An electron diffraction and bond valence sum study of the space group symmetries and structures of the photocatalytic 1:2 B site ordered A3CoNb2O9 perovskites (A = Ca2+, Sr2+, Ba2+)

Abstract

A careful investigation has been carried out into the space group symmetries, structures and crystal chemistries of the 1:2 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=ap, 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, ah=bp-cp, bh=cp-a p, ch=ap+bp+cp structure type while both the A=Ca and Sr compounds occur at room temperature in monoclinic P121/c1, am=[1̄1̄2]p, bm=[11̄0]p, cm=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. Show more