Abstract
The dissolution of α-FePO4 and the α-Ga0.75Fe0.25PO4 solid solution with α-quartz-type structures under hydrothermal conditions in 1 M HNO3 aqueous solution was investigated by in situ X-ray absorption spectroscopy (XAS) at the Fe K-edge. The solubility of α-FePO4 increases with temperature and is higher at 25 MPa than at 50 MPa. The Fe3+ cation in solution is 6-fold coordinated with an average Fe-O distance close to 2.0 Å. A similar experiment was performed with a solid solution of α-quartz-type Ga0.75Fe0.25PO4 as the starting phase under a pressure of 25 MPa. By varying the temperature from 303 K up to 573 K a single crystal was grown with 23% Fe3+ with the α-quartz-type structure. These results show that the crystallization of pure α-quartz-type FePO4 by the hydrothermal method is not possible due to the formation of very stable Fe3+ hexa-aquo complexes [Fe(H2O)6]3+ and to the absence of FeO4 tetrahedra in solution. Ga3+ cations in solution induce the formation of gallophosphate complexes at the solid-liquid interface, which are at the origin of the nuclei for crystallization. We propose a crystallization mechanism in which the Fe3+ substitutes Ga3+ with a 4-fold coordination in mixed (iron/gallo)-phosphate complexes that leads to the growth of an α-quartz-type Ga0.77Fe0.23PO4 single crystal.