Abstract
A new bismuth calcium vanadium oxide, BiCa 4V 3O 13, with hexagonal symmetry has been synthesized: space group P6 3 (#173), a = 9.819(2) Å, c = 7.033(2) Å, V = 587.2(3) Å ], and Z = 2. The structure was solved and refined from single crystal X-ray data leading to R = 0.055 and R w = 0.069 for 1076 unique reflections. Three different types of Ca sites were found: Ca(1) and Ca(2) coordinate to six O atoms, and Ca(3) coordinates to nine O atoms. There is no Bi site; instead, Bi partially occupies the Ca(1) and Ca(2) sites. The V atom is coordinated to four O atoms and forms a distorted tetrahedron with the V-O bond lengths ranging from 1.693(6) Å to 1.72(1) Å and O-V-O angles varying from 102.5(7)° to 114.7(6)°. The coordination polyhedra of Ca(1) and Ca(3) share faces forming chains along the c axis, and the coordination hexahedra of Ca(2) also from a chain along the c axis through sharing the corners among themselves. The VO 4 tetrahedra connect the two types of chains forming a three dimensional structure. The BiCa 4V 3O 13 formula may be written as A 5(VO 4) 3O to emphasize that this structure is essentially the same as the apatite structure, except that the inversion center is missing. Many compounds previously reported to have the apatite structure may actually have the lower symmetry version of this structure found in this study and thus be candidates for ferroelectricity.
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