A Study of the Hydration and Dehydration of Vanadyl Arsenate by X-ray Diffraction Analysis, Infrared and Raman Spectroscopy

摘要

The course of the intercalation and deinteralation of water molecules in vanadyl arsenate has been studied by X-ray diffraction analysis and by infrared and Raman spectroscopies. The formation of VOAsO_4 hydrates at ambient temperature has been found to depend on relative humidity (r.h.): VOAsO_4·5H_2O (basal spacing c = 10.48 A) is formed at r. h. above 76%, VOAsO_4·3H_2O (c = 8.03 A) at 43-76% r.h., VOAsO_4·2H_2O (C = 7.33 A) at 11-43% r.h.; dehydrated VOAsO_4 (c = 4.18 A) exists near 0% r. h. Like the thermal dehydration of VOPO_4·2H_2O, the thermal dehydration of VOAsO_4·3H_2O proceeds in a stepwise manner so that the dihydrate and monohydrate are formed en route to the anhydrous compound. The arsenate monohydrate is gradually dehydrated over a broad temperature range. The broad diffraction lines observed can be explained in terms of the existence of a disordered phase containing monohydrated and anhydrous forms of vanadyl arsenate. A similar phenomenon has been observed during the dehydration of VOAsO_4·3H_2O over phosphorus pentoxide at ambient temperature. The hydration of VOAsO_4 is different from that of VOAsO_4. The first step, i. e. the insertion of water that coordinates to the vanadium atoms, is very slow. On the contrary, the uptake of further water molecules with the formation of higher hydrates is fast. It thus seems likely that the filling of one interlayer space with water facilitates the intercalation of further water into neighboring interlayer spaces. Therefore, only higher hydrates together with the original anhydrous phase are observed. Impedance spectral measurements indicate that the conductivity of the trihydrate has a mixed ionic/electronic character.