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Page1of16Journal of Materials Chemistry CView Article OnlineDOI:10.1039/C6TC02737GSet of manganese ions activated fluoride phosphors (A,BF6:Mn4+, A=KNa, B=Si, Ge, Ti): Synthesis below O C and efficient room-temperaturephotoluminescenceFei Tang, Zhicheng Su," Honggang Ye," Mingzheng Wang," Xin Lan,' David LeePhillips,Yongge Cao, and Shijie XuaTransition-metal-ions activated solid-state phosphors are of particular interest for thedevelopment of LED-based white light sources. In addition to their relatively low cost,these luminescent materials show exceptionally high luminescence efficiencyespecially at room temperature and above,due to the involvement and promotion ofthermal phonons.Inthisarticle,wepresentacomprehensiveinvestigationonthesetof manganese ions doped fluoride phosphors (A2BF6:Mn*t, A=K, Na, B=Si, Ge, Ti),includingthesynthesisproceduresandvariouscharacterizationswiththeemphasisofoptical spectroscopic characterizations.All the phosphors synthesized at a temperatureof -16 C by chemical co-precipitation method exhibit intense red color emissions atroom temperature under the excitation of light with a wide range of wavelengths from450 nmto325nm.Inmost of the phosphors thephonon-assisted luminescencedominates in the spectra, which is evidenced by Raman scattering measurements.X-raydiffraction data of the samples reveal thatK2SiF6:Mn4+crystallizes in cubicphase, while the remaining crystals have hexagonal structures, but with differentsymmetries for K,TiF6:Mnttand NaSiF6:Mntt, respectively. More interestingly, thewell-resolved spectral splitting was observed on the major phonon-assistedluminescence signatures of both K2SiF6:Mn4+and K,TiF6:Mn*samples, indicatingthe occurrence of complicated but fascinating phonon-assisted transition processes inthe phosphors.IntroductionDriven by the big demand and rapid development of warm white light-emitting diodes(WLED), various Mn-activated alkali-metal hexafluoride red phosphors have beenexplored in recent years, primarily due to their distinct advantages in synthesis andluminescenceaspects.-Comparedwiththetraditional commercial redphosphorsusing Eu2+ ions as active centers, for example, CaAISiNg:Eu?+6. 7 which is usuallyaccompanied by a broad emission band, A2BF6:Mn4+ (i.e., A=K, Na, B=Si, Ge, Ti)phosphors exhibit sharp emission lines at red region with a high PL quantum yield(QY).Without use of expensive rare earth oxides and hard synthetic conditions, suchas high temperature and high pressure,the preparation of A,BF6:Mn4+phosphorsbecomes easier and much cheaper, which makes this kind of phosphors morecompetitive than the present commercial products. Nevertheless, the possibility oflarge varying chemical valance state of Mnelement from +7 to O also brings a greatDepartment of Physics, and Shenzhen Institute of Research and Innovation (SIRI),TheUniversity of Hong Kong, P. R. China.Corresponding author. Email: sjxu@hku.hkbDepartmentof Chemistry,TheUniversityofHongKong,P.R.China°Department of Physics, Renmin University of China, Beijing 100872, P. R. China1
1 Set of manganese ions activated fluoride phosphors (A2BF6:Mn4+, A=K, Na, B=Si, Ge, Ti): Synthesis below 0 oC and efficient room-temperature photoluminescence Fei Tang,a Zhicheng Su,a Honggang Ye,a Mingzheng Wang,a Xin Lan,b David Lee Phillips,b Yongge Cao,c and Shijie Xua* Transition-metal-ions activated solid-state phosphors are of particular interest for the development of LED-based white light sources. In addition to their relatively low cost, these luminescent materials show exceptionally high luminescence efficiency, especially at room temperature and above, due to the involvement and promotion of thermal phonons. In this article, we present a comprehensive investigation on the set of manganese ions doped fluoride phosphors (A2BF6:Mn4+, A=K, Na, B=Si, Ge, Ti), including the synthesis procedures and various characterizations with the emphasis of optical spectroscopic characterizations. All the phosphors synthesized at a temperature of -16 oC by chemical co-precipitation method exhibit intense red color emissions at room temperature under the excitation of light with a wide range of wavelengths from 450 nm to 325 nm. In most of the phosphors the phonon-assisted luminescence dominates in the spectra, which is evidenced by Raman scattering measurements. X-ray diffraction data of the samples reveal that K2SiF6:Mn4+ crystallizes in cubic phase, while the remaining crystals have hexagonal structures, but with different symmetries for K2TiF6:Mn4+and Na2SiF6:Mn4+, respectively. More interestingly, the well-resolved spectral splitting was observed on the major phonon-assisted luminescence signatures of both K2SiF6:Mn4+ and K2TiF6:Mn4+ samples, indicating the occurrence of complicated but fascinating phonon-assisted transition processes in the phosphors. Introduction Driven by the big demand and rapid development of warm white light-emitting diodes (WLED), various Mn4+-activated alkali-metal hexafluoride red phosphors have been explored in recent years, primarily due to their distinct advantages in synthesis and luminescence aspects.1-5 Compared with the traditional commercial red phosphors using Eu2+ ions as active centers, for example, CaAlSiN3:Eu2+ , 6, 7 which is usually accompanied by a broad emission band, A2BF6:Mn4+ (i.e., A=K, Na, B=Si, Ge, Ti) phosphors exhibit sharp emission lines at red region with a high PL quantum yield (QY).1 Without use of expensive rare earth oxides and hard synthetic conditions, such as high temperature and high pressure,8 the preparation of A2BF6:Mn4+ phosphors becomes easier and much cheaper, which makes this kind of phosphors more competitive than the present commercial products. Nevertheless, the possibility of large varying chemical valance state of Mn element from +7 to 0 also brings a great aDepartment of Physics, and Shenzhen Institute of Research and Innovation (SIRI), The University of Hong Kong, P. R. China. * Corresponding author. Email: sjxu@hku.hk bDepartment of Chemistry, The University of Hong Kong, P. R. China cDepartment of Physics, Renmin University of China, Beijing 100872, P. R. China Page 1 of 16 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
Journal ofMaterialsChemistryCPage2of16ViewArticleOnlinDOI:10.1039/C6TC02737Gchallenge in the synthetic process, which may result in undesired byproductscontaining Mnt(x+4)ions.Therefore, how to control the Mn valance state of being+4 during the reaction process is crucial to the preparation of A2BF:Mn4+ redphosphors withdesirableluminescence characteristics.Inthisstudy,wereporttherational svnthesis oftheseries of A,BF:Mn4+red phosphorsby usinga lowtraureowchmaltraonmdpftygouIt is known that in A2BF6:Mn4* red phosphors, B4ions are commonly coordinatedwith six Fions to form a regular octahedron,,l The introduction of Mnt ions intothe host lattice usually happens via substituting Btions in the view of their samevalence states and the lower formation energy.2 Thus, to avoid an excess latticedistortion, the choice of B4+ ions having similar ionic radius with Mn4+ is desired forsuch substitution process. In contrast, A element does not engage in the replacementbehavior of Mn+ ions and is generally alkali metal or alkaline earth metal element(A2).3, 14 On the other hand, the flexible selection of A element may offer us a goodprobability to turn the emission wavelength to meet our demand through adjusting theA-F bond strength to some extent which can lead to a large luminescence peakposition displacement.The two kinds of crystalline lattice structures,namelyhexagonal and cubicphases,havebeen verifiedforthisfamily of phosphorcrystals.Amongst them, K2SiF6:Mn+4crystal is found toform in the only cubic structure, butexhibit characteristic luminescence spectroscopic properties analogous to thehexagonal ones.15Thus far, several synthesis approaches have been developed to prepare this kind ofphosphors, including wet chemical etching,31617 cation exchange reactionoxydoreduction reaction's and hydrothermal reaction.1920 Each of them represents afeasiblepreparationrouteandshowssomeinterestingresults.However,basedontheconsideration of time consumption and the usage of strong corrosive HF solution, itstill remains a great challenge to develop a facile synthesis route for the low cost massproduction. Recently, Wei et al. proposed a low-temperature co-precipitation approachto synthesize K2MF6:Mn*t(M-Si and Ge) fluoride phosphors.10 This method candramatically shorten the preparation period.At the same time,low temperatureenvironment canefficientlydepresstheevaporationof HF solutionwhich may causesevere harm to human body.However, the synthesis temperature reported by themwas around or even above the boiling point of HF (19.5 C), which may still result inmassive evaporated HF gas. Inspired by the low-temperature co-precipitation idea, weimprove the route to make the synthesis to be carried out at temperature of-16 c.Byemploying different compounds containing B4+ions,for example,BO2,H2BF6, andK,BF asa reactant,we synthesize set of A,BF6:Mn* phosphors, e.g.K2SiF6:Mn**(KSFM),K,TiF6:Mn4*(KTFM),Na2SiF6:Mn**(NaSFM),andK,GeFg:Mn**(KGFM). And then we do comprehensive characterizations, includingX-ray diffraction, scanning electron microscope, steady-state and time-resolvedphotoluminescence (PL) and Raman scattering, as well as necessary ab initiocalculations, on these samples. These study activities make us obtain an overallstate-of-the-art understanding of this new class of red phosphors.2
2 challenge in the synthetic process, which may result in undesired byproducts containing Mn+x(x≠4) ions. Therefore, how to control the Mn valance state of being +4 during the reaction process is crucial to the preparation of A2BF6:Mn4+ red phosphors with desirable luminescence characteristics. In this study, we report the rational synthesis of the series of A2BF6:Mn4+ red phosphors by using a low temperature slow chemical titration method proposed first by Liu group.9, 10 It is known that in A2BF6:Mn4+ red phosphors, B4+ ions are commonly coordinated with six F- ions to form a regular octahedron.1, 11 The introduction of Mn4+ ions into the host lattice usually happens via substituting B4+ ions in the view of their same valence states and the lower formation energy.12 Thus, to avoid an excess lattice distortion, the choice of B4+ ions having similar ionic radius with Mn4+ is desired for such substitution process. In contrast, A element does not engage in the replacement behavior of Mn4+ ions and is generally alkali metal or alkaline earth metal element (A2).13, 14 On the other hand, the flexible selection of A element may offer us a good probability to turn the emission wavelength to meet our demand through adjusting the A-F bond strength to some extent which can lead to a large luminescence peak position displacement. The two kinds of crystalline lattice structures, namely hexagonal and cubic phases, have been verified for this family of phosphor crystals. Amongst them, K2SiF6:Mn+4 crystal is found to form in the only cubic structure, but exhibit characteristic luminescence spectroscopic properties analogous to the hexagonal ones.15 Thus far, several synthesis approaches have been developed to prepare this kind of phosphors, including wet chemical etching,3,16,17 cation exchange reaction,1 oxydoreduction reaction18 and hydrothermal reaction.19,20 Each of them represents a feasible preparation route and shows some interesting results. However, based on the consideration of time consumption and the usage of strong corrosive HF solution, it still remains a great challenge to develop a facile synthesis route for the low cost mass production. Recently, Wei et al. proposed a low-temperature co-precipitation approach to synthesize K2MF6:Mn4+(M=Si and Ge) fluoride phosphors.10 This method can dramatically shorten the preparation period. At the same time, low temperature environment can efficiently depress the evaporation of HF solution which may cause severe harm to human body. However, the synthesis temperature reported by them was around or even above the boiling point of HF (19.5 oC), which may still result in massive evaporated HF gas. Inspired by the low-temperature co-precipitation idea, we improve the route to make the synthesis to be carried out at temperature of -16 oC. By employing different compounds containing B4+ ions, for example, BO2, H2BF6, and K2BF6 as a reactant, we synthesize set of A2BF6:Mn4+ phosphors, e.g. K2SiF6:Mn4+(KSFM), K2TiF6:Mn4+(KTFM), Na2SiF6:Mn4+(NaSFM), and K2GeF6:Mn4+(KGFM). And then we do comprehensive characterizations, including X-ray diffraction, scanning electron microscope, steady-state and time-resolved photoluminescence (PL) and Raman scattering, as well as necessary ab initio calculations, on these samples. These study activities make us obtain an overall state-of-the-art understanding of this new class of red phosphors. Journal of Materials Chemistry C Page 2 of 16 Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
Page3of16Journal ofMaterialsChemistryCView Article OnlineDOI:10.1039/C6TC02737GExperimentalCommercial available chemicals were used as reactants and precursors tabulated inTable sl of supporting information. All these materials were of analytical gradewithout further modification before usage.Considering the corrosive feature of HFthe plastic containers, instead of glass or metal ones, were required to have beenalwaysusedintheentiresynthesisprocess.Synthesis of MaterialsThe red phosphors of A,BF6:Mn4* were prepared using the two-step chemicalco-precipitation approach. In the entire synthesis process the surrounding temperaturewas kept at -16 c. The synthesis of KMF compound was completed in the first stepThe major procedures were schematically depicted in Fig. 1(a). The synthesized KMFcompound will be used as the Mn* ions precursor in the next step of preparation. Forthe synthesis ofKMF compound,vigorous stirringoperationwasfirstly carried out inmixing HF solution and KHF2 powder to form a colorless solution, followed byaddingKMnO4powdersto form a black uniform mixture solution.Herein,HOsolution was employed as an efficient reducing agent to drop slowly into the abovesolution, usually accompanying with the generation of a large amount of oxygen.BeforefinishingtheadditionofH,O,a slowchangeof solutioncolorfromblacktobrown-yellowshallbeobserved,whichimpliesthereductionofchemical valancestate of Mn element from +7 to +4. It is required to note that the addition rate of H,O2solution shall be carefully controlled to prevent formation of other valance states ofMn ions in the system.The reaction process may be described by,2KMnO,+8HF+3H,0,+2KHE,→2K,MnF.+8H0+30,个(1)After KMF was fully precipitated from the reaction solution, vacuum filtrationoperation was proceeded to filter KMF precipitation, then followed by washingprocesses for three times to eliminate other impurities.The drying process was carriedout at 80C before the achievement of KMF powder.In the next major process the red-phosphor A,BFg:Mn*+ powders were synthesized.As illustratedinFig.l(b),theKMFpowderobtainedinthelast stepwasemployedasa reactant in this preparation process.It was first added into HF solution undervigorous stirring to form a uniform gold-yellow solution.Then the B elementcompounds were subsequently dropped into the solution undera continuous stirringoperation.Herein, it shall benoted that the different compounds ofB elements areselected for the synthesis of different red phosphors. For KTFM, KGFM, KSFM, theirB element precursors were chosen to beK,TiF6, GeO2,and H2SiF6,respectively.InthecaseofNaSFM,H2SiF6wasalsoadoptedasSi+sourcematerialjustlikeKSFMAfter some reaction time, yellow precipitation can be observed at the bottom ofcontainer.Then viatheprocessesof filtration,washing and drying in sequence,thefinal phosphors were obtained.3
3 Experimental Commercial available chemicals were used as reactants and precursors tabulated in Table s1 of supporting information. All these materials were of analytical grade without further modification before usage. Considering the corrosive feature of HF, the plastic containers, instead of glass or metal ones, were required to have been always used in the entire synthesis process. Synthesis of Materials The red phosphors of A2BF6:Mn4+ were prepared using the two-step chemical co-precipitation approach. In the entire synthesis process the surrounding temperature was kept at -16 oC. The synthesis of KMF compound was completed in the first step. The major procedures were schematically depicted in Fig. 1(a). The synthesized KMF compound will be used as the Mn4+ ions precursor in the next step of preparation. For the synthesis of KMF compound, vigorous stirring operation was firstly carried out in mixing HF solution and KHF2 powder to form a colorless solution, followed by adding KMnO4 powders to form a black uniform mixture solution. Herein, H2O2 solution was employed as an efficient reducing agent to drop slowly into the above solution, usually accompanying with the generation of a large amount of oxygen. Before finishing the addition of H2O2, a slow change of solution color from black to brown-yellow shall be observed, which implies the reduction of chemical valance state of Mn element from +7 to +4. It is required to note that the addition rate of H2O2 solution shall be carefully controlled to prevent formation of other valance states of Mn ions in the system. The reaction process may be described by, 4 2 2 2 2 6 2 2 2KMnO +8HF+3H O +2KHF 2K MnF +8H O+3O → ↑ . (1) After KMF was fully precipitated from the reaction solution, vacuum filtration operation was proceeded to filter KMF precipitation, then followed by washing processes for three times to eliminate other impurities. The drying process was carried out at 80 oC before the achievement of KMF powder. In the next major process the red-phosphor A2BF6:Mn4+ powders were synthesized. As illustrated in Fig. 1(b), the KMF powder obtained in the last step was employed as a reactant in this preparation process. It was first added into HF solution under vigorous stirring to form a uniform gold-yellow solution. Then the B element compounds were subsequently dropped into the solution under a continuous stirring operation. Herein, it shall be noted that the different compounds of B elements are selected for the synthesis of different red phosphors. For KTFM, KGFM, KSFM, their B element precursors were chosen to be K2TiF6, GeO2, and H2SiF6, respectively. In the case of NaSFM, H2SiF6 was also adopted as Si4+ source material just like KSFM. After some reaction time, yellow precipitation can be observed at the bottom of container. Then via the processes of filtration, washing and drying in sequence, the final phosphors were obtained. Page 3 of 16 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
JournalofMaterialsChemistryCPage4of16ViewArticleOnlirDOI:10.1039/C6TC02737GsStirtK-MnFs(a)(b)(A=KorNa.B=SiorGeorTi)Fig. 1 Schematic diagrams of the synthesis of (a) K,MnF6 powder and (b)A2BF6:Mn++(A=K orNa,B=Ti,Si, or Ge)red phosphors via low-temperatureco-precipitation methods.MaterialscharacterizationX-raydiffraction(XRD)patternsofthefinalproductswererecordedatascanrateof0.02 °swith an X-ray diffractometer (Type D8 Advance ECO, Bruker, UK). In thisdiffractometer a Cu K-alpha line was used as the irradiation source.The acceleratingvoltage and applied current were set at 40 kV and 80 mA, respectively, for the XRDmeasurements. The morphology of the samples studied was analyzed by using aSU-8010 cold-field emission scanning electron microscope (HITACHI UHR, Japan)at an accelerating voltage of 5 kV.High-resolution PL spectra were measured at room temperature on a home-madePL setup. The 476 nm line from an Ar-Kr ion mixed gas laser (Coherent Innova-70)was used as theexcitation sourceforPLmeasurements.The luminescence signal wasdispersedbyamonochromator(Spex75oM)beforetransformingintoanelectricalsignal withaphotomultipliertube(HamamatsuR928).Alock-inamplifier(StanfordResearch SR830)togetherwitha standardoptical chopperwasemployedtogainahigh signal to noise ratio. Finally, a data acquire module was used to convert theelectrical signal into digital data that can be accepted by computer.Room-temperatureRamanspectraofthesamplesweremeasuredonaconfocalmicro-Raman system (WITech-Alpha) by using the 514.5 nm line of an Ar ion laseras the excitation light source.The laser beam was focused on the samples via asingle-modeopticalfiber.Thescattered signal waspassedthroughalongpassfiltertoeliminate the Rayleigh line before light dispersing in a monochromator (ActonA
4 Fig. 1 Schematic diagrams of the synthesis of (a) K2MnF6 powder and (b) A2BF6:Mn4+ (A=K or Na, B=Ti, Si, or Ge) red phosphors via low-temperature co-precipitation methods. Materials characterization X-ray diffraction (XRD) patterns of the final products were recorded at a scan rate of 0.02 o s -1 with an X-ray diffractometer (Type D8 Advance ECO, Bruker, UK). In this diffractometer a Cu K-alpha line was used as the irradiation source. The accelerating voltage and applied current were set at 40 kV and 80 mA, respectively, for the XRD measurements. The morphology of the samples studied was analyzed by using a SU-8010 cold-field emission scanning electron microscope (HITACHI UHR, Japan) at an accelerating voltage of 5 kV. High-resolution PL spectra were measured at room temperature on a home-made PL setup. The 476 nm line from an Ar-Kr ion mixed gas laser (Coherent Innova-70) was used as the excitation source for PL measurements. The luminescence signal was dispersed by a monochromator (Spex 750M) before transforming into an electrical signal with a photomultiplier tube (Hamamatsu R928). A lock-in amplifier (Stanford Research SR830) together with a standard optical chopper was employed to gain a high signal to noise ratio. Finally, a data acquire module was used to convert the electrical signal into digital data that can be accepted by computer. Room-temperature Raman spectra of the samples were measured on a confocal micro-Raman system (WITech-Alpha) by using the 514.5 nm line of an Ar ion laser as the excitation light source. The laser beam was focused on the samples via a single-mode optical fiber. The scattered signal was passed through a longpass filter to eliminate the Rayleigh line before light dispersing in a monochromator (Acton Journal of Materials Chemistry C Page 4 of 16 Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
Page5of16Journal ofMaterialsChemistryCView Article OnlingDOI:10.1039/C6TC02737GSP2300i). Finally, the signal was grasped by a thermoelectrically cooled CCDdetector (Andor).Room-temperature time-resolved photoluminescence (TRPL) spectra of thessamples wererecorded witha nanosecond time-resolved spectrometer(LP920 laserflash spectrometer,EdinburghInstruments Ltd).In the TRPLmeasurements,aQ-switched Nd:YAG laser(3th harmonic line at >=355 nm, 10 ns) was employed toilluminate the samples inside a 10 mm quartz cell.At a right angle the luminescencesignal was collected and guided into a monochromator before it was detected by a fastphotomultiplier tube, and recorded on a Tektronix model TDS 3012C digital signalanalyzer.Resultsand discussionAs mentioned earlier, four kinds of red phosphors were prepared in the present studyTheir synthesis may be described in the following reaction equations, respectively:for KTFM phosphor,xK,MnF,+(1-x)K, TiF。H>K,Tif,Mn,F.,(2)for KSFM phosphor,xK,MnF+2(1-x)KHE,+(1-x)H,SiFHF→K,Si-r)Mn,F,+4(1-x)HF,(3)forKGFM phosphor;xK,MnF,+2(1-x)KHF,+2(1-x)HF+(1-x)GeO, HF→K,Ge()Mn,F+2(1-x)H,O, (4)and for NaSFM phosphor,xK,MnF+2NaHF,+(1-x)H,SiF。HF→Na,Si(d-r)Mn,F+2xKF+(4-2x)HF,(5)in which x represents the concentration of Mn*+ ions.From the chemical equation (2)to (4), it can be seen that the B element compounds are different to each other, leadingtothreedifferentcrystallizationmechanisms.AsfortheK,TiF.Mn+alloy,it canbesynthesized by using K2TiF6 via a direct reaction with KMF in which cation exchangebetween Ti4+ and Mn++ takes place. This simpler reaction process can beaccomplished in a short time without using KHF2 powder as reagent or catalyzer'It isasimplebutefficientwaytoproduceKTFMphosphor.However,suchamethod doesnot efficiently work on the preparation of KSFM phosphor.Alternatively,H2SiFwaschosen as a B compound, and was assumed to spontaneouslyreact with Mn4+ and Kions before theformationof final product.In the caseofKGFM,GeO2powder waschosen as a B compound, and the relevant chemical reaction processes are morecomplicated compared with the former ones. It is anticipated that both the cationexchange and non-cation exchange reactions probably co-exist in the reaction process,andfinallyresultsinthegenerationofKGFMphosphors.ThepreparationofNaSFMis similar to that of KSFM except that KHF2 was replaced by NaHF2 as a reactant andcatalyzer, asshowninreaction equation(5)Shown in Fig. 2 are the XRD patterns of all the synthesized powders measured at5
5 SP2300i). Finally, the signal was grasped by a thermoelectrically cooled CCD detector (Andor). Room-temperature time-resolved photoluminescence (TRPL) spectra of the samples were recorded with a nanosecond time-resolved spectrometer (LP920 laser flash spectrometer, Edinburgh Instruments Ltd). In the TRPL measurements, a Q-switched Nd:YAG laser (3th harmonic line at λ=355 nm, 10 ns) was employed to illuminate the samples inside a 10 mm quartz cell. At a right angle the luminescence signal was collected and guided into a monochromator before it was detected by a fast photomultiplier tube, and recorded on a Tektronix model TDS 3012C digital signal analyzer. Results and discussion As mentioned earlier, four kinds of red phosphors were prepared in the present study. Their synthesis may be described in the following reaction equations, respectively: for KTFM phosphor, HF K MnF +(1- )K TiF K Ti Mn F 2 6 2 6 2 1- 6 x x x x → , (2) for KSFM phosphor; HF K MnF 2(1 ) KHF (1 ) H SiF K Si Mn F +4(1- )HF 2 6 2 2 6 2 (1- ) 6 x x x x x x + − + − → , (3) for KGFM phosphor; HF K MnF +2(1- )KHF +2(1- )HF+(1- )GeO K Ge Mn F +2(1- 2 6 2 2 2 (1- ) 6 2 x x x x x → x x )H O , (4) and for NaSFM phosphor; HF K MnF +2NaHF +(1- )H SiF Na Si Mn F +2 KF+(4-2 )HF 2 6 2 2 6 2 (1- ) 6 x x x x x x → , (5) in which x represents the concentration of Mn4+ ions. From the chemical equation (2) to (4), it can be seen that the B element compounds are different to each other, leading to three different crystallization mechanisms. As for the K2TiF6:Mn+4 alloy, it can be synthesized by using K2TiF6 via a direct reaction with KMF in which cation exchange between Ti4+ and Mn4+ takes place. This simpler reaction process can be accomplished in a short time without using KHF2 powder as reagent or catalyzer.1 It is a simple but efficient way to produce KTFM phosphor. However, such a method does not efficiently work on the preparation of KSFM phosphor. Alternatively, H2SiF6 was chosen as a B compound, and was assumed to spontaneously react with Mn4+ and K+ ions before the formation of final product. In the case of KGFM, GeO2 powder was chosen as a B compound, and the relevant chemical reaction processes are more complicated compared with the former ones. It is anticipated that both the cation exchange and non-cation exchange reactions probably co-exist in the reaction process, and finally results in the generation of KGFM phosphors. The preparation of NaSFM is similar to that of KSFM except that KHF2 was replaced by NaHF2 as a reactant and catalyzer, as shown in reaction equation (5). Shown in Fig. 2 are the XRD patterns of all the synthesized powders measured at Page 5 of 16 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
JournalofMaterialsChemistryCPage6of 16ViewArticle OnlinDOI:10.1039/C6TC02737G20 ranging from 10° to 80°.As seen in the figure, the obtained diffraction peaks canbe well indexed to the hexagonal structure of KMF (JCPDS 77-2133), the cubicstructureofKSFM(JCPDS75-0694)thehexagonalstructureofNaSFM(JCPDS33-1280)and thehexagonal structureof KTFM(JCPDS 08-0488),However, thediffraction patterns ofKGFM indicatea complexlattice structure probably consistingof two hexagonal phases with space groups of P3ml and P63mc respectively. It is0eworthnoticing that in a previous report, the double phases ofKGFMwas observedfora much higher synthesis temperature of above 473 k 21 We thus wish to say that acomplicatedcrystallization mechanismisoccurring inthe chemical synthesis ofKGFM phosphor.When GeO2 was added into the mixture solution of HE, KHF2 andKMF, two kinds of reactions probably simultaneously occur, and lead to thegeneration of H,GeFandK,GeF.In thiscase,taking into account the sameioniceeradii of Mn4+(0.054nm)and Ge4+(0.054nm)22,thegeneration ofKGFM could berealized through the two routes being similar as reaction equations (2) and (3)However,thedifferentreactionmechanismsmaydirectlyresultinvariationof crystalstructure of KGFM phosphor.(202)K2MnF6(203)(100)omo(104NB1910(111)(222)K2SiF6:Mn4(220)()(400)adaoin(311)(422)51)53142012(301)美Na2SiF6:Mn4+(11)20(30BOB(520)oof-(002)K2TiF6:Mn4101(2002)(100)20101K2GeF6:Mn4+(202-EOL红102030607080AO20 (50leunoFig.2XRDdifraction patternsof K,MnF6,K,SiFg:Mntt,NaSiF:MnK,TiF6:Mn4+andK2GeF6:Mn4+phosphors.In sharp contrast to the case of KGFM, the remaining three phosphors display highpurity in lattice structure (composition). For example, no KMF phase was identifiedfromtheirXRDpatterns.AninterestingcomparisoncomesfromcubicKSFMandhexagonal NaSFM.Thecrystal structureoftheformercompoundbelongsto thespace6
6 2θ ranging from 10o to 80o . As seen in the figure, the obtained diffraction peaks can be well indexed to the hexagonal structure of KMF (JCPDS 77-2133), the cubic structure of KSFM (JCPDS 75-0694), the hexagonal structure of NaSFM (JCPDS 33-1280) and the hexagonal structure of KTFM (JCPDS 08-0488). However, the diffraction patterns of KGFM indicate a complex lattice structure probably consisting of two hexagonal phases with space groups of P3m1 and P63mc respectively. It is worth noticing that in a previous report, the double phases of KGFM was observed for a much higher synthesis temperature of above 473 K.21 We thus wish to say that a complicated crystallization mechanism is occurring in the chemical synthesis of KGFM phosphor. When GeO2 was added into the mixture solution of HF, KHF2 and KMF, two kinds of reactions probably simultaneously occur, and lead to the generation of H2GeF6 and K2GeF6. In this case, taking into account the same ionic radii of Mn4+ (0.054nm) and Ge4+ (0.054nm)22, the generation of KGFM could be realized through the two routes being similar as reaction equations (2) and (3). However, the different reaction mechanisms may directly result in variation of crystal structure of KGFM phosphor. Fig. 2 XRD diffraction patterns of K2MnF6, K2SiF6:Mn4+, Na2SiF6:Mn4+ , K2TiF6:Mn4+ and K2GeF6:Mn4+ phosphors. In sharp contrast to the case of KGFM, the remaining three phosphors display high purity in lattice structure (composition). For example, no KMF phase was identified from their XRD patterns. An interesting comparison comes from cubic KSFM and hexagonal NaSFM. The crystal structure of the former compound belongs to the space Journal of Materials Chemistry C Page 6 of 16 Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
Page7of16JournalofMaterialsChemistryCViewArticleOnlinDOI:10.1039/C6TC02737Ggroup of Fm3m, whereas the latter to the space group of P321. Differing from theformer's case, the reaction solution of NasFM contains two At ions, i.e. KtoriginatingfromKMFandNainNaHF2,whichprobably producethemixture ofsKSFMandNaSFM.However,theXRDresultrevealsthatonlyNa2SiF6phaseisdetected. It seems that Kions were not engaged in the final product. Both KSFM andNaSFM exhibit strong and sharp diffraction peaks, indicating that they possess highquality crystallization.This result implies that the synthesized scheme adoptingH,SiF6 as B compound is much efficient for preparing both the samples. For theas-synthesized KTFM powder, it also shows a hexagonal structure with space groupofP3ml.Unliketheprevioustwosampleswhoserelativeintensitiesofthediffractionpeaks are consistent with those of their host standard patterns, our KTFM sampleexhibits the unusual XRD patterns having significant difference in the relativeintensitywithrespect to its corresponding hoststandard patterns.For example.thestrongest peak intensity occurs along the crystal face(O02)for KTFM instead ofthe(101) or (201) crystal faces for standard K2TiF6, although their peak positionscoincide with each other. This result suggests that the cation exchange processbetween Ti4 and Mn*+ may considerably alter the growth orientation of crystal facewhilethebasichostlatticestructurewasunaffectedFig.3 Crystal structures and as-synthesized phosphor photos of K2SiF6:Mn*+K,TiF6:Mn*t, Na2SiF6:Mn4+and K,GeF6:Mn*+Fig. 3 depicts the crystal structures of each sample together with the photos of theas-synthesizedphosphors.AsidentifiedbytheXRDdata,KSFMcrystalbelongstothe face-centered cubic structure, while the as-prepared powder shows bright
7 group of Fm3m, whereas the latter to the space group of P321. Differing from the former’s case, the reaction solution of NaSFM contains two A+ ions, i.e. K+ originating from KMF and Na+ in NaHF2, which probably produce the mixture of KSFM and NaSFM. However, the XRD result reveals that only Na2SiF6 phase is detected. It seems that K+ ions were not engaged in the final product. Both KSFM and NaSFM exhibit strong and sharp diffraction peaks, indicating that they possess high quality crystallization. This result implies that the synthesized scheme adopting H2SiF6 as B compound is much efficient for preparing both the samples. For the as-synthesized KTFM powder, it also shows a hexagonal structure with space group of P3m1. Unlike the previous two samples whose relative intensities of the diffraction peaks are consistent with those of their host standard patterns, our KTFM sample exhibits the unusual XRD patterns having significant difference in the relative intensity with respect to its corresponding host standard patterns. For example, the strongest peak intensity occurs along the crystal face (002) for KTFM instead of the (101) or (201) crystal faces for standard K2TiF6, although their peak positions coincide with each other. This result suggests that the cation exchange process between Ti4+ and Mn4+ may considerably alter the growth orientation of crystal face while the basic host lattice structure was unaffected. Fig. 3 Crystal structures and as-synthesized phosphor photos of K2SiF6:Mn4+ , K2TiF6:Mn4+, Na2SiF6:Mn4+ and K2GeF6:Mn4+ . Fig. 3 depicts the crystal structures of each sample together with the photos of the as-synthesized phosphors. As identified by the XRD data, KSFM crystal belongs to the face-centered cubic structure, while the as-prepared powder shows bright Page 7 of 16 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
Journal of Materials Chemistry CPage8of16ViewArticleOnlirDOI:10.1039/C6TC02737Ggold-yellow color as shown in the third row in the right part of the figure. By contrast,both KTFM and KGFM are found to possess similar hexagonal structuralcharacteristics, and their as-synthesized powders show deep tin-yellow color for theeformer and brown yellow color for the latter,respectively.When their surfaces weredropped by water, it can befound that the surface colors of these two samples changeto some extent, indicating that theymaybe soluble in water.Asfor NaSFMphosphor,itdisplayslight yellowcolor and hashighchemicalstability.Thesurfacemorphologies of all the samples wereobserved with the field-emission SEMmicroscopeandtheobtainedimagesareshowninFig.4.Inaddition,aphotooftheKMF phosphor precipitation in the tube solution is also shown in Fig. 4(f). As seen inFig.4(a), the prepared KMF powder has a surface morphology of hexagonal pyramidwith geometric size of around 30 μm. But for NaSFM phosphor in Fig. 4(b), it has anirregular shape with typical size of about 10 μm. However, the powder of KSTMshows a great regular shape with crystal edges being clearly observed, and its crystalsize attains to about 20 μm, as shown in Fig. 4(c). Again, the powder of KTFM has anirregularshapewithawidesizedistributionfrom 10 umto100 um,asdepicted inFig4(e). In the case of KGFM, its morphology features by a large plate shape with planesizeofabout 100 um andthickness of around 10 um.The differentgeometric shapesand surface morphologies may reflect the difference in detailed crystallizationmechanismforthese samples in their synthesizingprocesses.Fig. 4 FESEM images of (a) K2MnF6, (b) Na2SiF6:Mn*t, (c) K,SiF:Mn*t, (d)K,GeF6:Mn*+,(e)K,TiF6:Mn*and (f)thephotoofK2MnF6precipitation.In order to gain a deep understanding on the crystallization mechanism during theco-precipitation reaction process,the first-principles calculations were performed withthe CASTEP module ofthe Materials Studio package,23and the final enthalpy of eachsample was achieved on the basis oftheir cubic and hexagonal structures,as presentedin Table 1. The calculated results indicate that for NaSF host, the enthalpy value islowerby63lmeVinhexagonal structurethanthatincubicstructure,implyingthatthe former structure is more stable than the latter. Likewise, the hexagonal KTF hasformation energy of 158 meV lower than its cubic phase. In contrast to these twocases,thecubicKsFismorethermodynamicallytablethanitshexagonalcounterpartThese theoretical results are in good agreement with the measured XRD results shown9
8 gold-yellow color as shown in the third row in the right part of the figure. By contrast, both KTFM and KGFM are found to possess similar hexagonal structural characteristics, and their as-synthesized powders show deep tin-yellow color for the former and brown yellow color for the latter, respectively. When their surfaces were dropped by water, it can be found that the surface colors of these two samples change to some extent, indicating that they may be soluble in water. As for NaSFM phosphor, it displays light yellow color and has high chemical stability. The surface morphologies of all the samples were observed with the field-emission SEM microscope and the obtained images are shown in Fig. 4. In addition, a photo of the KMF phosphor precipitation in the tube solution is also shown in Fig. 4(f). As seen in Fig. 4(a), the prepared KMF powder has a surface morphology of hexagonal pyramid with geometric size of around 30 µm. But for NaSFM phosphor in Fig. 4(b), it has an irregular shape with typical size of about 10 µm. However, the powder of KSTM shows a great regular shape with crystal edges being clearly observed, and its crystal size attains to about 20 µm, as shown in Fig. 4(c). Again, the powder of KTFM has an irregular shape with a wide size distribution from 10 µm to 100 µm, as depicted in Fig. 4(e). In the case of KGFM, its morphology features by a large plate shape with plane size of about 100 µm and thickness of around 10 µm. The different geometric shapes and surface morphologies may reflect the difference in detailed crystallization mechanism for these samples in their synthesizing processes. Fig. 4 FESEM images of (a) K2MnF6, (b) Na2SiF6:Mn4+, (c) K2SiF6:Mn4+, (d) K2GeF6:Mn4+, (e ) K2TiF6:Mn4+ and (f) the photo of K2MnF6 precipitation. In order to gain a deep understanding on the crystallization mechanism during the co-precipitation reaction process, the first-principles calculations were performed with the CASTEP module of the Materials Studio package,23 and the final enthalpy of each sample was achieved on the basis of their cubic and hexagonal structures, as presented in Table 1. The calculated results indicate that for NaSF host, the enthalpy value is lower by 631 meV in hexagonal structure than that in cubic structure, implying that the former structure is more stable than the latter. Likewise, the hexagonal KTF has formation energy of 158 meV lower than its cubic phase. In contrast to these two cases, the cubic KSF is more thermodynamically table than its hexagonal counterpart. These theoretical results are in good agreement with the measured XRD results shown Journal of Materials Chemistry C Page 8 of 16 Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G
Page9of16Journal ofMaterialsChemistryCViewArticleOnlinDOI:10.1039/C6TC02737Gand discussed above. For KGF host crystal, a small thermal enthalpy deviation of 7meV is obtained for the two lattice phases, implying that there is a large possibility forKGF crystal to crystallize in the two lattice phases. However, in the case of KGFMphosphor studied in the present work, the room temperature XRD data reveals that itsmajority lattice phase is still hexagonal. This result suggests that doping Mn** into theKGFhost crystal may favor constructing a stable hexagonal crystal structure.Table 1. Calculated final enthalpy and deviation of each sample for their cubic andhexagonal structuresFluoride MaterialsCubic (E./eV)Hexagonal (Er/eV)E=(En-E.)/meVNa2SiF6:Mn*+-6.7028416×103-631-6.7022104×103K,SiF6:Mn4+111-5.6532261×103-5.6531152×103-5.6494413(5)x103 *K2GeF6:Mn4+7-5.6494480x103K,TiFg:Mn*+-7.1497440×103-7.1499016x103-158*This value indicates the different final thermal enthalpies of two possible hexagonal structuresfor KGF host crystal.For the set of red phosphors synthesized in this work, their luminescence behaviorsare the remaining major subject of this study. Shown in Fig. 5 are the high-resolutionPL spectraofthe samplesmeasured at roomtemperature.As seenfromthePL spectraall they emit the red color luminescence featured by intenseStokes lines andrelatively weaker anti-Stokes lines. Stokes emission means photon generationaccompanying by phonon emission, while anti-Stokes emission is accompanied byphonon absorption.Boththeemissions basicallysymmetricallylocated atthetwosides of the zero phonon line (ZPL). Depending on sample, different samples shallhave their respective ZPL lines usually having different energetic locations due to thevariations of constituting element,composition and structure.Indeed the set offluoridephosphors having different chemical compositions arefound tohavedifferentZPL lines whose energetic positions are given in the figure and are marked by thedashed vertical lines. For the major emission lines, they have been simply attributedtothephonon-assistedtransitions involvingtheMnF-octahedralvibrationalmodes(thlrau4odinththuthere are six normal modes of vibration in an octahedral structure.25 They includethree threefold degenerate odd-parity vibrations, one of Tzu symmetry and two ofTiu symmetry, and these modes may be all involved within the characteristictransitions between the *A2g ground state and the 2Eg excited state of the Mn+ion.26 With respect to the ZPL emission behaviors, the phonon sidebands (e.g., theStokes lines) exhibit much stronger intensity in most of the samples. Interestingly,only one-order phonon sidebands are observed in these fluoride phosphorssynthesized at temperature of -16 C, which is sharply different from the emissioncharacteristics of semiconductors where one-order and higher order phonon sidebandsare generally observed.In the view of peak position, the ZPL lines tend to shifttoward higher energy direction in the order of KTFM, KGFM, and KSFM. However,9
9 and discussed above. For KGF host crystal, a small thermal enthalpy deviation of 7 meV is obtained for the two lattice phases, implying that there is a large possibility for KGF crystal to crystallize in the two lattice phases. However, in the case of KGFM phosphor studied in the present work, the room temperature XRD data reveals that its majority lattice phase is still hexagonal. This result suggests that doping Mn4+ into the KGF host crystal may favor constructing a stable hexagonal crystal structure. Table 1. Calculated final enthalpy and deviation of each sample for their cubic and hexagonal structures. Fluoride Materials Cubic (Ec/eV) Hexagonal (Eh/eV) △E=(Eh-Ec)/meV Na2SiF6:Mn4+ -6.7022104×103 -6.7028416×103 -631 K2SiF6:Mn4+ -5.6532261×103 -5.6531152×103 111 K2GeF6:Mn4+ -5.6494480×103 -5.6494413(5)×103 * 7 K2TiF6:Mn4+ -7.1497440×103 -7.1499016×103 -158 *This value indicates the different final thermal enthalpies of two possible hexagonal structures for KGF host crystal. For the set of red phosphors synthesized in this work, their luminescence behaviors are the remaining major subject of this study. Shown in Fig. 5 are the high-resolution PL spectra of the samples measured at room temperature. As seen from the PL spectra, all they emit the red color luminescence featured by intense Stokes lines and relatively weaker anti-Stokes lines. Stokes emission means photon generation accompanying by phonon emission, while anti-Stokes emission is accompanied by phonon absorption. Both the emissions basically symmetrically located at the two sides of the zero phonon line (ZPL). Depending on sample, different samples shall have their respective ZPL lines usually having different energetic locations due to the variations of constituting element, composition and structure. Indeed, the set of fluoride phosphors having different chemical compositions are found to have different ZPL lines whose energetic positions are given in the figure and are marked by the dashed vertical lines. For the major emission lines, they have been simply attributed to the phonon-assisted transitions involving the MnF6 2-octahedral vibrational modes (V3, V4, V6) in the literatures.3,15,24 According to the relevant theory of space group, there are six normal modes of vibration in an octahedral structure.25 They include three threefold degenerate odd-parity vibrations, one of symmetry and two of symmetry, and these modes may be all involved within the characteristic transitions between the ground state and the excited state of the Mn4+ ion.26 With respect to the ZPL emission behaviors, the phonon sidebands (e.g., the Stokes lines) exhibit much stronger intensity in most of the samples. Interestingly, only one-order phonon sidebands are observed in these fluoride phosphors synthesized at temperature of -16 oC, which is sharply different from the emission characteristics of semiconductors where one-order and higher order phonon sidebands are generally observed. In the view of peak position, the ZPL lines tend to shift toward higher energy direction in the order of KTFM, KGFM, and KSFM. However, Page 9 of 16 Journal of Materials Chemistry C Journal of Materials Chemistry C Accepted Manuscript Published on 19 September 2016. Downloaded by Cornell University Library on 20/09/2016 06:17:28. View Article Online DOI: 10.1039/C6TC02737G