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 ox ygen sharing WO6 octahedra belonging to infinite chains running along the non equivalent a and a+/-b directions. X-ray diffuse scattering ex periments show that these chains are subject to charge density wave (C DW) instabilities. It is found that the leading CDW instability change s from chains along a in the m = 4 member to chains along a+/-b for la rger 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 nesti ng 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 lar ge number of harmonics, and in m = 7 its amplitude shows unexpected hy steresis phenomena. It is suggested that these features could be the s tructural fingerprint of an electron localization, possibly due to the reduction of screening effects when the average number of electron pe r tungsten, 2/m, decreases with increasing m.