Charge Density Wave Properties of Quasi Low-Dimensional Transition Metal Oxide Bronzes

Several families of transition metal oxide bronzes including the quasi one-dimensional molybdenum blue bronzes A$_{0.3}$MoO$_3$, the quasi two-dimensional (2D) Mo purple bronzes AMo$_6$O$_{17}$ and the quasi 2D monophosphate tungsten bronzes (PO$_2$)$_4$(WO$_3$)$_{2m}$ show Peierls transitions leading to commensurate or incommensurate charge density wave (CDW) states. The properties of the monophosphate tungsten bronzes (PO$_2$)$_4$(WO$_3$)$_{2m}$ are reviewed. These series of compounds provide a model system where both the low-dimensional character and the average electron concentration are function of the $m$ parameter. The low $m$ compounds ($m=4,\,6$) show conventional CDW instabilities as well as giant positive magnetotransport and quantum oscillations at low temperature. Detailed information on the Fermi surface in the CDW state is obtained from transport data, the size of the pockets left by the CDW gap openings being found of the order of 1% of the high temperature two-dimensional Brillouin zone. The $m=5$ compound, which may be considered as an intergrowth of $m=4$ and 6, shows also instabilities with slightly different properties. The absence of CDW long range order affects the properties of the $m=8$ member which shows low temperature properties possibly due to weak localisation effects. Finally, studies on the $4\leq m \leq 14$ compounds show that the Peierls temperatures are increasing with $m$, reaching high values, up to 550 K, for $m=13$. This is attributed to a 2D character increasing with $m$. The increase of the room temperature resistivity with $m$ is discussed in terms of enhanced electron-phonon or electron-electron interactions.