Dielectric Loss in Vanadyl Pnictates
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DIELECTRIC LOSS IN VANADYL PNICTATES
Joseph F. Lomax, John J. Fontanella", Mary C. Wintersgill'" and Anthony Kotarski'" " Chemistry Department, United States Naval Academy, Annapolis, MD 21402 Physics Department, United States Naval Academy, Annapolis, MD 21402 ABSTRACT The results of audio frequency complex impedance studies for vanadyl phosphate and its hydrates (VOPO4 .nH 2O; n--0,1,2) are reported. Measurements were made at seventeen frequencies between 10 and I05 Hz over the temperature range 5.5-380K. Two lowtemperature features were observed and are correlated with particular water species between the VOPO, layers. In light of chemical evidence and complex impedance measurements on related compounds, it is speculated that VOPcO,.nH 20 (Pc = P, As) can be thought of as a mixed nonstoichiometric compound [VO(HPcO4)]x[VOPcO4Q,._.nH 20 and the solid Bronsted acid nature of the compound is related to proton movement between layer pnictates and intralayer waters. INTRODUCTION Vanadium phosphorous oxides are used as selective, mild oxidation catalysts (e.g. conversion of butane to maleic anhydride)[l]. Vanadyl phosphate (VOPO4 ) is a layered compound that can be intercalated by a host of materials[2,3,4]. The wide variety of reaction that vandyl phosphate can accomplish is made possible by virtue of its available coordination site (for Lewis acid/base reactions), and its reversible VI/V' redox couple. The vanadium is seated in a distorted octahedron of oxygens where each of the equatorial oxygens is from a comer-shared phosphate. The double bonded axial oxygen fills the final coordination spot in the adjacent layer in the dehydrated form (interlayer spacing = 4.434 A). In the hydrated forms the final coordination spot is filled by a water. The second water in the VOPO,.2H 20 [5,6] is found in the vicinity of the phosphate, as seen in Figure 1 (interlayer spacing = 7.41 A). Clearly water has a profound effect on the VOPO4*2H20 structure, and, one would expect, the chemical and physical properties of not only the this compound. but il, Looking down the c.a.is
c-axis T
"on-Coordinated it--
Coordinated Water
Figure 1.
Coordinated Water
Water
Vanadyl Onygen Oxygen
View of part of a Vandyl Phosphate layer a) down the c-axis and b) with a view of the intralayer waters. Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
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intercalation compounds. The nature of the water between the layers has been a point of discussion in the literature[7,8]. It has been speculated that the acidic nature of VOPO, 212H20 extends past the Lewis acidity of the vanadium to proton donation. The spectroscopy hints of protons associated with one or more of waters (to form a hydronium ion), vanadyl oxygens or phosphate oxygens. Within and between the layers, any proton movement must be a dynamic process of protons being donated and accepted. AC impedance measurements are ideally suited for investigating such dynamic ion processes. It has a wide dynamic range, it can provide information the nature and energetics of
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