Abstract
The perovskite-related phase Ca 3Nb 2O 8, when grown as single crystals from a calcium vanadate flux, incorporates a small amount of vanadium from the flux to form the composition Ca 3Nb 2− x V x O 8 with x=0.025. The crystals have pseudo-cubic symmetry with a=6× a c(perovskite). The actual symmetry is rhombohedral, space group R3, with a h=16.910(1) Å, c h=41.500(2) Å. 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□ 24] A [Ca 42Nb 117V 3] B[O 480□ 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 3×3×3 blocks of ( a + a + a +) tilts are periodically twinned on the pseudo-cubic {1 0 0} c planes.
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