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A New Octahedral Tilt System in the Perovskite Phase Ca3Nb2O8



L M. Cranswick, W G. Mumme, I E. Grey, Robert S. Roth, P Bordet


The perovskite-related phase Ca3Nb2O8, when grown as single crystals from a calcium vanadate flux, incorporates a smallamount of vanadium from the flux to form the composition Ca3Nb2-xVxO8 with x = 0.025 . The crystals have pseudo-cubicsymmetry with a = 6 x ac(perovskite). The actual symmetry is rhombohedral, space group R3, with ah = 16.910(l) , ch = 41.500 (2) . 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 [Ca138 24]A [Ca42Nb117V3]8[0480 6}, withvacancies 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-centred octahedra alternate with layers containing both Nb-centred and Ca-centred 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 3x3x3 blocks of (a+a+a+) tilts are periodically twinned on the pseudo-cubic l 00}c planes.
Journal of Solid State Chemistry
No. 1


Ca<sub>3</sub>Nb<sub>2</sub>O<sub>8</sub>, CaO-Nb<sub>2</sub>O<sub>5</sub>, crystal structure, neutron diffractin, perovskite, x-ray diffraction


Cranswick, L. , Mumme, W. , Grey, I. , Roth, R. and Bordet, P. (2003), A New Octahedral Tilt System in the Perovskite Phase Ca<sub>3</sub>Nb<sub>2</sub>O<sub>8</sub>, Journal of Solid State Chemistry (Accessed April 16, 2024)
Created March 31, 2003, Updated October 12, 2021