Electronic Instabilities and Localization Effects in the Quasi-TwoDimensional Monophosphate Tungsten Bronzes (PO2)4(WO3)2m and KxP4W8O32

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 lowdimensional 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 KxP4W8O32 with pseudo-hexagonal tunnels that show electronic instabilities with critical temperatures depending on x.