A New Hexagonal 12-Layer Perovskite-Related Structure:  Ba₆R₂Ti₄O₁₇ (R = Nd and Y)

Abstract

The crystal structure of Ba 6 R 2 Ti 4 O 17 (R = Nd and Y) was determined by combined analysis of X-ray and neutron powder diffraction data. Both materials crystallize in hexagonal symmetry with space group P 6 3 / mmc, Z = 2. The unit cell parameters at room temperature are a = 5.99283(9) Å and c = 29.9289(8) Å for Nd and a = 5.93055(4) Å and c = 29.5239(3) Å for Y. The structure can be described as stacking of cubic (c) and hexagonal (h) [BaO 3 ] layers as well as ordered, oxygen-deficient, pseudo-cubic (c‘) [BaO 2 ] layers in the sequence of (c ‘ cchcc) 2 along the c- axis and belongs to the 12-layer hexagonal perovskite-related family. The structural framework exhibits face-sharing TiO 6 octahedral layers (Ti 2 O 9 ), RO 6 octahedral layers, and TiO 4 tetrahedral layers. The TiO 4 subunits form around the c ‘ layers.