STUDY OF THE STRUCTURE OF AMORPHOUS CALCIUM PHOSPHATE BY THE RADIAL DISTRIBUTION FUNCTION OF ATOMS

Abstract

This work presents the results of a study on the structure of amorphous calcium phosphate (ACP), synthesized by precipitation from aqueous solutions with the stoichiometric Ca/P ratios ranging from 1 to 2.13. This material represents an important intermediate phase in the formation of crystalline hydroxyapatite (HA) in solution. At the same time, HA is the main inorganic component of the bone tissue in humans and animals. It is widely used in biomedical technologies. Due to the structural features of amorphous substances, the X-ray diffraction patterns of ACP exhibit broad diffuse maxima, which makes it impossible to use traditional methods of X-ray structure analysis. To obtain detailed information about the spatial arrangement of atoms in ACP samples, a pair distribution function (PDF) was constructed based on experimental X-ray diffraction data processed using the PDFGetX software. The study have been showed that the PDFs of all samples have a similar appearance: the main maxima are located within interatomic distances up to 10 Å, and their positions change only slightly with increasing Ca/P ratio. This indicates that all samples have similar local structure regardless of their chemical composition, synthesis method, precipitation conditions, and processing parameters. Comparison with literature data confirmed the Posner model, according to which both ACP and HA are composed of the same cluster elements – spherical nanoclusters of Ca₉(PO₄)₆.

Thus, even in the amorphous state, ACP exhibits local ordering characteristic of crystalline HA. The obtained results are important for understanding the mechanisms of HA nucleation and growth from the amorphous phase and can be used in the development of new biocompatible materials with controlled properties, particularly in the fields of implantology, orthopedics, regenerative medicine, dentistry, pharmaceutical chemistry, nanotechnology, and bone tissue engineering.