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

Abstract:

The monophosphate tungsten bronzes K_xP₄W₈O₃₂ 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 K_xP₄W₈O₃₂ 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 K_xP₄W₈O₃₂ do not seem to be conventional Peierls instabilities in contrast with those observed in pure P₄W₈O₃₂.