STRUCTURAL FEATURES OF THE LOW DIMENSIONAL POLYPHOSPHATE TUNGSTEN BRONZES - P4W12O44 AND CS1-XP8W8O40

We present a X-ray diffuse scattering investigation of new series of monophosphate P4W12O44 and disphosphate Cs1-xP8W8O40 tungsten bronzes with two dimensional (2D) and one dimensional (1D) electronic properties respectively. P4W12O44 exhibits a charge density wave (CDW) instability at 183 K with a critical wave vector which has doubly incommensurate components in the 2D conducting plane. We suggest that the CDW is stabilized by a unique inter-Fermi-surface nesting mechanism. In Cs1-xP8W8O40 the Cs+ ions are partly ordered in channels delimited by the W4O12 chains linked by the P2O7(4-)ions. The Cs+ ions are disordered between the channels at ambient temperature and order progressively with decreasing temperature. We propose that, by reducing the interchannel disorder component, this local ordering accounts for the gradual transition from a semiconducting to a metallic regime observed in resistivity measurements.