Phase formation, structural and microstructural characterization of novel oxynitride–perovskites synthesized by thermal ammonolysis of (Ca,Ba)MoO 4 and (Ca,Ba)MoO 3

Abstract

Reactions of AMoO 4 and AMoO 3 ( A=Ca 2+, Ba 2+) with ammonia were investigated at 873 K< T<1123 K with the particular intention to synthesize novel oxynitride–perovskites of the general composition AMo(O,N) 3 and to study their crystal structure. CaMo(O,N) 3 and BaMo(O,N) 3 were prepared by thermal ammonolysis of the corresponding CaMoO 3 and BaMoO 3 precursors at T=898 and 998 K, respectively. The structural parameters of the oxynitrides were obtained from Rietveld refinements of X-ray and neutron powder diffraction data. CaMo(O,N) 3 crystallizes in the GdFeO 3 distorted perovskite structure with orthorhombic space group Pbnm and a=5.5029(1) Å, b=5.5546(1) Å, c=7.8248(1) Å as determined by X-ray powder diffraction. Its O/N content refined from the neutron diffraction data corresponds to the composition CaMoO 1.7(1)N 1.3(1). BaMo(O,N) 3 crystallizes in the cubic perovskite structure with space group Pm3¯ m and a=4.0657(1) Å as determined by X-ray powder diffraction. Transmission electron microscopy reveals a complex microstructure for both CaMoO 3 and CaMoO 1.7(1)N 1.3(1) represented by twin domains of different orientation.