Electronic instabilities in the quasi-two-dimensional conducting monophosphate tungsten bronzes K P W O ( )

Abstract

The monophosphate tungsten bronzes KxP4W8O32 are quasi-two-dimensional conductors. The parent compound P4W8O32 shows two charge density wave instabilities (CDW). The layered structure of the doped compounds contains pseudo hexagonal tunnels stabilised by the insertion of potassium for 0.75<x<2. In order to study the role of the band filling in the CDW instabilities, we have performed resistivity, magnetoresistance, Hall effect, thermopower and specific heat measurements on the doped compounds with 0.8<x<1.94. Anomalies at a temperature \(\) depending on x appear on all the transport properties. From transport data, we have obtained a phase diagram \(\) which shows surprisingly a maximum at 170 K for x=1.3. In the whole range of temperature studied (4.2 K-300 K), the transport properties of KxP4W8O32 show a change for \(\). The diagram \(\) is discussed in term of non-monotonous behaviour for the density of states versus the energy and in relation to previous X-ray studies. The transitions in KxP4W8O32 do not seem to be conventional Peierls instabilities in contrast with those observed in pure P4W8O32.