Structures of ordered tungsten- or molybdenum-containing quaternary perovskite oxides

The room temperature crystal structures of six A2MMoO6 and A2MWO6 ordered double perovskites were determined from X-ray and neutron powder diffraction data. Ba2MgWO6 and Ba2CaMoO6 both adopt cubic symmetry (space group Fm3̄m, tilt system a0a0a0). Ba2CaWO6 has nearly the same tolerance factor (t=0.972) as Ba2CaMoO6 (t=0.974), yet it surprisingly crystallizes with I4/m symmetry indicative of out-of-phase rotations of the MO6 octahedra about the c-axis (a0a0c−). Sr2ZnMoO6 (t=0.979) also adopts I4/m symmetry; whereas, Sr2ZnWO6 (t=0.976) crystallizes with monoclinic symmetry (P21/n) with out-of-phase octahedral tilting distortions about the a- and b-axes, and in-phase tilting about the c-axis (a−a−c+). Ca2CaWO6 (t=0.867) also has P21/n symmetry with large tilting distortions about all three crystallographic axes and distorted CaO6 octahedra. Analysis of 93 double perovskites and their crystal structures showed that while the type and magnitude of the octahedral tilting distortions are controlled primarily by the tolerance factor, the identity of the A-cation acts as the secondary structure directing factor. When A=Ba2+ the boundary between cubic and tetragonal symmetries falls near t=0.97, whereas when A=Sr2+ this boundary falls somewhere between t=1.018 and t=0.992.