EXAFS of Amorphous Vanadium Phosphate Oxides: Correlation Between Hopping Conductivity and Local Structure

Abstract

The vanadium phosphate glasses have attracted a large interest as characteristic systems for hopping conductivity [1–5]. The binary (V2 O5)x (P2 O5)1_x glasses exhibit semiconducting properties arising from electron transfer between V 4+ and V 5+ ions. The presence of V 4+ ions is due to the loss of oxygen compensated by vanadium ions in the lower oxidation state. Therefore for a glass of fixed composition x mole% V2 O5 different redox ratio of V 4+ on the total number of vanadium ions C=V 4+ /V are possible using different glass preparation procedures. The d. c. conductivity follows an exponential law σ = σo exp(−W h /KT) predicted by the small polaron theory, where the polarization of the lattice around V 4+ ion follows the diffusion of the electron between vanadium ions in different valence states. W h is the activation energy, which is approximately equal to half the polaron binding energy [6]. The activation energy is depending on the distance between vanadium sites (V-O-V), on the structure of vanadium coordination shell VO n and on the arrangement of VO n poliedra in the glass. A variation of the conductivity and of the activation energy [3] ranging from 0.3 eV to 0.42 eV for glasses with different composition x and redox ratio C has been observed [4]. In spite of the large number of studies on the electronic properties of this class of binary glasses, very little is known on local atomic structure. Moreover V 4+ is silent to most of spectroscopical methods.