An electron diffraction and bond valence sum study of the space group symmetries and structures of the photocatalytic 1:1 ordered A2InNbO 6 double perovskites ( A=Ca 2+, Sr 2+, Ba 2+)

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 A 2InNbO 6 ( A=Ca 2+, Sr 2+, Ba 2+) 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 In 3+ and Nb 5+ ions onto the perovskite B-site positions of these compounds and hence Pm 3 ̄ m ( 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 =2 c 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 P12 1/ n1 ( a = a p + b p , b =− a p + b p , c =2 c p ) perovskite-related superstructure phases while the A=Ba compound occurs in the Fm 3 ̄ m , a=2 a 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.