An electron diffraction and bond valence sum study of the space group symmetries and structures of the photocatalytic 1 : 1 ordered A(2)InNbO(6) double perovskites (A = Ca2+, Sr2+, Ba2+)

摘要

A careful investigation has been carried Out into the space group symmetries, structures and crystal chemistries of the 1:1 B-site oredered double perovskites A(2)InNbO(6) (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 it random distribution of In3+ and Nb5+ ions onto the perovskite B-site positions of these compounds and hence Pm (3) over barm (a = a(p), subscript p for parent perovskite substructure) space group symmetry for the A = Ba and Sr compounds and Pmma (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 P12(1)1 (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 Fm (3) over barm, 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. (C) 2003 Elsevier Inc. All rights reserved. [References: 16]