Synthesis, structural and vibrational studies of lacunar apatite type NaPb2CaSr(PO4)3 and application to phosphoric acid demetallization

NaPb2CaSr(PO4)3 lacunar apatite was synthesized by the solid‐state reaction method. The compound was characterized by X‐ray diffraction, Raman and infrared spectroscopies. The crystal structure of this new compound has been determined by Rietveld method, adopting the space group P63/m (No. 176), with a = b = 9.6684(3) Å and c = 7.1104(3) Å. The analysis shows that the M(2) sites are shared by four cations Ca, Sr, Pb and Na. While the M(1) sites are occupied by the Ca, Sr and Pb cations. The structure was described as built up from [PO4]3‐ tetrahedra and Pb2+/Ca2+/Sr2+ of six fold coordination cavities (6h positions), which delimit void hexagonal tunnels running along [0 0 1]. The tunnels are connected by cations of mixed sites (4f) half occupied by Pb2+/Ca2+/Sr2+ and half by Na+ mixed alkali cation. The results of vibrational spectroscopy are in good agreements with the XRD measurements, the assignments of internal modes of (PO4)3‐ tetrahedra have been made and corroborate well with factor group analysis for the symmetry P63/m. An attempt to purify phosphoric acid is done using our apatite and the results are promising.