Low-temperature formation of Ln silicate oxyapatite (Ln = la and Nd) by the water-based sol-gel method

Low-temperature formation routes of lanthanum silicate and neodymium silicate oxyapatites were analyzed by a combination of powder X-ray diffraction and thermogravimetric/differential thermal analysis by using precursors prepared from the water-based sol-gel method. Oxyapatite phase was found to form at about 1073 K in the case of lanthanum silicate and at about 1273 K in the case of neodymium silicate - temperatures much lower than those for a conventional solid-state reaction method. In each system, the oxyapatite phase was formed after decomposition of the lanthanoid oxycarbonate or metastable lanthanoid oxide. In particular, decomposition of lanthanum oxycarbonate was found to play an important role in lanthanum silicate oxyapatite formation. In addition to proposing a crystal structure for the lanthanoid oxycarbonate, we hypothesize an in-situ chemical pulverization process due to the delamination of lanthanoid oxide by carbon dioxide dissociation.