Abstract
Three perovskite-like phases of the CaNbO system have been isolated, Ca x NbO 3 (0.9 ⩽ x ⩽ 1), Ca(Ca x Nb 1−x) O 3 (0 ⩽ x ⩽ 1 3 ) , and Ca 3Nb 2O 8− x . X-ray powder diffraction and electron microscopy and diffraction were used to obtain crystallographic data of these phases. The high temperature form of Ca(Ca x Nb 1− x )O 3 perovskites, where calcium ions occupy partially the octahedral sites, presents an orthorhombic symmetry and quasi-periodic twin bands like Ca x NbO 3 compounds. Two monoclinic forms (for x = 1 3 and x = 1 4 ) were found by heating Ca(Ca x Nb 1− x )O 3 compounds at lower temperature for long periods; ordering of calcium and niobium ions in octahedral sites is proposed for that structure. The Ca 3Nb 2O 8− x compound presents a quadratic cell: ordering of the cations derived from (NH 4) 3FeF 6 structure is proposed in agreement with the observed space group P4 nnc . A new multiple-phase family, with general formula Ca n Nb n O 3 n+2 , was obtained for n = 4.25, 4.5, and 5; electron microscopy shows intergrowth phenomena.
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