Investigation of the Alkali-Metal Vanadium Oxide Xerogel Bronzes: A_αV₂O₅.nH₂O (A = K and Cs)

The synthesis of bronze-like A_xV₂O₅·nH₂O xerogels (A = K and Cs, 0.05 < x < 0.6) and systematic characterization of their chemical, structural, spectroscopic magnetic, and chargetransport properties are reported. These materials were prepared by the reaction of V₂O₅·nH₂O with various amounts of alkali iodide (KI and CSI) in acetone under N₂ atmosphere for 3 days. X-ray diffraction and spectroscopic data indicate that the V₂O₅ framework in A_xV₂O₅·nH₂O maintains the pristine V₂O₅ xerogel structure. The increased V⁴⁺ (d¹) concentration in the V₂O₅ framework causes the disappearance of EPR hyperfine structure and the increase of magnetic susceptibility and electrical conductivity. The optical diffuse reflectance spectra of these compounds show characteristic absorption bands due to inter-valence (V⁴⁺/V⁵⁺) charge-transfer transitions. The magnetic behavior is best described as Curie-Weiss type coupled with temperature-independent paramagnetism (TIP). The Curie constant and EPR peak width of the A_xV₂O₅·nH₂O materials show unusual behavior consistent with strong antiferromagnetic coupling of neighboring V⁴⁺ centers. The electrical conductivity slightly increases with V⁴⁺ concentration, and its temperature dependence indicates a thermally activated process. The thermoelectric power of the A_xV₂O₅·nH₂O materials is negative and becomes less negative with increasing V⁴⁺ concentration (i.e., increasing x).