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Free keywords:
high-pressure synthesis; X-ray powder diffraction; crystal structure; calcium orthovanadate
Abstract:
A new polymorph of calcium orthovanadate Ca-3(VO4)(2) has been
synthesized at 11 GPa and 1373 K and recovered to ambient
conditions. It crystallizes in a monoclinic cell (space group
C2/m, Z = 2) with a = 9.6715(2), b = 5.43276(7), c = 7.0713(1)
Angstrom, beta = 116.949(1)degrees. The crystal structure has
been solved ab initio from X-ray powder diffraction data using
direct methods, The oxygen atoms form a hexagonal close
packing. The VO43- tetrahedra are radially and angularly
distorted. The two crystallographically independent Ca atoms
are in deformed octahedral and ten-fold coordinations. Unlike
in the palmierite-derived parent Ba-3(VO4)(2) (R (3) over barm,
Z = 1) and Ca-3(VO4)(2) (R3c, Z = 7), this network is three-
dimensional. The new monoclinic structure of calcium
orthovanadate is discussed in relation to those of related
orthophosphates and orthovanadates. It is suggested that the
pressure-induced amorphization of Ca-3(VO4)(2) (R3c, Z = 7) at
10 GPa and room temperature could be due to kinetically
inhibited changes in the dimensionality of the crystal
structure. (C) 2002 Editions scientifiques et medicales
Elsevier SAS. All rights reserved.
