Cesium ordering and electron localization-delocalization phenomena in the quasi-one-dimensional diphosphate tungsten bronzes: Cs1‒xP8W8O40

We present a structural investigation of the family of quasi-one-dimensional (quasi-1D) conductors, which exhibit intriguing charge transport properties where, for x small, the conductivity exhibits a crossover from a semiconducting to a metallic like regime when the temperature decreases. In these materials the double zig-zag chains, together with the diphosphate groups, delimit channels which are partially filled with the ions. It is found, from an X-ray diffuse scattering investigation, that at room temperature the ions are locally ordered on a lattice of well-defined sites in the channel direction and not ordered between neighboring channels. These ions form 1D incommensurate concentration waves whose periodicity depends on the stoichiometry. In upon cooling, the intrachannel order increases significantly, and an interchannel order between the 1D concentration waves develops. But, probably because of kinetic effects, no tridimensional (3D) long range order of the ions is achieved at low temperature. The 3D low-temperature local order has been determined and it is found that the phase shift between the concentration waves minimizes their Coulomb repulsions. This local order is increasingly reduced as the Cs concentration diminishes. We interpret the intriguing features of the electrical conductivity in relationship with the thermal evolution of the Cs ordering effects. We suggest that in , for x small, a localization-delocalization transition of the Anderson type occurs due to the thermal variation of the Cs disorder. When x increases, the enhancement of the disorder leads to a localization of the electronic wave function in the whole temperature range measured. Finally, and probably because of the disorder, no charge density wave instability is revealed by our X-ray diffuse scattering investigation. Received: 10 October 1997 / Received in final form: 11 December 1997 / Accepted: 16 December 1997