Small polarons in V2O5single crystal doped with WO3

Electron spin resonance (ESR) and DC conductivity measurements show that the free and bound polarons model can be applied to WO 3 -doped V 2 O 5 . At low temperature, small polarons are trapped in the electrostatic field due to the W 6+ impurity and conductivity only arises from the long-range hopping of bound polarons between localised defects. At high temperature, the small polaron is freed from its trap and a thermally activated hopping along vanadium ions is observed. ESR experiments show, however, that two bound polaron states occur: the first one, at very low temperature, when the electron is trapped on a single vanadium site adjacent to the W 6+ ion; the second one, at higher temperature, when the unpaired electron still remains in the electrostatic field of the W 6+ ion, but is allowed to jump among several non-equivalent sites surrounding the impurity.