Low-Temperature Formation and Identification of Biphasic Calcium Carbonate Phosphates

Abstract

Biphasic calcium carbonate phosphates comprised of amorphous calcium carbonate phosphate and carbonated hydroxyapatite were synthesized by wet precipitation at pH 9, Ca/P = 1.67, and ${\text{CO}}_{3}^{{2-}}$ concentrations of 0.60–0.90 mol/L under varying precipitation conditions. The wet formation of biphasic calcium carbonate phosphates includes a partial conversion of the amorphous phase into a mixture of carbonated hydroxyapatite and calcite, followed by leaching of calcite and constitutional Ca²⁺ and ${\text{CO}}_{3}^{{2-}}$ ions during long-term decantation. The identification of biphasic calcium carbonate phosphates involves complex analysis using a spectroscopic method to elucidate carbonate substitution schemes, an X-ray diffraction method to determine the degree of apatite amorphization, and thermoanalytical methods to detect the effects of crystallization of the amorphous phase. Variations in the preparation parameters provide a means for controlling the contents of both amorphous calcium carbonate phosphate and constitutional ${\text{CO}}_{3}^{{2-}}$ions in biphasic calcium carbonate phosphates, which amounts determine their resorbability and thermal stability.