Structural study of the cation ordering in the ternary oxide Ba_8Ti_3Nb_4O_(24)

摘要

The ternary oxide Ba_8Ti_3Nb_4O_(24) was synthesized and, based on a transmission electron microscopy study, its crystal structure was reexamined using single crystal X-ray diffraction. Ba_(8)Ti_3Nb_4O_(24) is a B-site deficient "hexagonal" perovskite with a (hccc)_2-type stacking sequence of the BaO_3 layers. In addition to the previous structural studies, the existence of a superstructure is clearly demonstrated (a = 10.068(4) A, c = 18.917(2) A, SG: P6_3/mcm, Z = 3). This superstructure is associated with the complex ordering of the Ti atoms, Nb atoms and vacancies on the B-sites of the h.c.p. part of the structure, which can be itself related to the necessity to minimize electrostatic repulsion between cations located in adjacent face-sharing octahedral layers.