Electronic Instabilities and Localization Effects in the Quasi-Two-Dimensional Monophosphate Tungsten Bronzes (PO₂)₄(WO₃)_2m and K_xP₄W₈O₃₂

The monophosphate tungsten bronzes (PO₂)₄(WO₃)_2m with pentagonal tunnels are quasi-two-dimensional conductors that show charge density wave type electronic instabilities. These series of compounds provide a model system where the low-dimensional character and the average electron concentration are functions of the m parameter. The low m compounds (m=4, 6) show conventional charge density wave instabilities. The m=5 compound exists with two different crystal structures and shows instabilities with slightly different properties. We report measurements of transport properties for the compounds m=5, 7, 8, 9. We show that, for m>7, these compounds exhibit an upturn of resistivity and field dependence of the magnetoresistance characteristic of quantum interference effects. We also report transport properties of the compounds K_xP₄W₈O₃₂ with pseudo-hexagonal tunnels that show electronic instabilities with critical temperatures depending on x.