High-temperature synthesis and structures of perovskite and n=1 Ruddlesden-Popper tantalum oxynitrides

The perovskite- and K2NiF4-structure group 5 oxynitrides SrTaO2N, BaTaO2N, Sr2TaO3N, and Ba2TaO3N, previously prepared by reacting oxide precursors with ammonia, may also be prepared by the reaction between the appropriate alkaline earth oxide and TaON at 1500 degreesC under 1 atm of pure nitrogen gas for a few hours. The nitrogen atmosphere is essential to prevent reduction of Ta-V, and the method of inductive heating ensures that the nitrogen atmosphere above the sample is maintained free of oxygen or water. SrTaO2N crystallizes in I4/mcm with a = 5.694 11(7) Angstrom, b = 8.0658(2) Angstrom, and Z = 4. The oxide and nitride anions are almost completely disordered, as observed when the material is made by nitridation of a pure oxide precursor using ammonia at 900-1000 degreesC, but contrasting with the full O/N order observed when such nitridation is mineralizer assisted. The detailed structure of Ba2TaO3N is reported for the first time: this compound adopts the K2NiF4 type structure with full O/N order in space group I4/mmm. with a = 4.115 08(3) Angstrom, c = 13.3778(l) Angstrom, and Z = 2 and is isostructural with the strontium analogue (Sr2TaO3N: I4/mmm, a = 4.038 99(4) c = 12.6007(3) Angstrom and Z = 2). O/N disorder is favored in perovskite-structure oxynitrides, but in layered K2NiF4-structure materials O/N ordering is the norm, with the ordering scheme dictated by the competition between cations of differing electronegativity for the anions.