Transformation of tricalcium phosphate into apatite by ammonia treatment

Abstract

Tricalcium phosphate, Ca₃(PO₄)₂, was treated in dry ammonia at temperatures between 1100°C and 1300°C for different times of up to 16 h. X-Ray diffraction and ³¹P-NMR studies showed a phase transformation into apatite when treated at temperatures between 1100 and 1200°C. Nitrogen was detected in the treated samples with a maximum content of 2.9 wt%. ³¹P-NMR-analysis indicated no direct bonding of nitrogen to phosphorous. The presence of graphite during ammonia treatment was required for nitrogen incorporation. Infrared spectra showed increasing intensities of four new bands at 3250, 2016, 1966 and 700 cm⁻¹ attributed to the occurrence of cyanamide ions. Taking into account the results of XRD, ³¹P-NMR and FTIR spectrometry, it is concluded that tricalcium phosphate transforms into calcium deficient cyanamidapatite by ammonia treatments according to the reaction: 3Ca₃(PO₄)₂ + H₂CN₂ → Ca₉(PO₄)₅(HPO₄)(HCN₂).