Location of Nd dopant in fluorapatite, Ca₅(PO₄)₃F: Nd

A doping-vehicle dependence of the relative substitution of Nd at the two non-equivalent Ca sites in fluorapatite has been verified. By implication, similar differences may be expected for other rare-earth dopants in other apatites. Detailed structure refinements based on especially reliable single-crystal X-ray diffraction data from (i) undoped, (ii) Nd₂O₃-doped, and (iii) NdF₃-doped synthetic fluorapatite were used. An analysis of ratios of site-occupancy factors, based on Nd³⁺-for-Ca²⁺ as the substitution model, led to the conclusions that (i) Nd substitutes at both Ca(l) and Ca(2) sites in approximately equal atomic fractions in the NdF₃-doped material and (ii) Nd substitutes only at the Ca(2) site in the Nd₂O₃-doped material. These results correlate well with reports that neodymium-doped fluorapatite crystals prepared as solid-state laser hosts differ in spectra, growth rate, and laser quality with the source of dopant (NdF₃ or Nd₂O₃).