Institut Charles Gerhardt Montpellier, Equipe C2M, UMR CNRS-UM2-ENSCM-UM1 5253, Université Montpellier 2, Place Eugène Bataillon, CC1504, 34095 Montpellier Cedex 5, France.
Inorg Chem. 2012 Nov 5;51(21):11811-9. doi: 10.1021/ic3017485. Epub 2012 Oct 11.
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 Å. 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(2)O)(6) 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(0.77)Fe(0.23)PO(4) single crystal.
在 1 M HNO3 水溶液中，通过原位 X 射线吸收光谱（XAS）在 Fe K 边研究了α-FePO4 和具有α-石英型结构的α-Ga(0.75)Fe(0.25)PO4 固溶体在水热条件下的溶解情况。α-FePO4 的溶解度随温度升高而增加，在 25 MPa 下比在 50 MPa 下更高。溶液中的 Fe(3+)阳离子以 6 配位方式存在，平均 Fe-O 距离接近 2.0 Å。在 25 MPa 的压力下，以α-石英型 Ga(0.75)Fe(0.25)PO4 固溶体作为起始相进行了类似的实验。通过将温度从 303 K 升高到 573 K，可以在α-石英型结构下生长出具有 23%Fe(3+)的单晶。这些结果表明，由于非常稳定的 Fe(3+)六水合配合物[Fe(H2O)6]3+的形成以及溶液中不存在 FeO4 四面体，因此通过水热法不可能结晶出纯的α-石英型 FePO4。溶液中的 Ga3+阳离子在固液界面处诱导形成膦酸盐配合物，这是结晶核的起源。我们提出了一种结晶机制，其中 Fe(3+)取代 Ga(3+)，形成具有 4 配位的混合（铁/镓）磷酸盐配合物，导致α-石英型 Ga(0.77)Fe(0.23)PO4 单晶的生长。
