An electron diffraction and bond valence sum study of the space group symmetries and structures of the photocatalytic 1:1 ordered A ₂InNbO₆ double perovskites ( a =Ca²⁺, Sr ²⁺, Ba²⁺)

A careful investigation has been carried out into the space group symmetries, structures and crystal chemistries of the 1:1 B-site ordered double perovskites A₂InNbO₆ (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 In³⁺ and Nb⁵⁺ ions onto the perovskite B-site positions of these compounds and hence Pm3̄m (a=a_p, subscript p for parent perovskite sub-structure) space group symmetry for the A=Ba and Sr compounds and Pnma (a=a _p+b_p, b=-a_p+b_p, c=2c_p) space group symmetry for the A=Ca compound. A careful electron diffraction study, however, shows that both the A=Ca and Sr compounds occur at room temperature in P12₁/n1 (a=a_p+b_p, b=-a _p+b_p, c=2c_p) perovskite-related superstructure phases while the A=Ba compound occurs in the Fm3̄m, a=2a_p, elpasolite structure type. Bond valence sum calculations are used to explain why this should be so as well as to provide a useful first-order approximation to the structures of each of the compounds.