Preparation and characterization of carbonated barium-calcium hydroxyapatite solid solutions

Particles of carbonated barium-calcium hydroxyapatite solid solutions (BaCaHap) with different Ba/(Ba + Ca) (XBa) atomic ratios were prepared by a wet method at 100 °C and characterized by various means. The crystal phases and structures of the products strongly depended on the composition of the starting solution, that is, the Ba/(Ba + Ca) atomic ratio ([XBa]) and H3PO4 concentration ([H3PO4]) in the solution. BaCaHap with XBa⩽0.43 could be prepared at [XBa]⩽0.7 by changing [H3PO4], but could never be obtained at [XBa]=0.8-0.95 regardless of [H3PO4]. The carbonated calcium hydroxyapatite particles prepared at [XBa]=0 were fine and short rod-shaped particles (ca. 14 × 84 nm). With increasing [XBa] from 0 to 0.8, the particles obtained became large spherical agglomerates. The carbonated barium hydroxyapatite particles formed at [XBa]=1 were long rod-shaped agglomerates (ca. 0.2 × 2 μm) of fine primary particles. The amount of CO2 adsorbed irreversibly on a series of BaCaHaps showed a minimum at (Ba + Ca)/(P + C) atomic ratio of around 1.56, which agreed well with the minimum cation/P ratio obtained for the other hydroxyapatites, as already reported.