Phase transitions and thermal expansion of apatite-structured compounds

The phase transitions and thermal expansion of apatite-structured compounds with the general formula M₅^II(A^VO₄)₃L (M^II = Ca, Sr, Cd, Ba, Pb; A^V = P, V; L = F, Cl) have been studied by high-temperature X-ray diffraction and differential thermal analysis. The Pb-containing apatites are shown to undergo phase transitions involving a reduction in unit-cell symmetry from hexagonal to monoclinic. The thermal expansion anisotropy in the hexagonal phases increases in the order Ca < Sr < Ba < Pb < Cd, and the monoclinic phases are less anisotropic but have larger thermal expansion coefficients in comparison with the hexagonal phases.