INVESTIGATION OF THE ALKALI-METAL VANADIUM-OXIDE XEROGEL BRONZES - A(X)V(2)O(5)CENTER-DOT-NH(2)O (A=K AND CS)

The synthesis of bronze-like A(x)V(2)O(5) . nH(2)O xerogels (A = K and Cs, 0.05 < x < 0.6) and systematic characterization of their chemical, structural, spectroscopic magnetic, and charge transport properties are reported. These materials were prepared by the reaction of V2O5 . nH(2)O with various amounts of alkali iodide (KI and CSI) in acetone under N-2 atmosphere for 3 days. X-ray diffraction and spectroscopic data indicate that the V2O5 framework in A(2)V(2)O(5) . nH(2)O maintains the pristine V2O5 xerogel structure. The increased V4+ (d(1)) 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(x)V(2)O(5) . nH(2)O 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(x)V(2)O(5) . nH(2)O materials is negative and becomes less negative with increasing V4+ concentration (i.e., increasing x).