A study on the thermal conversion of scheelite-type ABO4 into perovskite-type AB(O,N)3

Abstract

Phase-pure scheelite AMoO₄ and AWO₄ (A = Ba, Sr, Ca) were thermally treated under an ammonia atmosphere at 400 to 900 °C. SrMoO₄ and SrWO₄ were shown to convert into cubic perovskite SrMoO₂N and SrWO_1.5N_1.5, at 700 °C and 900 °C respectively, and to form metastable intermediate phases (scheelite SrMoO_4−xN_x and SrWO_4−xN_x), as revealed by X-ray diffraction (XRD), elemental analysis and FTIR spectroscopy. High-temperature oxide melt solution calorimetry reveals that the enthalpy of formation for SrM(O,N)₃ (M = Mo, W) perovskites is less negative than that of the corresponding scheelite oxides, though the conversion of the scheelite oxides into perovskite oxynitrides is thermodynamically favorable at moderate temperatures. The reaction of BaMO₄ with ammonia leads to the formation of rhombohedral Ba₃M₂(O,N)₈ and the corresponding binary metal nitrides Mo₃N₂ and W_4.6N₄; similar behavior was observed for CaMO₄, which converted upon ammonolysis into individual oxides and nitrides. Thus, BaMO₄ and CaMO₄ were shown to not provide access to perovskite oxynitrides. The influence of the starting scheelite oxide precursor, the structure distortion and the degree of covalency of the B-site-N bond are discussed within the context of the formability of perovskite oxynitrides.