Cesium ordering and electron localization-delocalization phenomena in the quasi-one-dimensional diphosphate tungsten bronzes: Cs_1-xP₈W₈O₄₀

We present a structural investigation of the Cs_1-xP₈W₈O₄₀ family of quasi-one-dimensional (quasi-1D) conductors, which exhibit intriguing charge transport properties where, for cursive chi small, the conductivity exhibits a crossover from a semiconducting to a metallic like regime when the temperature decreases. In these materials the W₄O₁₈ double zig-zag chains, together with the P₂O₇ diphosphate groups, delimit channels which are partially filled with the Cs⁺ ions. It is found, from an X-ray diffuse scattering investigation, that at room temperature the Cs⁺ ions are locally ordered on a lattice of well-defined sites in the channel direction and not ordered between neighboring channels. These Cs⁺ ions form 1D incommensurate concentration waves whose periodicity depends on the Cs⁺ stoichiometry. In CsP₈W₈O₄₀ upon cooling, the intrachannel order increases significantly, and an interchannel order between the 1D Cs⁺ concentration waves develops. But, probably because of kinetic effects, no tridimensional (3D) long range order of the Cs⁺ 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 Cs⁺ 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 Cs_1-xP₈W₈O₄₀, for cursive chi small, a localization-delocalization transition of the Anderson type occurs due to the thermal variation of the Cs disorder. When cursive chi 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.