Publication
Crystal structure of calcium vanadate-phosphate fluoride
Publisher:
Cambridge University Press (CUP)
Date:
18-03-2019
DOI:
10.1017/S0885715619000150
Abstract: Apatite-type materials A I 4 A II 6 (BO 4 ) 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 (V x P 1−x O 4 ) 6 F 2 obtained through the crystal structure characterization using Rietveld refinement and high-resolution transmission electron microscopy. The quantification of vanadium hosphorus partitioning in BO 4 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 °C) was employed. In comparison with the apatites with different ionic radii at A I and A II sites, Ca 10 (V x P 1−x O 4 ) 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 BO 4 tetrahedra when A site cation is fixed.