Charge density wave phenomena in the family of two dimensional monophosphate tungsten bronzes : (PO₂)₄(WO₃)_{2 m}

Abstract
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 experirnents 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 1D 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 harrnonics, 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.