Synthesis and morphological control of Ca5(PO4)3Cl and Ca2PO4Cl via the phase transformation of amorphous calcium phosphate in molten chlorides

In the present work, the phase conversion of amorphous calcium phosphate (ACP) in different molten chlorides (LiCl, NaCl, KCl, CaCl2) was investigated in detail. The main synthesis parameters influencing the phase purity and morphological features of the products include the chemical composition of molten salts, the heat treatment temperature and the ACP-to-flux ratio. The selective synthesis of single-phase Ca5(PO4)3Cl or Ca2PO4Cl depends on the content of CaCl2 in the reaction medium. The morphology control of Ca5(PO4)3Cl powders was achieved by varying the KCl/CaCl2 ratio in the flux, resulting in the formation of the particles of different size and shape. The KCl-rich fluxes led to the formation of relatively small nearly spherical particles, whereas the CaCl2-rich fluxes promoted an anisotropic growth of the Ca5(PO4)3Cl crystals resulting in the formation of monodispersed hexagonally-shaped microrods. Whereas the anisotropic growth was observed at relatively low temperature (750 °C) the increase of the reaction temperature up to 1200 °C significantly reduced this effect leading to the formation of the particles with obviously low aspect ratio. The phase crystallinity and purity were analyzed using powder X-ray diffraction, FTIR spectroscopy as well as 31P, 35Cl and 1H solid-state NMR. The morphological features and chemical composition of the synthesized products were studied by SEM/EDX analysis.