Synthesis, structure, and thermal expansion of the Sr5(AO4)3L (A = P, V, Cr; L = F, Cl, Br) apatites

We report structure refinement results for Sr5(VO4)3Br (a = 10.2793(2) Å, c = 7.32089(8) Å, V = 669.92(2) Å3; Rwp = 3.17%; Rp = 2.39%) and thermal expansion data for nine strontium apatites. An increase in the ionic radius of the halogen in the apatites is accompanied by a change in its crystallographic site and, as a consequence, an increase in linear thermal expansion coefficients and thermal expansion anisotropy parameter (αa/αc). A monoclinic phase of Sr5(CrO4)3Cl has been identified for the first time, which has negative linear thermal expansion coefficients along the crystallographic axis c. The volume expansivity of the strontium apatites correlates with their anisotropy parameter.