Investigation of Phosphorus Site Condensation in CaHPO4 by Analysis of 31P MAS-NMR Tensor and X-Ray Powder Patterns

CaHPO4 was obtained by slow evaporation at room temperature. Seven samples, obtained at different annealing temperatures, were characterized by X-ray diffraction and 31P MAS-NMR spectroscopy. All NMR spectra were analyzed using a DMFIT program. At room temperature, the observed 31P NMR chemical shifts for the title compound were −1.59, −0.36 and 1.26 ppm with the relative intensities 39%, 10% and 51%, revealing the presence of three non-equivalent phosphorus sites in the structure. The investigation of the NMR tensor shift of all spectra shows that the abounding HPO2− 4 anion was progressively transformed into P2O 4− 7 when the temperature increased.