CHARGE-DENSITY-WAVE TRANSITIONS IN 2-DIMENSIONAL TRANSITION-METAL BRONZES AND OXIDES

The structural instabilities towards the formation of a charge density wave (CDW) ground state exhibited by several layered Mo and W bronzes and oxides are reviewed. It is shown that in these two-dimensional (2D) metals, including the purple bronzes AxMo6O17 (A = K, Na, Tl; x almost-equal-to 1), the gamma and eta phases of MO4O11 and the monophosphate tungsten bronzes with pentagonal tunnels (PO2)4(WO3)2m (m = 4,6,7), the CDW instability can be associated with particular chains of MoO6 or WO6 octahedra of the ReO3 type slabs along which there is a strong overlap of the t2g orbitals. The CDW critical wave vectors of the purple bronzes, Mo4O11 and the tungsten bronzes with m = 4 and 6 lead to a common nesting between differently oriented 1D Fermi surfaces. It is suggested that the anharmonic CDW modulation, which occurs in the tungsten bronzes with m greater-than-or-equal-to 7, could be the structural fingerprint of electron localization effects.