Effect of nominal Ca/P ratios on the growth of calcium phosphate phases: An in-situ and ex-situ XRD investigation

Abstract

In this study; the effect of nominal Ca/P ratios has been investigated to build a relationship among the synthesis, structural, compositional, thermal stability, and electronic structure assets of calcium phosphate phases. The in-situ and ex-situ X-ray diffraction (XRD) measurements and quantitative phase analysis are performed to investigate the structural phase evolution and transformation under heat treatment. The low-temperature (80 °C) synthesis employed in this work has resulted in calcium-deficient hydroxyapatite (CDHA; pentacalcium dihydrogen-phosphate tris-phosphate hydroxide) phase formation for all the nominal Ca/P ratios (1.4–1.8). It is evidenced that the single phase β-Ca3(PO4)2 (β-tricalcium diorthophosphate; β-TCP) is stabilized for nominal Ca/P = 1.4 at 1000–1100 °C. The in-situ and ex-situ XRD results revealed that the Ca10(PO4)6(OH)2 (i.e., hydroxyapatite; HA) and β-TCP are the stable phases in the nominal Ca/P ratios of 1.5 and 1.6 containing samples. In the intermediate Ca/P ratios range (i.e., 1.67–1.7), the tri-phasic calcium phosphates (β-TCP, HA, and α-TCP) have been stabilized. Excitingly, the significant proportion of the α-TCP phase has been stabilized along with the dominating HA phase, at higher nominal Ca/P ratios (1.8). FE-SEM results show porous morphology, despite the nominal Ca/P ratio variation. The phase identification and dissimilarity in the O 2p and Ca 3d orbital hybridization, in the various phases of calcium phosphate, is also investigated by employing the element-specific X-ray absorption near edge structure (XANES) at O K-edge and Ca L-edge.