Structural and spectroscopic characterization of a series of potassium- and/or sodium-substituted β-tricalcium phosphate

Abstract

In this paper, we report X-ray diffraction investigations as well as Raman and solid-state 31P and 23Na magic angle spinning nuclear magnetic resonance (NMR) characterization of three series of calcium orthophosphates. The general formulae of the studied compounds are Ca10.5−x/2Mx(PO4)7, where M=K or Na and x=0, 0.25, 0.50, 0.75, 1.0; and Ca10KxNa1−x(PO4)7, where x=0, 0.25, 0.5, 0.75, 1.0. These calcium orthophosphates are found to be isostructural with β-tricalcium phosphate (β-TCP, Ca3(PO4)2) with the substitution of some calcium sites by potassium and/or sodium cations. The unit cell parameters vary continuously with the level of substitution, a characteristic of these solid solutions. The Raman spectra show the different vibrational bands of the phosphate groups PO4, while the NMR chemical shifts are sensitive to the non-equivalent phosphorus and sodium ions present in these substituted samples. As both Raman and NMR spectroscopies are local probes, they offer tools to distinguish between these different phosphorus and phosphate groups, according to their structural site and local environment, especially the type of cation substituent. A convenient decomposition of the Raman and NMR spectra into Gaussian–Lorentzian components leads us to propose an assignment of the main observed bands of these substituted β-TCPs.