New Ca2.90(Me2+)0.10(PO4)2 β-tricalcium Phosphates with Me2+ = Mn, Ni, Cu: Synthesis, Crystal-Chemistry, and Luminescence Properties

Ca2.90Me2+0.10(PO4)2 (with Me = Mn, Ni, Cu) β-tricalcium phosphate (TCP) powders were synthesized by solid-state reaction at T = 1200 °C and investigated by means of a combination of scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy, and luminescence spectroscopy. SEM morphological analysis showed the run products to consist of sub spherical microcrystalline aggregates, while EDS semi-quantitative analysis confirmed the nominal Ca/Me composition. The unit cell and the space group were determined by X-ray powder diffraction data showing that all the compounds crystallize in the rhombohedral R3c whitlockite-type structure, with the following unit cell constants: a = b = 10.41014(19) Å, c = 37.2984(13) Å, and cell volume V = 3500.53(15) Å3 (Mn); a = b = 10.39447(10) Å, c = 37.2901(8) Å; V = 3489.22(9) Å3 (Ni); a = b = 10.40764(8) Å, c = 37.3158(6) Å, V = 3500.48(7) Å3 (Cu). The investigation was completed with the structural refinement by the Rietveld method. The FTIR spectra are similar to those of the end-member Ca β-tricalcium phosphate (TCP), in agreement with the structure determination, and show minor band shifts of the (PO4) modes with the increasing size of the replacing Me2+ cation. Luminescence spectra and decay curves revealed significant luminescence properties for Mn and Cu phases.