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 Ca₃Nb_2−xV_xO₈ 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 Ca₁₃₈□₂₄]_A Ca₄₂Nb₁₁₇V₃]_BO₄₈₀□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.