Crystallographic studies of the role of Mg as a stabilizing impurity in β-Ca3(PO4)2. II. Refinement of Mg-containing β-Ca3(PO4)2

The crystal structures of five samples of Mg-containing β-tricalcium orthophosphate, β-Ca3(PO4)2, have been refined and the Mg-impurity distribution has been examined. Typically, ∼7500 reflections were measured and merged into a unique set of ∼2500. Least-squares refinements with anisotropic temperature factors (and third cumulants for some atoms) produced weighted residuals, Rw(F), in the region of 0.02 to 0.04. Three of the five cation sites are always completely filled with calcium ions, a fourth site with Ca and Mg ions, and the fifth site contains Ca, Mg, and vacancies. Local order may exist because the sizes of the coordinations about the two Mg-containing cation sites are directly related by the orientation of phosphate group P(2)O4 such that a small ion in one site favors the incorporation of a small ion in the other site.