QUASI-ONE-DIMENSIONAL CHARGE-DENSITY-WAVE INSTABILITIES IN THE FAMILYOF MONOPHOSPHATE TUNGSTEN BRONZES - (PO2)4(WO3)2M

We present a X-ray diffuse scattering investigation of the members m=4 ,6,7 of a new series of tungsten bronzes (PO2)4(WO3)2m, which exhibit two-dimensional electronic properties. Suprisingly, at ambient tempera ture these materials show quasi-one-dimensional (1D) charge density wa ve (CDW) fluctuations, as proved by computer simulations of the X-ray patterns. These fluctuations can be associated with particular chains of WO6 octahedra, contributing to flat portions of the Fermi surface o f these materials. In P4W12O44 these fluctuations condense into succes sive phase transitions at 120 K and 62K where the electrical resistivi ty shows giant anomalies. In P4W14O50 they condense at 189 K into a tr iply incommensurate and non-sinusoidal modulation exhibiting harmonics up to the 6th order. Below 47 K another triply incommensurate and non -sinusoidal modulation is stabilized. Different quasi-ID fluctuations are involved in the 80 K and 52 K CDW phase transitions of P4W8O32. Th e origin of these ID CDW instabilities and their 3D ordering at the va rious phase transitions are discussed in the framework of nesting mech anisms of the Fermi surface of these bronzes.