The role of the chemical composition of monetite on the synthesis and properties of α-tricalcium phosphate.

There has been a resurgence of interest in alpha-tricalcium phosphate (α-TCP), with use in cements, polymer composites and in bi- and tri-phasic calcium phosphate bone grafts. The simplest and most established method for preparing α-TCP is the solid state reaction of monetite (CaHPO4) and calcium carbonate at high temperatures, followed by quenching. In this study, the effect of the chemical composition of reagents used in the synthesis of α-TCP on the local structure of the final product is reported and findings previously reported pertaining to the phase composition and stability are also corroborated. Chemical impurities in the monetite reagents were identified and could be correlated to the calcium phosphate products formed; magnesium impurities favoured the formation of β-TCP, whereas single phase α-TCP was favoured when magnesium levels were low. Monetite synthesised in-house exhibited a high level of chemical purity; when this source was used to produce an α-TCP sample, the α-polymorph could be obtained by both quenching and by cooling to room temperature in the furnace at rates between 1 and 10°C/min, thereby simplifying the synthesis process. It was only when impurities were minimised that the 12 phosphorus environments in the α-TCP structure could be resolved by (31)P nuclear magnetic resonance; samples containing chemical impurity showed differing degrees of line-broadening. Reagent purity should therefore be considered a priority when synthesising/characterising the α-polymorph of TCP.