Polar Structure Formation in Solid Solution of Strontium-Substituted Fluorapatite-Gelatin Composites: From Structural and Morphogenetic Aspects to Pyroelectric Properties

Strontium-substituted apatite-(CaF)-gelatin composites have been synthesized within a gelatin gel using the double-diffusion technique. All experimental parameters were kept constant while systematically varying the strontium/calcium molar ratio in solution. The effect of the presence of strontium ions in the growth solution on composition, morphogenesis, and morphology as well as pyroelectric properties of synthetic aggregates was systemically analyzed. It was shown that strontium ions significantly inhibit the growth process of composite aggregates and increase growth anisotropy along [0001], which were also confirmed and explained using molecular dynamic simulations. Furthermore, the promotion of the crystal branching processes and spherulite formation were observed. Pyroelectric microscopy (SPEM) measurements on mixed substituted apatite-(CaSrF)-gelatin composite aggregates showed an increase in polar properties, suggesting a lowering of the crystal symmetry. This was verified by Rietveld refinement of synchrotron powder X-ray diffraction, which revealed the noncentrosymmetric P6(3) apatite crystal structure. These data could shed new light on understanding piezoelectric and pyroelectric properties of apatite-based biological hard tissues.