Crystal structure of calcium vanadate-phosphate fluoride

Apatite-type materials A(4)(I)A(6)(II)(BO4)(6)X-2 have two unique cations sites A(I) and A(II), which can host large mono-, di- tri- and tetra-valent cations. The average cation radii will affect the twist angle and lattice constants. However, there are few reports on the influence of B site substitutions on the twist angle and lattice parameters. It is believed that the lattice constant variation as a function of B site substitutions may not follow the same twist-angle model as proposed for A site. This work reports our results on the crystal chemistry of synthetic apatite Ca-10 (VxP1-xO4)(6)F-2 obtained through the crystal structure characterization using Rietveld refinement and high-resolution transmission electron microscopy. The quantification of vanadium/phosphorus partitioning in BO4 tetrahedra showed that equilibrium with more than 70% substitution of phosphorous by vanadium was difficult to achieve unless longer annealing (about 1 week at 900 degrees C) was employed. In comparison with the apatites with different ionic radii at A(I) and A(II) sites, Ca-10 (VxP1-xO4)(6)F-2 apatites with different ionic radii at B site show little twist angle variation for the whole series, which indicates that the dilation of unit cell constants is mainly because of the expansions of BO4 tetrahedra when A site cation is fixed. (C) 2019 International Centre for Diffraction Data.