The crystal structure of Ba8Ta6NiO24: Cation ordering in hexagonal perovskites

The new complex perovskite-like oxide Ba8Ta6NiO24 was synthesized and its crystal structure was studied using X-ray powder diffraction, electron diffraction, and high resolution electron microscopy. The Ba8Ta6NiO24 structure is based on a 8H (cchc)(2) close-packed stacking of BaO3 layers (space group P6(3)cm, a = 10.07458(6) Angstrom, c = 19.0122(2) Angstrom.). The structural refinement gave the following values of reliability factors: R(F) = 0.028, R(P) = 0.042, R(wP) = 0.056. The main feature of this structure is the existence of two different pairs of face-sharing octahedra. In the first pair one octahedron is occupied by Ta whereas the other one is empty. In the second pair one of the two octahedra is occupied by Ta and a cation vacancy and its neighbor is occupied by Ni, Ta, or a vacancy. This distribution of cations was completely confirmed by electron microscopy. The formation of a Ba8Ta6+0.4xNi1-xO24 solid solution with a narrow homogeneity range with (-0.2 less than or equal to x less than or equal to 0.2) was observed. (C) 1996 Academic Press, Inc.