Lanthanum Oxide Effects on the Structure of Calcium Phosphate Glasses

Calcium phosphate glasses, in which part of calcium oxide was replaced by lanthanum oxide, were prepared by using the conventional melt quench method. The structures of xLa₂O₃ · (50-x)CaO · 50P₂O₅ (x = 0, 1, 3, 6, 12 mol%) samples were investigated by X-ray diffraction (XRD), Raman spectrum, Fourier transform infrared spectrum (FTIR), and differential scanning calorimetry (DSC). The results show that lanthanum oxide acts as network modifier in the network space of glass structure. The glass formation occurs at an O/P ratio of about 3.0–3.12. At larger values, the crystalline phases calcium pyrophosphate (Ca₂P₂O₇) and calcium lanthanum phosphate Ca₉La(PO₄)₇] are detected in the samples. Raman and FTIR spectra indicate that the structure of lanthanum-free sample is chain P–O–P bond metaphosphate–based Q² units. Glass structure will change to Q² and Q¹ units when the lanthanum oxide content is less then 6 mol%. When lanthanum oxide content increases to 9 and 12 mol% more nonbridging oxygen in the glass, resulting in the depolymerization of the phosphate network, the network of glass transforms to Q², Q¹, and Q⁰ units mixture. Based upon DSC results, T_g slightly decreases because of the depolymerization of microstructure. Endothermal peak of DSC curves indicate that crystal phases separate out from vitreous body with the addition of lanthanum oxide content.