TRANSFORMATION BEHAVIOR AND STRUCTURE OF A SERIES OF AMORPHOUS CALCIUM CARBONATE/PHOSPHATE HYBRIDS

Synthetic amorphous calcium carbonate (ACC) and amorphous calcium phosphate (ACP) both have applications to human health, the former as a potential source of bioavailable calcium and the latter in a variety of forms from dental products to surface coatings on prosthetic joints. It has been proposed that phosphate has a stabilizing effect on biogenic ACC, but the stabilization mechanisms are presently unknown. Conversely, carbonate is a known component of crystalline bone minerals but the structural effects of carbonate incorporation on ACP, a proposed amorphous precursor, are not well understood. Phosphate doped ACC and carbonate doped ACP act as single component analogs for the biogenic materials, but these synthetics are currently understudied.

A series of phosphate doped ACC samples and carbonate doped ACP samples were synthesized from aqueous solutions at low temperature and pressure (PACC series). Phosphate was observed to partition preferentially into the amorphous solid over carbonate. Thermal analysis revealed an increasing thermal stability of the solid with increasing phosphate content. Local and intermediate range structure was investigated using X-ray total scattering and pair distribution function (PDF) analysis. PDFs of the PACC series show a progressive change from ACC-like to ACP-like structural features, with intermediate composition samples containing pair correlations at unique distances compared to pure ACC or ACP. Preliminary time-resolved aging studies in high humidity environments indicate that the low-phosphate content ACC is more stable compared to undoped ACC. These results and the partitioning behavior are strong indicators for phosphate doped ACC as a potential bioavailable Ca source. The distinct local and intermediate structure of the intermediate carbonate content PACC samples reveals a new hybrid material with distinct properties from ACC and ACP and potential for novel medical applications. This study provides new insight on the role of anion dopants in these medically useful materials.