Analysis of the effects of thermal treatments on CaHPO4 by 31P NMR spectroscopy

The compound CaHPO4 was obtained by slow evaporation at room temperature. The sample was characterised by X-ray diffraction, differential scanning calorimetry and 31P MAS NMR spectroscopy at different annealing temperature. At room temperature, the observed values of the 31P NMR chemical shift for the title compound are found to be −1.6, −0.4 and 1.4 ppm with the proportions 1/4, 1/2 and 1/4, respectively, revealing the presence of three non-equivalent phosphorus sites in the structure. The 31P NMR investigation at different annealing temperatures points to a conversion of (HPO42−) into (P2O74−) at high temperature.