Synthesis and thermodynamic stability of Ba2B ' B '' O-6 and Ba3B*B-2 '' O-9 perovskites using the molten salt method

A number of mixed perovskites of the types Ba2B'B"O-6 (BaB'B-1/2"O-1/2(3) a nd Ba3B*B-2"O-9 (BaB1/3*B-2/3"O-3) where B' = Gd, La, Nd, Sm, or Y; B" = Nb and B* = Ca were synthesized by a conventional calcination process, as wel l as by the molten salt method, The former consists of calcining appropriat e mixtures of oxide or carbonate precursors in air at elevated temperatures (similar to 1250 degrees C), The latter method consists of adding appropri ate mixtures of oxide or carbonate precursors to a molten salt bath at rela tively low temperatures ton the order of 300 to 500 degrees C) so that the requisite compound is formed by dissolution-reprecipitation. X-ray diffract ion confirmed the formation of a single-phase perovskite in each case with calcination at 1250 degrees C, In a molten salt bath, however, all except B a2LaNbO6, and Ba2NdNbO6 formed the perovskite structure. On the contrary, p owders of Ba2LaNbO6 and Ba2NdNbO6 formed by a high-temperature calcination process readily decomposed when introduced into the molten salt bath. The f ormation of the requisite perovskite at a temperature as low as 350 degrees C in a molten salt suggests that: (a) The perovskite is stable at 350 degr ees C, (b) The molten salt exhibits sufficient precursor solubility for the dissolution-reprecipitation process to occur in a reasonable time. Similar ly, the decomposition of Ba2LaNbO6 and Ba2NdNbO6 in a molten salt bath show s that these materials are thermodynamically unstable at the temperature of the molten salt bath, (C) 1999 Academic Press.