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A Study of the Hydration and Dehydration of Vanadyl Arsenate by X‐ray Diffraction Analysis, Infrared and Raman Spectroscopy
Author(s) -
Beneš Ludvík,
Melánová Klára,
Zima Vítězslav,
Trchová Miroslava,
Uhlířová Eva,
Matějka Pavel
Publication year - 2000
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(200005)2000:5<895::aid-ejic895>3.0.co;2-#
Subject(s) - chemistry , raman spectroscopy , arsenate , infrared , dehydration , infrared spectroscopy , x ray crystallography , diffraction , crystallography , spectroscopy , x ray , analytical chemistry (journal) , inorganic chemistry , arsenic , organic chemistry , optics , biochemistry , physics , quantum mechanics
The course of the intercalation and deintercalation 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 · 5 H 2 O (basal spacing c = 10.48 Å) is formed at r.h. above 76%, VOAsO 4 · 3 H 2 O ( c = 8.03 Å) at 43–76% r.h., VOAsO 4 · 2 H 2 O ( c = 7.33 Å) at 11–43% r.h.; dehydrated VOAsO 4 ( c = 4.18 Å) exists near 0% r.h. Like the thermal dehydration of VOPO 4 · 2 H 2 O, the thermal dehydration of VOAsO 4 · 3 H 2 O 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 · 3 H 2 O over phosphorus pentoxide at ambient temperature. The hydration of VOAsO 4 is different from that of VOPO 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.