Mechanism of the Solid-State Transformation of a Calcium-Deficient Hydroxyapatite (CDHA) into Biphasic Calcium Phosphate (BCP) at Elevated Temperatures

Three types of poorly crystallized calcium-deficient hydroxyapatites (CDHA) with Ca/P molar ratios 1.50, 1.58, and 1.67 were prepared from CaHPO 4 ·2H 2 O and KOH. These were sintered at 1050 °C for 4 h. Well-crystallized β-tricalcium phosphate (β-TCP), biphasic calcium phosphate (BCP), and stoichiometric hydroxyapatite (HA) were produced, respectively. The sintered and unsintered calcium phosphates were studied by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. In addition, Ca/P molar ratios were determined by a chemical analysis. Thermogravimetric analysis (TGA) revealed a loss of water for the period of sintering. On the basis of the experimental data, a structure of BCP was suggested. After applying the numerical values of ionic (OH -, Ca 2+, and PO 4 3 - ) diffusion coefficients at 1000 °C, the solid-state transformation mechanism of CDHA into BCP was proposed.