Insights into new calcium phosphosilicate xerogels using an advanced characterization methodology

Amorphous calcium phosphosilicate xerogels of high phosphate content were synthesized by a new sol–gel route. Their structural characterization was achieved through the combination of complementary analytical methods, including advanced solid state NMR and scattering techniques. Two representative compositions, with similar P contents but a different Ca:Si ratio, have been chosen for detailed study. Using 43Ca solid state NMR and Ca K-edge XANES, the Ca local environment in the samples was characterized, revealing that it is similar for both compositions. It was found that POSi linkages are present in both compounds after calcination at 350 °C. However, for the sample with the lower Si content, a higher fraction of unusual 6-coordinated silicon was observed. Interestingly, calcium was also found to enhance the network connectivity and to enter the phosphosilicate network without the need for calcination at high temperature, which is advantageous in comparison with syntheses performed previously on similar compounds.

► Calcium cations enter gel network at low temperature with the help of phytic acid.
► Calcium cations are mainly located in the vicinity of phosphate tetrahedron.
► Gel network is highly crosslinked between phosphate and silicate.
► Calcium enhances the overall network connectivity.
► High coordinated silicon at high phosphate content.