Single-crystal analysis of La-doped pyromorphite [Pb5(PO4)3Cl]

Abstract

Abstract Rare earth elements (REE) in calcium apatite have been widely described in the literature. Based on the investigations of minerals and their synthetic analogs, the mechanism of substitution of REE3+ for Ca2+ and their structural positions are well established. Although the presence of REE in natural pyromorphite has been reported, the structural response of substitution of REE3+ for Pb2+ is not established. A better understanding of REE-rich Pb-apatite may facilitate the potential use of this mineral in industrial processes. Two La-doped pyromorphite analogs [Pb5(PO4)3Cl] and two control pyromorphite analogs (with the absence of La) were synthesized from aqueous solutions at 25 °C. Na+ and K+ were used as charge-compensating ions to facilitate the incorporation of trivalent REE cations (La3+ + Na+ ↔ 2Pb2+ and La3+ + K+ ↔ 2Pb2+). Microprobe analysis, scanning electron microscopy, and Raman spectroscopy were used to confirm the purity of obtained phases. High-precision crystal structure refinements (R1 = 0.0140–0.0225) of all four compounds were performed from single-crystal X-ray diffraction data. The La content varied from 0.12(1) to 0.19(1) atoms per formula unit with the counter ions of K+ and Na+, respectively. Both substituting ions were accommodated at the Pb1 site only. By comparing the La-doped pyromorphite analogs with their control samples, it was possible to detect small changes in bond distances and polyhedral volumes caused by the La substitution. Variations in individual and mean interatomic distances reflected the cumulative effect of both the amount of substitution and ionic radii of substituting ions (La3+, Na+, and K+).