Peculiarities of the formation, structural and morphological properties of zinc whitlockite (Ca18Zn2(HPO4)2(PO4)12) synthesized via a phase transformation process under hydrothermal conditions

Abstract

Zinc whitlockite (Zn-WH, Ca₁₈Zn₂(HPO₄)₂(PO₄)₁₂) is a new synthetic analog of the second most abundant biomineral – magnesium whitlockite (Mg-WH, Ca₁₈Mg₂(HPO₄)₂(PO₄)₁₂), which is considered as a potential material for bone regeneration. In the present work, the formation of Zn-WH via a dissolution–precipitation process under hydrothermal conditions was investigated in detail. The influence of medium pH, reaction time, temperature and concentration of precursors on the formation of the single-phase material was studied. It was demonstrated that the phase-pure material can be obtained in the pH range from 5.6 to 6.2 in a very short period of time – 1 h. Variation of chemical composition in terms of the Ca-to-Zn ratio was determined to be possible in a narrow range. The structural properties of the synthesized Zn-WH were investigated by powder X-ray diffraction analysis along with ¹H and ³¹P solid-state NMR spectroscopy, FTIR spectroscopy, Raman spectroscopy and XPS. Rietveld refinement confirmed that the synthesized material has a rhombohedral structure with the R3c space group. The NMR study confirmed the presence of three inequivalent phosphorus atoms in the crystal structure, whereas only one PO₄³⁻ tetrahedron was determined to be protonated. The size and shape of the particles can be controlled by the concentration of the precursors as well as by the initial Ca-to-Zn ratio in the reaction mixture.