Example 1 Molten salt Fluxes ASrCO,+ALO, Ta, 0,+Sr AITao. (SrCL, flux, 900C) Powder sample, wash away SrCI with weakly acidic HO a Synthesis needs to be carried out at a temperature T>1400° C and Sr2Ta2O7 here the flux is a liquid Purity problems can arise, requires due to incorporation of the molten salt ions in product. This can be overcome either by using a salt ontaining cations and/or anions which are also Volatility of potassium plagues direct reaction resent in the desired product (i.e. synthesis of Example 3 Sra AITaO, in a SrCl, flux), or by g salts wher K2Te+Cu→K2Cu3Te3(K2 aTe Flux,350°0 the desired product (i.e. synthesis of PbZrO3 in a B,O, flux flux. In this approach the flux acts not only as a solvent but been made in the past decade using this approaegunds have also as a reactant. A large number of Precursor Routes Precursor Routes Approach: Decrease diffusion distances through intimate mixing of cations a Methods: With the exception of using mixed ADvantages: Lower reaction temperatures, possibly cation reactants, all precursor routes involve the stabilize metastable phases, eliminate intermediate a Mixing the starting reagents together in crystallites/high surface area dIsadvantages: Reagents are more difficult to work a Removal of the solvent, leaving behind an with. can be hard to control exact stoichiometry in amorphous or nanocrystaline mixture of cations certain cases, sometimes it is not possible to find Nba patible reagents(for example ions such as Ta+and Nb*immediately hydrolyze and precipitate in aqueous he result, e, oxalate, aving anions: acetate, reaction to the desired product Mixed Cation Synthesis of Na, ZrTeO Coprecipitation Synthesis of ZnFe,O attempting to make Na ZrTeO from Na, CO, ZrO mount -700-750.C, I began to form my desired product, but there of HO decreases a mixed Zn/Fe acetate(probably was also a considerable amount of ZrO, still present, together with some Na TeO Increasing the annealing temp e Na zrteo 950°C)did ntration, bu Fe, Zn(Coo2:>ZnFe 0,+ 4C0+4C02 before all of the ZrO, would react I began to volatilize a This method is easy and effective when it works. It is not tellurium species. tried pre-reacting the Na CO, and zro at 1000.C. I then reacted C to form single phase Na ZrTeOe These limitations make this route unpractical for man Na2CO3+ZrO2→Na2z stoichiometric ratios may not always be maintained Na2O3()+TeO2→Na2ZreO。(s)15 Molten Salt Fluxes Synthesis needs to be carried out at a temperature where the flux is a liquid. Purity problems can arise, due to incorporation of the molten salt ions in product. This can be overcome either by using a salt containing cations and/or anions which are also present in the desired product (i.e. synthesis of Sr2AlTaO6 in a SrCl2 flux) , or by using salts where the ions are of a much different size than the ions in the desired product (i.e. synthesis of PbZrO3 in a B2O3 flux). ß Example 1 4SrCO3 + Al2O3 + Ta2O5 → Sr2AlTaO6 (SrCl2 flux, 900°C) ß Powder sample, wash away SrCl2 with weakly acidic H2O ß Direct synthesis requires T > 1400°C and Sr2Ta2O7 impurities persist even at 1600°C ß Example 2 La2O3 + CuO + KOH→ La2-xKxCuO4 (KOH flux, 380°C) ß Volatility of potassium plagues direct reaction ß Example 3 K2Tex + Cu → K2Cu5Te5 (K2Tex Flux, 350°C) ß Example of reactive A2Qx (A = alkali metal, Q = S, Se, Te) flux. In this approach the flux acts not only as a solvent but also as a reactant. A large number of new compounds have been made in the past decade using this approach. Precursor Routes Approach : Decrease diffusion distances through intimate mixing of cations. Advantages : Lower reaction temperatures, possibly stabilize metastable phases, eliminate intermediate impurity phases, produce products with small crystallites/high surface area. Disadvantages : Reagents are more difficult to work with, can be hard to control exact stoichiometry in certain cases, sometimes it is not possible to find compatible reagents (for example ions such as Ta5+ and Nb5+ immediately hydrolyze and precipitate in aqueous solution). Precursor Routes Methods : With the exception of using mixed cation reactants, all precursor routes involve the following steps: Mixing the starting reagents together in solution. Removal of the solvent, leaving behind an amorphous or nanocrystaline mixture of cations and one or more of the following anions: acetate, citrate, hydroxide, oxalate, alkoxide, etc. Heat the resulting gel or powder to induce reaction to the desired product. Mixed Cation Synthesis of Na2 ZrTeO6 I was attempting to make Na2ZrTeO6 from Na 2CO3 , ZrO2 and TeO2 , using a conventional heat and beat approach. At ~700 -750°C, I began to form my desired product, but there was also a considerable amount of ZrO 2 still present, together with some Na2TeO4 . Increasing the annealing temperature (850 - 950°C) did lead to an increase in the Na 2ZrTeO6 concentration, but before all of the ZrO2 would react I began to volatilize a tellurium species. To circumvent this problem I tried pre-reacting the Na 2CO3 and ZrO2 to form Na2ZrO3 at ~1000°C. I then reacted Na2ZrO3 with TeO2 at 750°C to form single phase Na2ZrTeO6 . Na2CO3 + ZrO2®Na2ZrO3 Na2ZrO3 (s) + TeO2 (s) ® Na2ZrTeO6 (s) Coprecipitation Synthesis of ZnFe2O4 ß Mix the oxalates of zinc and iron together in water in a 1:1 ratio. Heat to evaporate off the water, as the amount of H2O decreases a mixed Zn/Fe acetate (probably hydrated) precipitates out. ß Fe2 ((COO)2 ) 3 + Zn(COO)2 ® Fe2Zn((COO)2 ) 5 ·xH2O ß After most of the water is gone, filter off the precipitate and calcine it (1000°C). ß Fe2Zn((COO)2 ) 5 ® ZnFe2O4 + 4CO + 4CO2 ß This method is easy and effective when it works. It is not suitable when 1. Reactants of comparable water solubility cannot be found. 2. The precipitation rates of the reactants is markedly different. ß These limitations make this route unpractical for many combinations of ions. Furthermore, accurate stoichiometric ratios may not always be maintained