Chemically pure beta-tricalcium phosphate powders: Evidence of two crystal structures

The question whether beta-tricalcium phosphate (beta-TCP) can form a solid solution with beta-calcium pyrophosphate (beta-CPP) and/or hydroxyapatite (HA) has still not been solved. For this reason, wet-chemically synthesized beta-TCP powders with only 20 ppm Sr (among 43 tested elements) and with different HA and beta-CPP contents, or in other words Ca/P molar ratios, were used. The graphical relationship between these various Ca/P molar ratios determined by X-ray diffraction and by inductively-coupled plasma mass spectrometry showed no discontinuity, indicating the absence of a solid solution between beta-TCP and beta-CPP or HA. Analysis of the beta-TCP lattice parameters as a function of the Ca/P molar ratio revealed a discontinuity at a Ca/P molar ratio of 1.500 and a maximum microstrain. These results indicated that at least two beta-TCP structures co-existed, with variable mixing ratios depending on the Ca/P molar ratio, and with a distinct jump at a Ca/P molar ratio of 1.500.