A new octahedral tilt system in the perovskite phase Ca3Nb2O8

The perovskite-related phase Ca3Nb2O8, when grown as single crystals from a calcium vanadate flux, incorporates a small amount of vanadium from the flux to form the composition Ca3Nb2-xVxO8 with x = 0.05. The crystals have pseudo-cubic symmetry with a = 6 x a(c)(perovskite). The actual symmetry is rhombohedral, space group R3, with a(h) = 16.910(1) Angstrom, C-h = 41.500(2) Angstrom. The structure was solved using a combination of single-crystal methods together with constrained refinements of powder X-ray and neutron powder data. The unit-cell composition is [Ca(138)square(24)]A [Ca42Nb117V3](B)[O(480)square(6)], With vacancies in both the anion sites and A-cation sites. The Ca and Nb atoms are fully ordered in the B-sites such that (001) layers containing only Nb-centered octahedra alternate with layers containing both Nb-centered and Ca-centered octahedra. At the origin B-site, ordered oxygen vacancies result in the octahedron being transformed to a tetrahedron, which, in the single crystals, is occupied by vanadium. The structure displays a new type of octahedral tilt system in which 3 x 3 x 3 blocks of (a(+)a(+)a(+)) tilts are periodically twinned on the pseudo-cubic {100}(c) planes. (C) 2003 Elsevier Science (USA). All rights reserved.