An electron diffraction and bond valence sum study of the space group symmetries and structures of the photocatalytic 1:2 B site ordered A ₃CoNb₂O₉ perovskites (A=Ca²⁺, Sr²⁺, Ba²⁺)

A careful investigation has been carried out into the space group symmetries, structures and crystal chemistries of the 1:2 B-site ordered triple perovskites A₃Co²⁺Nb⁵⁺₂O₉ (A=Ca²⁺, Sr²⁺, Ba²⁺) using a combination of bond valence sum calculations, powder XRD and electron diffraction. A recent investigation of these compounds by Yin et al. reported a random distribution of Co²⁺ and Nb⁵⁺ ions onto the perovskite B-site positions and hence Pm3̄m (a=a_p, subscript p for parent perovskite sub-structure) space group symmetry for the A=Ba and Sr compounds and a monoclinically distorted variant thereof (although no space group was given) in the case of the A=Ca compound. An electron diffraction study, however, shows that the A=Ba compound occurs in a 1:2 B site ordered trigonal P3̄m1, a_h=b_p-c_p, b_h=c_p-a _p, c_h=a_p+b_p+c_p structure type while both the A=Ca and Sr compounds occur at room temperature in monoclinic P12₁/c1, a_m=[1̄1̄2]_p, b_m=[11̄0]_p, c_m=3[110]_p, perovskite-related structure types. The latter monclinic structure type can be described as a modulated variant of the 1:2 B site ordered trigonal P3̄m1 structure type with the additional modulations associated with a b ^-b^-c⁺ octahedral tilt sequence. Bond valence sum calculations are used to explain why this should be so as well as to provide a useful zeroth-order approximation to the structures of each of the compounds.