An electron diffraction and bond valence sum study of the space group symmetries and structures of the photocatalytic 1:1 ordered A2InNbO6 double perovskites (A = Ca2+, Sr2+, Ba2+)

Abstract

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=ap, subscript p for parent perovskite sub-structure) space group symmetry for the A=Ba and Sr compounds and Pnma (a=a p+bp, b=-ap+bp, c=2cp) 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=ap+bp, b=-a p+bp, c=2cp) perovskite-related superstructure phases while the A=Ba compound occurs in the Fm3̄m, a=2ap, 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. Show more