as temperature induced great increase in emission probability due to the deep participation of thermal phonons.26 These outstanding theoretical studies provide a solid base and guideline for developing high-efficiency and temperaturerobust impurity-activated phosphors for pc-WLEDs. In turn, the development of such phosphors also offers a great change for testing these theoretical predictions and pushing the further progress in relevant theoretical studies. Therefore, conducting a systematic investigation of Mn4+-activated fluoride phosphors of (KxNa1-x)2SiF6 with a varying alkali metal composition is of both technical and scientific significance. Page 1 of 8 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B
Page 2of 8Journal ofMaterials ChemistryCARTICLEJournal NameIn this article, we report a comprehensive study ofto form a golden-yellow solution. Then, both KHEa.and.NaHF2microstructural phasetransformationandresultantpromotionpowderswereadded into the aboveSolutfoi3heTFeeof phonon-induced luminescence in (KxNa1-x)2SiF:Mn4+ redcomposition of K and Na in compounds can be preciselyphosphorswithsystematicrelativecompositionvariationsofKcontrolled by changing the amount of KHF2 and NaHF2and Na metals for the first time. By using high-resolution x-raypowders.HzSiF6 solution was subsequently dropwised into thetdiffraction(XRD),high-resolutiontransmissionelectronobtainedsolutionandthesolutioncolorchangedfromgolden-P(TEM),andmicro-Ramanyellow to gold.Followed the filtration of thegolden solution,microscopelightscatteringEtechniques, we firmly show clear evidence for the structuralthe final redphosphorswithdifferent alkali metalCphase transformation from trigonal to orthorhombic andcompositions were prepared through washing and drying thesnurfurther to cubic phase in the phosphors for varying the relativeprecipitants for several times.composition of K with respect to Na from o to 1. MoreCharacterization techniques. XRD patterns of the powderexcitingly,thephonon-induced luminescence is boostedup sosignificantly that the best internal luminescence quantumsamples were measured on an X-ray diffractometer (Type D8?efficiency (QE) of 97.6% (external QE=73.0%) is obtained inAdvance EcO, Bruker,UK).Rietveld refinement work on the2K2SiF6:Mn4+ phosphor with cubic crystalline structure at roommeasured XRD data was carried out using the GSAS package.WeTEMtemperature.AalsoemployelectronparamagneticBoth low-and high-resolutionmeasurementswerearesonance (EPR)technique to examine the hyperfine states ofperformedonaJEOLJEM-2100TEMinstrumentoperatedate3d3 electrons of Mn ions within host local lattice structures.200okV.EPR spectra of thepowder samples were recorded on北Thetime-resolvedPLspectraof thisseriesof phosphorsalsoa BRUKER-BIOSPIN EPRinstrumentwithspectral resolutionofPoffersome consistent evidence for the boosting up of phonon-kHz.Photoluminescence excitation (PLE) spectra were1einduced light emission due to the lattice phase transition.measuredon ahome-assembled setup witha Xenon lampODifferent from the study on a particular fluoride phosphor,e.g.(Muller,Germany)as the excitation sourcezs by monitoring theUK2SiFg:Mn4+ or Na2SiFg:Mn4+, this work clearly shows us for thestrongest emission peak for each phosphor. High-resolution PLAfirst time the evolution progress of crystalline structure andspectra were measured at room-temperature on a home-related properties with the composition variation of K+ and Natassembled high-resolution PL setup29 using the 477 nm laserOions in red fluoride phosphorsbeam of an Ar+-Kr+ mixed gas laser as the excitation lightsource. Room-temperature Raman scattering spectrum ofKunsExperimentaleach phosphor was registered on a confocal micro-Ramansystem (WITech-Alpha) using the 514.5 nm line of an Ar+ ioneaeChemical and materials. The raw powders of potassiumgas laser as the excitation light source. Room-temperaturepermanganate (KMnOa), sodium hydrogen difluoride (NaHF2)time-resolved PL (TRPL) decay traces were recorded on aand potassiumhydrogen fluoride(KHF2)werepurchasedfromnanosecondtime-resolvedspectrometer(FLS920dSinopharm Chemical Reagent Co., China. Hydrofluoric acidspectrofluorimeter, Edinburgh Instruments Ltd).(HF), hydrogen peroxide solution (HzOz) and fluorosilicic acidResults anddiscussionsolution (H2SiFs) were purchased from Aladdin Reagent Co.(Shanghai). All these chemicals were directly used without anyfurther treatments.Mn4+-activated fluoride phosphors.SynthesisofAsdescribed previously elsewhere,13 the studied Mn4+-activatedfluoride phosphors were synthesized at low temperature of -16 oC by employing a two-step chemical co-precipitationmethod. Firstly, we prepared K2MnF6 powder as the Mn4+source material, and the specific preparation process is asfollows:KHF2powder was firstly dissolved into HF solutionunder vigorous stirring operation to form a uniform solution.Then,acertainamountofblackKMnO4powderwaspouredinto the above solution with further stirring operation for onehour to produce Modena solution.Followed by the slowly-dropping H2O2 solution, a brown-yellow solution will begenerated with some precipitant.After filtrating the obtainedsolution,a certain amount of brown precipitant can be gained.By drying and washing the participant for three times, weobtainedKzMnFs powder.The secondstepis toprepare(K.Na1-x)2SiF6:Mn4+ phosphors with x=1, 0.75, 0.5, 0.25, 0. Theprocedure is described as follows:The as-preparationsynthesizedKzMnF6powderwasfirstlymixedwithHFsolution21J.Name.,2012,00,1-3This journal is The Royal Society of Chemistry 20xxPlease donotadjust margin
ARTICLE Journal Name 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins In this article, we report a comprehensive study of microstructural phase transformation and resultant promotion of phonon-induced luminescence in (KxNa1-x)2SiF6:Mn4+ red phosphors with systematic relative composition variations of K and Na metals for the first time. By using high-resolution x-ray diffraction (XRD), high-resolution transmission electron microscope (TEM), and micro-Raman light scattering techniques, we firmly show clear evidence for the structural phase transformation from trigonal to orthorhombic and further to cubic phase in the phosphors for varying the relative composition of K with respect to Na from 0 to 1. More excitingly, the phonon-induced luminescence is boosted up so significantly that the best internal luminescence quantum efficiency (QE) of 97.6% (external QE=73.0%) is obtained in K2SiF6:Mn4+ phosphor with cubic crystalline structure at room temperature. We also employ electron paramagnetic resonance (EPR) technique to examine the hyperfine states of 3d3 electrons of Mn ions within host local lattice structures. The time-resolved PL spectra of this series of phosphors also offer some consistent evidence for the boosting up of phononinduced light emission due to the lattice phase transition. Different from the study on a particular fluoride phosphor, e.g. K2SiF6:Mn4+ or Na2SiF6:Mn4+, this work clearly shows us for the first time the evolution progress of crystalline structure and related properties with the composition variation of K + and Na+ ions in red fluoride phosphors Experimental Chemical and materials. The raw powders of potassium permanganate (KMnO4), sodium hydrogen difluoride (NaHF2) and potassium hydrogen fluoride (KHF2) were purchased from Sinopharm Chemical Reagent Co., China. Hydrofluoric acid (HF), hydrogen peroxide solution (H2O2) and fluorosilicic acid solution (H2SiF6) were purchased from Aladdin Reagent Co. (Shanghai). All these chemicals were directly used without any further treatments. Synthesis of Mn4+-activated fluoride phosphors. As described previously elsewhere,13 the studied Mn4+ -activated fluoride phosphors were synthesized at low temperature of - 16 oC by employing a two-step chemical co-precipitation method. Firstly, we prepared K2MnF6 powder as the Mn4+ source material, and the specific preparation process is as follows: KHF2 powder was firstly dissolved into HF solution under vigorous stirring operation to form a uniform solution. Then, a certain amount of black KMnO4 powder was poured into the above solution with further stirring operation for one hour to produce Modena solution. Followed by the slowlydropping H2O2 solution, a brown-yellow solution will be generated with some precipitant. After filtrating the obtained solution, a certain amount of brown precipitant can be gained. By drying and washing the participant for three times, we obtained K2MnF6 powder. The second step is to prepare (KxNa1-x)2SiF6:Mn4+ phosphors with x=1, 0.75, 0.5, 0.25, 0. The preparation procedure is described as follows: The assynthesized K2MnF6 powder was firstly mixed with HF solution to form a golden-yellow solution. Then, both KHF2 and NaHF2 powders were added into the above solution. The relative composition of K and Na in compounds can be precisely controlled by changing the amount of KHF2 and NaHF2 powders. H2SiF6 solution was subsequently dropwised into the obtained solution and the solution color changed from goldenyellow to gold. Followed the filtration of the golden solution, the final red phosphors with different alkali metal compositions were prepared through washing and drying the precipitants for several times. Characterization techniques. XRD patterns of the powder samples were measured on an X-ray diffractometer (Type D8 Advance ECO, Bruker, UK). Rietveld refinement work on the measured XRD data was carried out using the GSAS package. Both low- and high-resolution TEM measurements were performed on a JEOL JEM-2100 TEM instrument operated at 200 kV. EPR spectra of the powder samples were recorded on a BRUKER-BIOSPIN EPR instrument with spectral resolution of 1 kHz. Photoluminescence excitation (PLE) spectra were measured on a home-assembled setup with a Xenon lamp (Müller, Germany) as the excitation source28 by monitoring the strongest emission peak for each phosphor. High-resolution PL spectra were measured at room-temperature on a homeassembled high-resolution PL setup29 using the 477 nm laser beam of an Ar+ -Kr+ mixed gas laser as the excitation light source. Room-temperature Raman scattering spectrum of each phosphor was registered on a confocal micro-Raman system (WITech-Alpha) using the 514.5 nm line of an Ar+ ion gas laser as the excitation light source. Room-temperature time-resolved PL (TRPL) decay traces were recorded on a nanosecond time-resolved spectrometer (FLS920 spectrofluorimeter, Edinburgh Instruments Ltd). Results and discussion Journal of Materials Chemistry C Page 2 of 8 Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B
Page3of 8JournalofMaterialsChemistryCJournalNameARTICLE(a)(b)photographs of the synthesized powder phospboreletramwhichwe seethatthe samplecolorgraaualyzAdngesforX-0faint orange-yellow of NazSiFs:Mn4+ phosphor to bright goldenyellow of K2SiF6:Mn4+phosphor.This color change may implyLOsignificant change in both structure and PL properties, as1P321shown and evidenced later.x=0.25(a)NaSiFe:Mn"(b)PDF872-1115trigonalx=0.50(KaNaau)SiFMn()smixedPnma(KaNaSiFaMn)BE国PDFX81-0009童orthorhombic服工牌身x=0.75(KaNaus):SiFe:Mn*cubicKiSiFa:MnPDF81-2264cubic电20 (degrees)20 (degrees)x=1Fm-3mFigure 2. (a) XRD diffraction patterns of (KxNai-x)2SiF6:Mn4*redphosphors with x=1,0.75, 0.50, 0.25, and0 from bottom to top.(K.Na1-x)SiF6:Mn4+OKONaOSi OFOMn(b) Replotted XRD patterns at low diffraction angles.Structural5phase transformation can be clearly seen as the relativeFigure 1. (a)Three types of crystalline structures of K2SiF6:Mn4+,content of K and Na is systematically varied.KNaSiF6:Mn4+ and NazSiF6:Mn4+ phosphors with space groupsof Fm-3m,Pnma andP321.(b)Photographs of theas-synthesizedpowderphosphors(KNa1-x)zSiF:Mn4+(frombottomtotop:x=1,0.75,0.50,0.25,and0)Inordertoelucidatethestructuralphasetransformationinthe studied series of (K,Na1-x)2SiFg:Mn4+ red phosphors, fineXRD measurements were carried out, and the results areShown in Figure 1(a) are the three types of crystallineillustrated in Figure 2(a).The XRD patterns at low diffractionstructures of K2SiF6:Mn4+ (cubic), KNaSiFg:Mn4+ (orthorhombic)angles are enlarged in Figure 2(b),so thatthe solid evidenceAo uo persiedandNazSiFg:Mn4+phosphors(trigonal).Theirfull-solutionfor phasetransformation can be clearly seen.From thesynthesis process may be concisely described by the followingexperimental xRDpatterns,three majorlattice phaseSreactionequationsstructurescan be determined:cubic phase characterized byaxK2MnF6+2(1-x)KHF2+(1-x)H2SiF6HE_)spacegroup of Fm3mforK2siF6:Mn4+, trigonal phase witha?(1) space group of P321 for NazSiFg:Mn4+,and orthorhombicK2Si(lr)Mn.F6 + 4(1 x)HFphase with a space groupof Pnma for (Ko.5Nao.s)2SiFg:Mn4+or0simply KNaSiFg:Mn4+, For (Ko.25Nao.75)2SiFg:Mn4+ phosphor, itfor K2SiF6:Mn4+phosphor:possess a mixed phase of trigonal and orthorhombic structures.xK2MnF6+2NaHF2 +(1-x)H2SiF6—H)(2)Asshown inFigure2(a),theexperimental XRDpatternswereNa2Sil)Mn.F6+ 2xKF +2(2 - x)HFfinely simulated with standard base data (e.g.JCPDF 81-2264,JCPDF81-0009andJCPDF72-1115).Toexaminetheinfluencefor Na2SiFg:Mn4+ phosphor; andof alkali metal composition variation on the crystallineOxK2MnF6+(1-x)KHF2 +NaHF2 +(1-x)H2SiF6structures of the phosphors more accurately,we did fine(3) H>KNaSi()Mn.F6+ xKF+(4-3x)HFtheoretical simulations on the measured XRD result of eachsample with Rietveld method, as depicted in Figure S1-S5 inunfor KNaSiFa:Mn4+ phosphor.the supporting document. Obtained lattice parameters for theInaboveequations,xrepresentstheconcentrationofMn4+ion.three major types of crystalline structures are tabularized inNote that inK2iF6:Mn4+phosphor,K+ion originates frombothTable 1.K,MnFand KHFz,whereasNa element inthecombinatorialphosphors is mainly supplied by the NaHF2 compound. Moredetaileddescriptiononthelow-temperaturesynthesiscanbereferred to one previous publication.13 Figure 1(b) presentsThis journal is @TheRoyal Society of Chemistry2OxxJ.Name.,2013,00,1-33Please donotadjustmargins
Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3 Please do not adjust margins Please do not adjust margins Figure 1. (a) Three types of crystalline structures of K2SiF6:Mn4+ , KNaSiF6:Mn4+ and Na2SiF6:Mn4+ phosphors with space groups of Fm-3m, Pnma and P321. (b) Photographs of the assynthesized powder phosphors (KxNa1-x)2SiF6:Mn4+ (from bottom to top: x=1, 0.75, 0.50, 0.25, and 0). Shown in Figure 1(a) are the three types of crystalline structures of K2SiF6:Mn4+ (cubic), KNaSiF6:Mn4+ (orthorhombic) and Na2SiF6:Mn4+ phosphors (trigonal). Their full-solution synthesis process may be concisely described by the following reaction equations: HF 2 6 2 2 6 2 (1- ) 6 K MnF 2(1- )KHF (1- )H SiF K Si Mn F 4(1 )HF x x x x x x , (1) for K2SiF6:Mn4+ phosphor； HF 2 6 2 2 6 2 (1- ) 6 K MnF 2NaHF (1- )H SiF Na Si Mn F 2 KF 2(2 )HF x x x x x x , (2) for Na2SiF6:Mn4+ phosphor; and 2 6 2 2 2 6 HF (1- ) 6 K MnF (1- )KHF NaHF (1- )H SiF KNaSi Mn F KF (4-3 )HF x x x x x x x , (3) for KNaSiF6:Mn4+ phosphor. In above equations, x represents the concentration of Mn4+ ion. Note that in K2SiF6:Mn4+ phosphor, K + ion originates from both K2MnF6 and KHF2, whereas Na element in the combinatorial phosphors is mainly supplied by the NaHF2 compound. More detailed description on the low-temperature synthesis can be referred to one previous publication.13 Figure 1(b) presents photographs of the synthesized powder phosphors, from which we see that the sample color gradually changes from faint orange-yellow of Na2SiF6:Mn4+ phosphor to bright golden yellow of K2SiF6:Mn4+ phosphor. This color change may imply significant change in both structure and PL properties, as shown and evidenced later. Figure 2. (a) XRD diffraction patterns of (KxNa1-x)2SiF6:Mn4+red phosphors with x=1, 0.75, 0.50, 0.25, and 0 from bottom to top. (b) Replotted XRD patterns at low diffraction angles. Structural phase transformation can be clearly seen as the relative content of K and Na is systematically varied. In order to elucidate the structural phase transformation in the studied series of (KxNa1-x)2SiF6:Mn4+ red phosphors, fine XRD measurements were carried out, and the results are illustrated in Figure 2(a). The XRD patterns at low diffraction angles are enlarged in Figure 2(b), so that the solid evidence for phase transformation can be clearly seen. From the experimental XRD patterns, three major lattice phase structures can be determined: cubic phase characterized by a space group of Fm3̅m for K2SiF6:Mn4+ , trigonal phase with a space group of P321 for Na2SiF6:Mn4+, and orthorhombic phase with a space group of Pnma for (K0.5Na0.5)2SiF6:Mn4+ or simply KNaSiF6:Mn4+. For (K0.25Na0.75)2SiF6:Mn4+ phosphor, it possess a mixed phase of trigonal and orthorhombic structures. As shown in Figure 2(a), the experimental XRD patterns were finely simulated with standard base data (e.g. JCPDF 81-2264, JCPDF 81-0009 and JCPDF 72-1115). To examine the influence of alkali metal composition variation on the crystalline structures of the phosphors more accurately, we did fine theoretical simulations on the measured XRD result of each sample with Rietveld method, as depicted in Figure S1-S5 in the supporting document. Obtained lattice parameters for the three major types of crystalline structures are tabularized in Table 1. Page 3 of 8 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B
JournalofMaterialsChemistryCPage4of8ARTICLEJournalNameTable 1.Rietveldrefinedlatticeparameters for thebecauseof both strongewAuantumopticalpropertiesphosphorswiththreetypesofcrystallineconfinement effect and remarkable redGctin0/ong9rgesynthesizedelectron-phonon interaction.30 In particularly,it may favor thestructures.phonon-induced luminescence and thus may result in highluminescence efficiency at roomtemperature and even higher.PhospCrystalLatticeLatticeLatticeUnit CellhorsVolumeStructureParametParameterParameterb (A)c (A)(A3)era (A)8.1258288.1258288.125828536.541K2SiFe:CubicMn4+士土土±0.0180.0000900.0000900.000090KNasiF9.3304595.5093029.804473Orthorho503.9926:Mn**土土mbic土±0.051200nm200.nm500nm0.0003590.0002110.000342o(e)NazSiFtrigonal8.8671808.8671805.047439343.6956:Mn4+士±土±0.0110.0001160.0001160.0000892nm200nmInadditiontotheXRDdataandanalysis,weemployedhigh-Figure 3. (a), (b), and (c) show TEM images of the synthesizedresolution TEM+selected area electron diffraction (SAED)tocubic K2SiFg:Mn4+, orthorhombic KNaSiFg:Mn4+,and trigonaldo a direct microstructural characterization on the samples.NazSiFg:Mn4+,respectively. The inset figures in (a), (b), and (c)Figure 3(a), (b)and (c) show typical TEM images of cubicaopeooashowtheir corresponding SAED patterns.(d)High-resolutionKNaSiFg:Mn4+,K,SiF6:Mn4+orthorhombicandtrigonalTEM image of the K-richcombinatorial phosphor.The insetNa2SiFg:Mn4+ phosphors,respectively.Their respective SAEDillustratesafastFouriertransformationpatternofthisHR-TEMimage.(e)TEM image of theNa-rich phosphorand its SAEDpatterns are depicted as the inset figures in upper rightcorners of respective TEM images. Figure 3(d) depicts a high-patterns (the inset figure).resolutionTEMimageoftheK-richphosphor(K75%+Na25%)Its fast Fourier transformation patterns are shown in the insetZfigure.Two different crystallineplanes canbe identified withNaSiFs:Mntalmostequal interplanarspacingof~0.307nm,asmarkedintrigonalthe image. Nevertheless, the measured angle between thecrystalline planes differsfrom the theoretical value,suggesting(ne) is d(KasNans)SiFe:Mnthe appearance of serious lattice distortion in the K-richmixedphosphor. Such lattice distortion shall be induced by thepartial substitution of K by Na. As for the Na-rich phosphor (K(Ka.soNanso)SiFe:Mn*25%+Na75%),itsTEMandcorrespondingSAEDpatternsareAshown in Figure 3(e). Compared with the SAED patterns oforthorhombicother phosphors, the SAED patterns of the Na-rich phosphor(KasNa)SiFe:Mn"display more complicated features.Such results are expectedbecausetheNa-richphosphorpossessesamixedlatticephasecubicof trigonal and orthorhombic structures,justifiedfrom the XRDKasiFs:Mnt0data.Thecharacteristic SAEDpatternsconsistingof rings anddiscrete bright spots in Figure 3(a), (b), (c)and (e) indicate thatcubicAallthesynthesizedphosphorsseemofnano-scaled590600610620630640650660polycrystalline.However,considering the sharp xRD patternsWavelength (nm)and theSEM images(not shownhere,but referredto thepreviousonespublishedelsewhere13),suchaspecialmaterialFigure 4. PL spectra of (KxNa1-x)2SiF6:Mn4+ (with x=1, 0.75, 0.50,structure composed of distorted nanocrystals +amorphous0.25,and0)phosphorsmeasuredatroomtemperature.Thesurrounding matrix is believed to be most likely caused by thedashed downward arrows in the figure indicate the zero-destructiondueto the bombardment of high-energy electronphonon lines (ZPL).Dbeam on thefluoride phosphors in TEMmeasurement.If suchstructure is produced under the bombardment of high-energyelectron or laserbeam, and it may exhibit some extraordinary4/J.Name.,2012,00,1-3This journal is@TheRoyal Societyof Chemistry20xxPleasedonotadjust margins
ARTICLE Journal Name 4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins Table 1 ． Rietveld refined lattice parameters for the synthesized phosphors with three types of crystalline structures. Phosp hors Crystal Structure Lattice Paramet er a (Å) Lattice Parameter b (Å) Lattice Parameter c (Å) Unit Cell Volume (Å3 ) K2SiF6: Mn4+ Cubic 8.125828 ± 0.000090 8.125828 ± 0.000090 8.125828 ± 0.000090 536.541 ±0.018 KNaSiF 6:Mn4+ Orthorho mbic 9.330459 ± 0.000359 5.509302 ± 0.000211 9.804473 ± 0.000342 503.992 ±0.051 Na2SiF 6:Mn4+ trigonal 8.867180 ± 0.000116 8.867180 ± 0.000116 5.047439 ± 0.000089 343.695 ±0.011 In addition to the XRD data and analysis, we employed highresolution TEM + selected area electron diffraction (SAED) to do a direct microstructural characterization on the samples. Figure 3(a), (b) and (c) show typical TEM images of cubic K2SiF6:Mn4+, orthorhombic KNaSiF6:Mn4+, and trigonal Na2SiF6:Mn4+ phosphors, respectively. Their respective SAED patterns are depicted as the inset figures in upper right corners of respective TEM images. Figure 3(d) depicts a highresolution TEM image of the K-rich phosphor (K 75% + Na 25%). Its fast Fourier transformation patterns are shown in the inset figure. Two different crystalline planes can be identified with almost equal interplanar spacing of ~0.307 nm, as marked in the image. Nevertheless, the measured angle between the crystalline planes differs from the theoretical value, suggesting the appearance of serious lattice distortion in the K-rich phosphor. Such lattice distortion shall be induced by the partial substitution of K by Na. As for the Na-rich phosphor (K 25% + Na 75%), its TEM and corresponding SAED patterns are shown in Figure 3(e). Compared with the SAED patterns of other phosphors, the SAED patterns of the Na-rich phosphor display more complicated features. Such results are expected because the Na-rich phosphor possesses a mixed lattice phase of trigonal and orthorhombic structures, justified from the XRD data. The characteristic SAED patterns consisting of rings and discrete bright spots in Figure 3(a), (b), (c) and (e) indicate that all the synthesized phosphors seem of nano-scaled polycrystalline. However, considering the sharp XRD patterns and the SEM images (not shown here, but referred to the previous ones published elsewhere13 ), such a special material structure composed of distorted nanocrystals + amorphous surrounding matrix is believed to be most likely caused by the destruction due to the bombardment of high-energy electron beam on the fluoride phosphors in TEM measurement. If such structure is produced under the bombardment of high-energy electron or laser beam, and it may exhibit some extraordinary optical properties because of both strong quantum confinement effect and remarkable reduction of long-range electron-phonon interaction. 30 In particularly, it may favor the phonon-induced luminescence and thus may result in high luminescence efficiency at room temperature and even higher. Figure 3. (a), (b), and (c) show TEM images of the synthesized cubic K2SiF6:Mn4+, orthorhombic KNaSiF6:Mn4+, and trigonal Na2SiF6:Mn4+, respectively. The inset figures in (a), (b), and (c) show their corresponding SAED patterns. (d) High-resolution TEM image of the K-rich combinatorial phosphor. The inset illustrates a fast Fourier transformation pattern of this HR-TEM image. (e) TEM image of the Na-rich phosphor and its SAED patterns (the inset figure). Figure 4. PL spectra of (KxNa1-x)2SiF6:Mn4+ (with x=1, 0.75, 0.50, 0.25, and 0) phosphors measured at room temperature. The dashed downward arrows in the figure indicate the zerophonon lines (ZPL). Journal of Materials Chemistry C Page 4 of 8 Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B
Page5of 8Journal of Materials Chemistry CJournalNameARTICLEFigure 4 illustrates the measured PL spectra of the samplesbe observed in their Raman spectrum. In fact, alltbe,Rssihleat room temperature. In the figure, the dashed downwardvibrationalmodesareindeedobservedPtHestidied(KNaarrows indicate the zero-phonon lines (zPL)of vibronicx)2siF6:Mn4+ phosphors. Even for the silent mode (i.e., v6), atransitions.Both Stokesandanti-Stokesluminescencelocal vibration mode (i.e., Ve(Mn)) of Mn-F bonds is stillsidebandsofphonon-assistedtransitionssymmetricallyresolved in the several Raman spectra in Figure 5. For thefdistribute at both sides of the ZPL line for phonon modes ofmeasured PL spectra, as argued earlier, the three localpV3,V4,and v6.Evidently,our spectral dataverifythe theoreticalvibrational modes around Mn ions, namely vn(Mn)n=3, 4, 6,prediction on multimode phonon-induced optical transitions inare observed and are in good agreement with available dataimpurity-activatedphosphors,anditisingoodagreementwithreportedby other groups.31Notethat intheRamanosnuewthe previous result reported by Takahashi and Adachi.31in the spectra, vs(Mn) mode actually significantly overlaps with vi(Si)room-temperaturePLspectraofthesamples,themostmode, as decoupled in Figure 5. Beside the three majorsignificant change in spectral structure could be disappearancephonon peaks, Takahashi and Adachi also observed severalof theZPL lineinthephosphorswithhighKcontents.Insharpweak composite phonon peaks such as v3 +LA, V3+V6 etc.in thecontrast,the zPL lines produced by the pure electronicPLspectrumoftheKzSiF6:Mn4+phosphor.31Forthephosphorstransitions of Mn ions are still very strong in the (KxNai-studied here, such weak PL structures seem to exist too.x)2SiFg:Mn4+ phosphors with relatively high Na contents, aseseen in Figure4.Even intheNa-richphosphorwithamixedphase, double intense ZPL lines are observed. These resultsindicate that the parityforbidden transition rule is stillkept forV(Si)Mn ions in high-symmetric crystal structures with cubic phase,V:(Mn)(Mn)tswhereasitcanbepartlybrokenfor Mn ionsinlowerVs(Mn)V(Mn)crystallographic symmetry hosts.In fact,as shown anddiscussed later,significant enhancement of the total lightemissionisindeedconfirmedinthehighKfluoridephosphorsx5with cubic phase.() sV(Si)Theconsistentevolutionof luminescence spectral structureswith the crystalline phase transformation inthe phosphorsVMnVES/(MnVa(Mn<5V-(Mn)providesaunambiguousdemonstrationofdecisiverelationshipbetweenmatterstructureandoptical property.32-34 Besides the ZPL lines, the phonon-induced luminescencebands also show distinctive dependence on the crystallinestructures of phosphors, i.e,apparent shift and variation ofthe band peak position and lineshape, respectively,as seen inVi(SI)Figure 4. The apparent shift in peak position of the phonon-Vs(MnVi(SiVe(MnVs(Mn) vV(Mn)Va(Si)Vs(Mn)inducedluminescencemayimplythevariationinfrequencyofV(Mn)local lattice vibration modes (phonons)in thephosphors.AsshowninFigure 5,measuredmicro-Raman scatteringspectra2003005055675700O02RamanShift(cm)give clear signatures for this speculation. All six possiblephononmodesdenoted byVi,V2,V3,.,V6in thephosphorsFigure 5.Room-temperature micro-Raman scattering spectraare even well resolved in Figure 5. It is known that Ramanof the red phosphors of (K,Na1-x)2SiFg:Mn4+ with x=1, 0.75, 0.50,scattering is sensitive to even symmetry (gerade) modes of0.25, and o from bottom to top.Notethat in the Ramanvibration, while infrared radiation (IR) is sensitive to odd (un-spectra, Vs(Mn) mode actually significantly overlaps with vi(Si)gerade)modes.Therearetotal sixphononmodesexisting inmode,as decoupled in thefigure.octahedral crystal field, among which three gerade modes天(e.g.vi,V2,and vs)are Raman active, while the remaining un-Ogerade modes are divided into IR-active (vsand v4) and silentmodes (v6).35 However, the parity selection rule for RamanAfter examining thePL spectra, XRD patterns and phononeuscattering can partially relax or be significantly altered inmodes,aswell astheirinterplays,wearenow inpositionforhighly-defected crystal materials.36 As a result, someforbiddenlooking atthe valence electrons of Mn4+ion in the luminescent?Lmodes (e.g.vsand v4 modes in the studied Mn-activatedcompounds.As illustrated in Figure 6, EPR spectra of thephosphors)canbecomeRaman-active,andeven newlocalsamples were measured at room temperature via applying avibrationmodecanbeobserved.37Inaddition,itispossibletomicrowaveradiationof9.41x1o9HzandscanninganexternalOmagnetic field from 2800 Gto 3900 G.Because of 3d3observesomecompositeormulti-phononpeaksinRamanelectronic configuration of Mn4+ ion, total of six multilinescattering spectrum, although they are usually very weak. It ishyperfine structures may be observed.38,39 As expected, sixthusanticipated thatmost of thevibration modes inthetwofine resonance structures are indeed observed in the EPRoctahedral structures ([SiFs]2-and [MnF]2-) of phosphors shallThis journal is@TheRoyal Society of Chemistry2OxxJ.Name.,2013,00,1-35Please do not adjust margins
Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5 Please do not adjust margins Please do not adjust margins Figure 4 illustrates the measured PL spectra of the samples at room temperature. In the figure, the dashed downward arrows indicate the zero-phonon lines (ZPL) of vibronic transitions. Both Stokes and anti-Stokes luminescence sidebands of phonon-assisted transitions symmetrically distribute at both sides of the ZPL line for phonon modes of ,, and. Evidently, our spectral data verify the theoretical prediction on multimode phonon-induced optical transitions in impurity-activated phosphors, and it is in good agreement with the previous result reported by Takahashi and Adachi. 31In the room-temperature PL spectra of the samples, the most significant change in spectral structure could be disappearance of the ZPL line in the phosphors with high K contents. In sharp contrast, the ZPL lines produced by the pure electronic transitions of Mn ions are still very strong in the (KxNa1- x)2SiF6:Mn4+ phosphors with relatively high Na contents, as seen in Figure 4. Even in the Na-rich phosphor with a mixed phase, double intense ZPL lines are observed. These results indicate that the parity forbidden transition rule is still kept for Mn ions in high-symmetric crystal structures with cubic phase, whereas it can be partly broken for Mn ions in lower crystallographic symmetry hosts. In fact, as shown and discussed later, significant enhancement of the total light emission is indeed confirmed in the high K fluoride phosphors with cubic phase. The consistent evolution of luminescence spectral structures with the crystalline phase transformation in the phosphors provides a unambiguous demonstration of decisive relationship between matter structure and optical property.32- 34 Besides the ZPL lines, the phonon-induced luminescence bands also show distinctive dependence on the crystalline structures of phosphors, i.e., apparent shift and variation of the band peak position and lineshape, respectively, as seen in Figure 4. The apparent shift in peak position of the phononinduced luminescence may imply the variation in frequency of local lattice vibration modes (phonons) in the phosphors. As shown in Figure 5, measured micro-Raman scattering spectra give clear signatures for this speculation. All six possible phonon modes denoted by in the phosphors are even well resolved in Figure 5. It is known that Raman scattering is sensitive to even symmetry (gerade) modes of vibration, while infrared radiation (IR) is sensitive to odd (ungerade) modes. There are total six phonon modes existing in octahedral crystal field, among which three gerade modes (e.g.andare Raman active, while the remaining ungerade modes are divided into IR-active (and) and silent modes ().35 However, the parity selection rule for Raman scattering can partially relax or be significantly altered in highly-defected crystal materials.36 As a result, some forbidden modes (e.g.and modes in the studied Mn-activated phosphors) can become Raman-active, and even new local vibration mode can be observed.37 In addition, it is possible to observe some composite or multi-phonon peaks in Raman scattering spectrum, although they are usually very weak. It is thus anticipated that most of the vibration modes in the two octahedral structures ([SiF6] 2- and [MnF6] 2- ) of phosphors shall be observed in their Raman spectrum. In fact, all the possible vibrational modes are indeed observed in the studied (KxNa1- x)2SiF6:Mn4+ phosphors. Even for the silent mode (i.e.,6), a local vibration mode (i.e.,6(Mn)) of Mn-F bonds is still resolved in the several Raman spectra in Figure 5. For the measured PL spectra, as argued earlier, the three local vibrational modes around Mn ions, namelyn(Mn) n=3, 4, 6, are observed and are in good agreement with available data reported by other groups. 31 Note that in the Raman spectra,3(Mn) mode actually significantly overlaps with(Si) mode, as decoupled in Figure 5. Beside the three major phonon peaks, Takahashi and Adachi also observed several weak composite phonon peaks such as LA, etc. in the PL spectrum of the K2SiF6:Mn4+ phosphor. 31 For the phosphors studied here, such weak PL structures seem to exist too. Figure 5. Room-temperature micro-Raman scattering spectra of the red phosphors of (KxNa1-x)2SiF6:Mn4+ with x=1, 0.75, 0.50, 0.25, and 0 from bottom to top. Note that in the Raman spectra,3(Mn) mode actually significantly overlaps with(Si) mode, as decoupled in the figure. After examining the PL spectra, XRD patterns and phonon modes, as well as their interplays, we are now in position for looking at the valence electrons of Mn4+ ion in the luminescent compounds. As illustrated in Figure 6, EPR spectra of the samples were measured at room temperature via applying a microwave radiation of 9.41×109 Hz and scanning an external magnetic field from 2800 G to 3900 G. Because of 3d3 electronic configuration of Mn4+ ion, total of six multiline hyperfine structures may be observed.38,39 As expected, six fine resonance structures are indeed observed in the EPR Page 5 of 8 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B
JournalofMaterials Chemistry CPage6of8ARTICLEJournal Namespectra of (K,Na1-x)2SiFg:Mn4+phosphors with 50% K contentView Article Onlineand above.It strongly suggests the formation of coordinatedDOI:10.1039/C7TC04695B[MnF]2-octahedral cluster. However, one more resonanceIn the last part of this study, we would like to give a briefstructure appears in the EPR spectra of high Na compounds.discussion on the integrated PL intensities and luminescenceThe sevenfine resonance structuresmay be ascribedto thelifetimes of the phosphors at room temperature. Figure 7(a)coexistence of both interstitial Mn4+ ions and coordinatedpresents integrated PL intensities(solid spheres) of thepMn4+ions[MnF]2-clustersinNazSiF6andinphosphors atroomtemperature,while (b)shows measuredPL(Ko.25Nao.75)2SiF6:Mn4+phosphors.As discussed earlier,theTthelifetimes(solidspheres)ofphosphorsatroomintroductionof more Nations intohostlatticecauses someUtemperature. From Figure 7(a), it can be seen that the overallshrinkage of unit cells which may hinder the efficientSemissionintensityofcubicK,SiFs:Mn4+phosphorisincreasedreplacement of Si4+ by Mn4+ to some extent. As a result, somesnulby more than 5 folds with respect to that of trigonalMn4+ ions probably exist as interstitial impurities in both hostlattices with high Na concentrations.For the phosphor with 50% NazSiF6:Mn4+ phosphor.By using the improved integrating-Na and 50% K, the six fine resonance signatures aresphere method,41 we measured PL quantum yield of thesuperimposedbysomeweak structures.Recall thefactthatphosphors at room temperature and found that the bestthe structural transformation from cubic to orthorhombicquantumefficiencyofthecubicK,SiFg:Mn4+phosphorreachesphase occursforthis compound.The six EPRresonance signals97.6 %, which is comparable to that recently observed fromOwith some weak superimposed structures are understood forhigh-quality KzSiF6:Mn4+phosphor by Garcia-Santamariaetethe compound with orthorhombic phase.As the Na content isal.42 Figure 7(b) shows obtained PL lifetimes of the phosphorstfurther increased,another lattice phase transition fromatroomtemperature.Thephase structuresofthephosphorsOorthorhombic to trigonal phase takes place in the compounds.arealsodescribedinthefigure.Fromthefigure,itcanbeseeneIt has been theoretically argued that the multiline numbersthat the cubic K2SiF6:Mn4+ phosphor exhibits the longestand detailed lineshapes of EPR signal are sensitive to the localClatticeenvironmentofmetal ions in complicated compounds.40lifetime of 8.36 ms with respect to the rest phosphors.WhenUWe thus have a concluding remark:The EPR data are25%KwasreplacedbyNa,themeasuredlifetimebecomesAsupportively consisted with the PL spectra, XRD patterns andslightly smaller, e.g.8.15 ms.Interestingly, the lifetimeRamanscatteringresults.significantlydecreases to5.55msfor thephosphorwith50%K?and 50% Na. From the XRD and other experimental datadescribed and analyzed earlier,we know that a crystallineEphase transition from cubic to orthorhombic phase occursNaSiFa:Mnwhen5o%KwasreplacedbyNainthecompounds.ForSNazSiFg:Mn4+with trigonal structure, its PL lifetimewas 6.06ms, longer than that of (Ko.sNao.s)2SiF6:Mn*+ but shorter thanwtrigonalthat of K2SiF6:Mn4+.As for (Ko.25Nao.75)2SiF6:Mn4+ phosphor withese(KaasNaa.s)SiFs:Mna mixed phase structures, its lifetime was 6.02 ms.Obviously,the PL lifetimes of the Mn4+-activated fluoride phosphorsexhibit a distinctive dependence on the lattice structuremixedlikewise the steady-state PL, Raman scattering and even EPRsignal. Therefore, this comprehensive experimental studyee(KasNas)SiFa:Mn*consistently and firmly demonstrates a conclusive relationshipbetweenmatterstructureand optical property.orthorhombic(KarsNas).SiFe:Mn*WcubicKSiFe:MnWncubic300032003400380028003600Magnetic Field (G)Figure6.EPRspectraofthephosphorsof (KxNa1-x)2SiF6:Mn4+(x=1,0.75,0.50,0.25,and0from bottomtotop)measured atroom-temperature.Inthemeasurements,constantmicrowaveradiationof9.41x109Hz wasapplied.6J.Name.,2012,00,1-3This journal is@TheRoyal Societyof Chemistry20xxPlease donot adjustmargins
ARTICLE Journal Name 6 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins spectra of (KxNa1-x)2SiF6:Mn4+ phosphors with 50% K content and above. It strongly suggests the formation of coordinated [MnF6] 2- octahedral cluster. However, one more resonance structure appears in the EPR spectra of high Na compounds. The seven fine resonance structures may be ascribed to the coexistence of both interstitial Mn4+ ions and coordinated Mn4+ ions in [MnF6] 2- clusters in Na2SiF6 and (K0.25Na0.75)2SiF6:Mn4+ phosphors. As discussed earlier, the introduction of more Na+ ions into host lattice causes some shrinkage of unit cells which may hinder the efficient replacement of Si4+ by Mn4+ to some extent. As a result, some Mn4+ ions probably exist as interstitial impurities in both host lattices with high Na concentrations. For the phosphor with 50% Na and 50% K, the six fine resonance signatures are superimposed by some weak structures. Recall the fact that the structural transformation from cubic to orthorhombic phase occurs for this compound. The six EPR resonance signals with some weak superimposed structures are understood for the compound with orthorhombic phase. As the Na content is further increased, another lattice phase transition from orthorhombic to trigonal phase takes place in the compounds. It has been theoretically argued that the multiline numbers and detailed lineshapes of EPR signal are sensitive to the local lattice environment of metal ions in complicated compounds.40 We thus have a concluding remark: The EPR data are supportively consisted with the PL spectra, XRD patterns and Raman scattering results. Figure 6. EPR spectra of the phosphors of (KxNa1-x)2SiF6:Mn4+ (x=1, 0.75, 0.50, 0.25, and 0 from bottom to top) measured at room-temperature. In the measurements, a constant microwave radiation of 9.41×109 Hz was applied. In the last part of this study, we would like to give a brief discussion on the integrated PL intensities and luminescence lifetimes of the phosphors at room temperature. Figure 7(a) presents integrated PL intensities (solid spheres) of the phosphors at room temperature, while (b) shows measured PL lifetimes (solid spheres) of the phosphors at room temperature. From Figure 7(a), it can be seen that the overall emission intensity of cubic K2SiF6:Mn4+ phosphor is increased by more than 5 folds with respect to that of trigonal Na2SiF6:Mn4+ phosphor. By using the improved integratingsphere method, 41 we measured PL quantum yield of the phosphors at room temperature and found that the best quantum efficiency of the cubic K2SiF6:Mn4+ phosphor reaches 97.6 %, which is comparable to that recently observed from high-quality K2SiF6:Mn4+ phosphor by Garcia-Santamaria et al. 42 Figure 7(b) shows obtained PL lifetimes of the phosphors at room temperature. The phase structures of the phosphors are also described in the figure. From the figure, it can be seen that the cubic K2SiF6:Mn4+ phosphor exhibits the longest lifetime of 8.36 ms with respect to the rest phosphors. When 25% K was replaced by Na, the measured lifetime becomes slightly smaller, e.g. 8.15 ms. Interestingly, the lifetime significantly decreases to 5.55 ms for the phosphor with 50% K and 50% Na. From the XRD and other experimental data described and analyzed earlier, we know that a crystalline phase transition from cubic to orthorhombic phase occurs when 50 % K was replaced by Na in the compounds. For Na2SiF6:Mn4+ with trigonal structure, its PL lifetime was 6.06 ms, longer than that of (K0.5Na0.5)2SiF6:Mn4+ but shorter than that of K2SiF6:Mn4+. As for (K0.25Na0.75)2SiF6:Mn4+ phosphor with a mixed phase structures, its lifetime was 6.02 ms. Obviously, the PL lifetimes of the Mn4+-activated fluoride phosphors exhibit a distinctive dependence on the lattice structure likewise the steady-state PL, Raman scattering and even EPR signal. Therefore, this comprehensive experimental study consistently and firmly demonstrates a conclusive relationship between matter structure and optical property. Journal of Materials Chemistry C Page 6 of 8 Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B
Page7of8JournalofMaterialsChemistry CARTICLEJournal NameuAcknowledgementsView Article OnlineDOI:10.1039/C7TC04695BThis work was supported by a Hong Kong RGC-GRF Grant0(GrantNo.HKU705812P),NationalNaturalScienceFoundation of China (Grant No.11374247),HKU SRT on New8FC81Materials,as well as inpartbyHK-UGCAoEGrants(GrantNo.AoE/P-03/08).Notesandreferencesn1P.Pust, V.Weiler, C.Hecht, A. Tucks, A. S. Wochnic, A.-K.0.00.20.40.60.81.0Henb, D. Wiechert, C. Scheu, P. J. Schmidt, W. Schnick, Nat.xvaluein (KNal-r)2SiF6:Mn*Mater.2014,13,8912 H. H. Zhu, C. C. Lin, W. Q. Luo, S. T. Shu, Z. G. Liu, Y. S. Liu, J. T.(b)Kong,E. Ma, Y.G. Cao, R.S. Liu, X.Y.Chen, Nat. Commun.-8.5?2014,5,4312pa3X.Huang,Nat.Photonics,2014,8,7488.0ii4P.Pust,P.J.Schmidt,W.Schnick,Nat.Mater.2015,14,454.ii(su)ona7.5oo5 S. Reineke, Nat. Mater.2015,14, 459.0上6K.N.Shinde,S.J.Dhoble,H.C.Swart,K.Park,Phosphate7.0PhosphorsforSolid-State Lighting,Springer,2012.7 S. Adachi, H. Abe, R. Kasa, T. Arai, J. Electrochem. Soc. 2012,6.5159,J34OO8 S. P. Singh, M. Kim, W. B. Park, J. Lee, K. Sohn, Inorg. Chem.6.02016,55,10310.9 Y. -T. Tsai, C. Y. Chiang, W. Zhou, J. -F. Lee, H. -S. Sheu, R. S.5.5Liu,J.Am.Chem.Soc.2015,137,89360.00.20.40.60.81.0510 M. H. Fang, H. D. Nguyen, C. C. Lin, R. S. Liu, J. Mater. Chem. Cxvalue in (K.Nai-)2SiF6:Mn2015, 3, 7277.11 L L. Wei, c. C. Lin, M. H. Fang, M. G. Brik, S. F. Hu, H. Jiao, R. S.KsFigure7.(a)Integrated PL intensities (solid spheres)of (KxNai-Liu,J.Mater.Chem.C2015,3,1655.x)2SiFg:Mn4+ (with x=0, 0.25, 0.5, 0.75, and 1 from left to right)12J.H.Oh,H.Kang,Y.J.Eo,H.K.Park,Y.R.Do,J.Mater.Chem.siwphosphors measured at roomtemperature. Inserted imagesC2015,3,607.13 F. Tang, Z. C. Su, H. G. Ye, M. Z. Wang, X. Lan, D. L. Phillips, Y.aretheluminescencephotosofthephosphors.(b)MeasuredG.Cao,S.J.Xu,J.Mater.Chem.C2016,4,9561.PL lifetimes (solid spheres) of the phosphorsatroom14 Y. Jin, M. H. Fang, M. Grinberg, S. Mahlik, T. Lesniewsi, M. G.temperature. The thin dashed and dotted lines in (a) and (b),Brik, G. Y. Luo, J. G. Lin, R. S. Liu, ACS Appl. Mater. Interfaces,respectively,aredrawn toguideeyes.2016,8,11194.15 L. Huang, Y. Zhu, X. Zhang, R. Zou, F. Pan, J. Wang, M. Wu,Chem.Mater.2016,28,1495.16 H. D. Nguyen, R. S. Liu, J. Mater. Chem. C 2016, 4, 10759.17E.Song,J.Q.Wang,J.Shi,T.Deng,S.Ye,M.Peng,J.Wang,L.ConclusionsWondraczek, Q.Zhang,Acs Appl. Mater.Interfaces 2017,9,8805.In conclusion, a series of Mn4+-activated (K,Na1-x)2SiF6 red18T.Deng,E.Song,J.Sun,L.Wang,Y.Deng,S.Ye,J.Wang,Q.phosphors were prepared below o C with a full-solutionZhang,J.Mater.Chem.C2017,5, 291019 Y.Zhu, L. Huang,R.Zou,J.Zhang,J.Yu, M.Wu,J.Wang,Q.Su,chemical route. Their structural transformation is firmlyJ.Mater.Chem.C2016,4,5690.revealed with a variety of techniques including high-resolution20 H. Chen, H. Lin, Q. Huang, F. Huang, J. Xu, B. Wang, Z. Lin, J.XRD, TEM, micro-Raman scattering etc.From the evolution ofZhou,Y.Wang,J.Mater.Chem.C2016,4,2374PL emission intensities with structure, the most exciting21 K. Huang,A. Rhys, Proc. Roy.Soc. (London)1950,A204, 406.findingcouldbetheconclusivedemonstrationofpromotionof22F.Seitz,Rev.Mod.Phys.1951,23,328.o23 F.E.William,J. Chem. Phys. 1951, 19,457.phonon-induced luminescence by structural transformation.24 R. Kubo, Y.Toyozawa, Prog.Theor.Phys.1955,13,160.All theexperimental data are highlyconsistent,leadingto the25B.Henderson,G.F.Imbusch,Optical Spectroscopyoffirm conclusion.This systematic study is thus of both technicalInorganicSolids,UKOxford:Oxford UniversityPress,1989and scientific significance to the wide-ranging communities,26 T. P. Martin, W. B.Fowler, Phys. Rev. 1970, B2, 4221.Oespecially to the science and technology of high-efficiency27 S.J.Xu,J.Lo Phonon Assisted Excition Luminescence二Processes in HeteroepitaxialGaN Films,inII-Nitridesolidphosphorsforfabricationofpc-WLEDlightbulbs.5Semiconductors Optical Properties Il, Ed.byManasrehMOandJiang HX.Taylor &Francis Books,New York, 2002,Chapter 8,339.Conflictsofinteresta28 S.J. Xu, H.J. Wang, S. H. Cheung, Q. Li, X. Q. Dai, M. H. Xie, S.Y. Tong,Appl. Phys. Lett. 2003, 83, 3477.There are no conflicts to declare".29 s.J.Xu, W. Liu, M.F.Li, Appl. Phys. Lett. 2000, 77,3376This journal is @ The Royal Society of Chemistry 20xxJ.Name.,2013,00,1-3/7Please donot adjust margins
Journal Name ARTICLE This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7 Please do not adjust margins Please do not adjust margins Figure 7. (a) Integrated PL intensities (solid spheres) of (KxNa1- x)2SiF6:Mn4+ (with x=0, 0.25, 0.5, 0.75, and 1 from left to right) phosphors measured at room temperature. Inserted images are the luminescence photos of the phosphors. (b) Measured PL lifetimes (solid spheres) of the phosphors at room temperature. The thin dashed and dotted lines in (a) and (b), respectively, are drawn to guide eyes. Conclusions In conclusion, a series of Mn4+-activated (KxNa1-x)2SiF6 red phosphors were prepared below 0 °C with a full-solution chemical route. Their structural transformation is firmly revealed with a variety of techniques including high-resolution XRD, TEM, micro-Raman scattering etc. From the evolution of PL emission intensities with structure, the most exciting finding could be the conclusive demonstration of promotion of phonon-induced luminescence by structural transformation. All the experimental data are highly consistent, leading to the firm conclusion. This systematic study is thus of both technical and scientific significance to the wide-ranging communities, especially to the science and technology of high-efficiency solid phosphors for fabrication of pc-WLED lightbulbs. Conflicts of interest There are no conflicts to declare”. Acknowledgements This work was supported by a Hong Kong RGC-GRF Grant (Grant No. HKU 705812P), National Natural Science Foundation of China (Grant No. 11374247), HKU SRT on New Materials, as well as in part by HK-UGC AoE Grants (Grant No. AoE/P-03/08). Notes and references 1 P. Pust, V. Weiler, C. Hecht, A. Tücks, A. S. Wochnic, A. –K. Henb, D. Wiechert, C. Scheu, P. J. Schmidt, W. Schnick, Nat. Mater. 2014, 13, 891. 2 H. H. Zhu, C. C. Lin, W. Q. Luo, S. T. Shu, Z. G. Liu, Y. S. Liu, J. T. Kong, E. Ma, Y. G. Cao, R. S. Liu, X. Y. Chen, Nat. Commun. 2014, 5, 4312. 3 X. Huang, Nat. Photonics, 2014, 8, 748. 4 P. Pust, P. J. Schmidt, W. Schnick, Nat. Mater. 2015, 14, 454. 5 S. Reineke, Nat. Mater. 2015, 14, 459. 6 K. N. Shinde, S. J. Dhoble, H. C. Swart, K. Park, Phosphate Phosphors for Solid-State Lighting, Springer, 2012. 7 S. Adachi, H. Abe, R. Kasa, T. Arai, J. Electrochem. Soc. 2012, 159, J34. 8 S. P. Singh, M. Kim, W. B. Park, J. Lee, K. Sohn, Inorg. Chem. 2016, 55, 10310. 9 Y. –T. Tsai, C. Y. Chiang, W. Zhou, J. –F. Lee, H. –S. Sheu, R. S. Liu, J. Am. Chem. Soc. 2015, 137, 8936. 10 M. H. Fang, H. D. Nguyen, C. C. Lin, R. S. Liu, J. Mater. Chem. C 2015, 3, 7277. 11 L. L. Wei, C. C. Lin, M. H. Fang, M. G. Brik, S. F. Hu, H. Jiao, R. S. Liu, J. Mater. Chem. C 2015, 3, 1655. 12 J. H. Oh, H. Kang, Y. J. Eo, H. K. Park, Y. R. Do, J. Mater. Chem. C 2015, 3, 607. 13 F. Tang, Z. C. Su, H. G. Ye, M. Z. Wang, X. Lan, D. L. Phillips, Y. G. Cao, S. J. Xu, J. Mater. Chem. C 2016, 4, 9561. 14 Y. Jin, M. H. Fang, M. Grinberg, S. Mahlik, T. Lesniewsi, M. G. Brik, G. Y. Luo, J. G. Lin, R. S. Liu, ACS Appl. Mater. Interfaces, 2016, 8, 11194. 15 L. Huang, Y. Zhu, X. Zhang, R. Zou, F. Pan, J. Wang, M. Wu, Chem. Mater. 2016, 28, 1495. 16 H. D. Nguyen, R. S. Liu, J. Mater. Chem. C 2016, 4, 10759. 17 E. Song, J. Q. Wang, J. Shi, T. Deng, S. Ye, M. Peng, J. Wang, L. Wondraczek, Q. Zhang, ACS Appl. Mater. Interfaces 2017, 9, 8805. 18 T. Deng, E. Song, J. Sun, L. Wang, Y. Deng, S. Ye, J. Wang, Q. Zhang, J. Mater. Chem. C 2017, 5, 2910. 19 Y. Zhu, L. Huang, R. Zou, J. Zhang, J. Yu, M. Wu, J. Wang, Q. Su, J. Mater. Chem. C 2016, 4, 5690. 20 H. Chen, H. Lin, Q. Huang, F. Huang, J. Xu, B. Wang, Z. Lin, J. Zhou, Y. Wang, J. Mater. Chem. C 2016, 4, 2374. 21 K. Huang, A. Rhys, Proc. Roy. Soc. (London) 1950, A204, 406. 22 F. Seitz, Rev. Mod. Phys. 1951, 23, 328. 23 F. E. William, J. Chem. Phys. 1951, 19, 457. 24 R. Kubo, Y. Toyozawa, Prog. Theor. Phys. 1955, 13, 160. 25 B. Henderson, G. F. Imbusch, Optical Spectroscopy of Inorganic Solids, UK Oxford: Oxford University Press, 1989. 26 T. P. Martin, W. B. Fowler, Phys. Rev. 1970, B2, 4221. 27 S. J. Xu, J. LO Phonon Assisted Excition Luminescence Processes in Heteroepitaxial GaN Films, in III-Nitride Semiconductors Optical Properties II, Ed. by Manasreh MO and Jiang HX. Taylor & Francis Books, New York, 2002, Chapter 8, 339. 28 S. J. Xu, H. J. Wang, S. H. Cheung, Q. Li, X. Q. Dai, M. H. Xie, S. Y. Tong, Appl. Phys. Lett. 2003, 83, 3477. 29 S. J. Xu, W. Liu, M. F. Li, Appl. Phys. Lett. 2000, 77, 3376. Page 7 of 8 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B
Page8of8JournalofMaterialsChemistryCARTICLEJournalName30 J. Q. Ning, C. C. Zheng, X. H.Zhang, S.J. Xu, Nanoscale 2015, 7,View Article Online17482.DOI:10.1039/C7TC04695B31T.Takahashi, S.Adachi,J.Electrochem.Soc.2008,155,E183.32 R.Kasa,S.Adachi,J.Electrochem.Soc.2012,159,J8933 S. K. Lim, M. Brewster, F. Qian, Y. Li, C. M. Lieber, S. Gradecak,Nano Lett.2009, 9, 3940.e34 Z. C. Su, C. C. Zheng, G. Cheng, C. M. Che, S. J. Xu, J. Mater.Chem.C2017,5,4488.35 R. Hoshino, S. Adachi, J. Appl. Phys. 2013, 114, 213502.36 J. Q. Ning, C. C. Zheng, L. X. Zheng, S.J. Xu, J. Appl. Phys. 2015,118,07310137 J. Q. Ning, S. J. Xu, D. P. Yu, Y. Y. Shan, S. T. Lee, Appl. Phys.Lett.2007,91,103117.38 W.R.Hagen, Dalton Trans.2006, 37,441539 A. Haddy, Photosynth.Res. 2007,92, 357.40 M.Zheng,G. C. Dismukes, Inorg. Chem. 1996, 35, 3307.41 J. C. de Mello, H. F. Wittmann, R. H. Friend, Adv. Mater. 1997,9, 230.42 F. Garcia-Santamaria, J. E. Murphy,A. A. Setlur, S. P. Sista, ECSeJ.SolidStateSci.Technol.2018,7,R3030eunor8|J.Name.,2012,00,1-3This journal is @The Royal Societyof Chemistry20xxPleasedonotadjustmargins
ARTICLE Journal Name 8 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins 30 J. Q. Ning, C. C. Zheng, X. H. Zhang, S. J. Xu, Nanoscale 2015, 7, 17482. 31 T. Takahashi, S. Adachi, J. Electrochem. Soc. 2008, 155, E183. 32 R. Kasa, S. Adachi, J. Electrochem. Soc. 2012, 159, J89. 33 S. K. Lim, M. Brewster, F. Qian, Y. Li, C. M. Lieber, S. Gradecak, Nano Lett. 2009, 9, 3940. 34 Z. C. Su, C. C. Zheng, G. Cheng, C. M. Che, S. J. Xu, J. Mater. Chem. C 2017, 5, 4488. 35 R. Hoshino, S. Adachi, J. Appl. Phys. 2013, 114, 213502. 36 J. Q. Ning, C. C. Zheng, L. X. Zheng, S. J. Xu, J. Appl. Phys. 2015, 118, 073101. 37 J. Q. Ning, S. J. Xu, D. P. Yu, Y. Y. Shan, S. T. Lee, Appl. Phys. Lett. 2007, 91, 103117. 38 W. R. Hagen, Dalton Trans. 2006, 37, 4415. 39 A. Haddy, Photosynth. Res. 2007, 92, 357. 40 M. Zheng, G. C. Dismukes, Inorg. Chem. 1996, 35, 3307. 41 J. C. de Mello, H. F. Wittmann, R. H. Friend, Adv. Mater. 1997, 9, 230. 42 F. Garcia-Santamaria, J. E. Murphy, A. A. Setlur, S. P. Sista, ECS J. Solid State Sci. Technol. 2018, 7, R3030. Journal of Materials Chemistry C Page 8 of 8 Journal of Materials Chemistry C Accepted Manuscript Published on 01 November 2017. Downloaded by University of Newcastle on 03/11/2017 06:08:29. View Article Online DOI: 10.1039/C7TC04695B