CHARGE-DENSITY-WAVE PHENOMENA IN THE FAMILY OF 2-DIMENSIONAL MONOPHOSPHATE TUNGSTEN BRONZES - (PO2)4(WO3)2M

The monophosphate tungsten bronzes with pentagonal tunnels (PO2)4(WO3)2m form a family of 2D conductors composed of ReO3-type layers of m oxygen sharing WO6 octahedra belonging to infinite chains running along the non equivalent a and a+/-b directions. X-ray diffuse scattering experiments show that these chains are subject to charge density wave (CDW) instabilities. It is found that the leading CDW instability changes from chains along a in the m = 4 member to chains along a+/-b for larger values of m. In the m = 4 and m = 6 bronzes the CDW wave vector, stabilized below each Peierls like transition, leads to a common nesting between differently oriented ID Fermi surfaces (FS) associated with these chains. Two and three successive CDW transitions, corresponding to a hierarchy of FS nestings, are observed in these members. The CDW modulation observed in the m = 7, 9, 10 and 11 bronzes presents a large number of harmonics, and in m = 7 its amplitude shows unexpected hysteresis phenomena. It is suggested that these features could be the structural fingerprint of an electron localization, possibly due to the reduction of screening effects when the average number of electron per tungsten, 2/m, decreases with increasing m.