Structural insights of glass-reinforced hydroxyapatite composites by Rietveld refinement

Phase transformations and interstitial and/or substitution of trace elements during the liquid-phase sintering process of P2O5–CaO–MgO glass-reinforced hydroxyapatite (GR-HA) composites were examined by X-ray diffraction and Rietveld analyses. Using the Rietveld method for structure refinement, changes in the lattice parameters of the two main phases of the composites, hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP), as well as changes in several bond lengths and in the occupancy of the hydroxyl oxygen site in the HA phase structure were assessed. The glasses gave rise to formation of between approximately 45 and 50% of β-TCP, with evidence for the Mg2+ enhancing the formation of β-TCP. Between 1300 and 1350°C, the β-TCP inverts to α-TCP, without further decomposition of the residual HA. The glasses showed evidence for stabilisation of the hydroxyl group located in the hydroxyl channels. This is supported by measurements of the hydroxyl channel radius (Rc), the Ca2–OH bond length and the hydroxyl oxygen occupancy (Oocc). Results showed that the Mg2+ containing glasses induced the β-TCP phase formation in the structure of GR-HA composites and retarded the β-TCP into α-TCP transformation at higher temperatures. The chemical composition of the P2O5 glasses also induces modifications in the lattice parameters of the crystallographic phases present in the microstructure of the composites. This suggests some substitution of Mg2+-for-Ca2+ in the β-TCP structure during the liquid-phase sintering process.