Study of Ga ~(3+)-induced hydrothermal crystallization of an α-Quartz Type Ga _(1-x_Fe _xPO _4 single crystal by in situ x-ray absorption spectroscopy (XAS)

摘要

The dissolution of α-FePO _4 and the α-Ga _(0.75)Fe _(0.25)PO _4 solid solution with α-quartz-type structures under hydrothermal conditions in 1 M HNO _3 aqueous solution was investigated by in situ X-ray absorption spectroscopy (XAS) at the Fe K-edge. The solubility of α-FePO _4 increases with temperature and is higher at 25 MPa than at 50 MPa. The Fe ~(3+) cation in solution is 6-fold coordinated with an average Fe-O distance close to 2.0 ?. A similar experiment was performed with a solid solution of α-quartz-type Ga _(0.75)Fe _(0.25)PO _4 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% Fe ~(3+) with the α-quartz-type structure. These results show that the crystallization of pure α-quartz-type FePO _4 by the hydrothermal method is not possible due to the formation of very stable Fe ~(3+) hexa-aquo complexes [Fe(H _2O) _6] ~(3+) and to the absence of FeO _4 tetrahedra in solution. Ga ~(3+) 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 Fe ~(3+) substitutes Ga 3+ with a 4-fold coordination in mixed (iron/gallo)-phosphate complexes that leads to the growth of an α-quartz-type Ga 0.77Fe 0.23PO _4 single crystal.