Sliding charge density wave in the monophosphate tungsten bronze(PO2)4(WO3)2mwith alternate stacking ofm=4andm=6WO3layers

Abstract

The monophosphate tungsten bronzes $({\mathrm{PO}}_{2}{)}_{4}({\mathrm{WO}}_{3}{)}_{2m}$ form family of two-dimensional metals which exhibit charge density wave (CDW) instabilities. These materials are generally built by the regular stacking of $(a,b)$ layers in which chains made of segments of m ${\mathrm{WO}}_{6}$ octahedra directed along the $\mathbf{a}$ and $\mathbf{a}\ifmmode\pm\else\textpm\fi{}\mathbf{b}$ directions are delimited. Their electronic structure thus originates from quasi-one-dimensional (1D) bands located on these chains. As a consequence their Fermi surface (FS) exhibits large flat portions whose nesting gives rise to successive CDW instabilities. Here we present a structural study of the CDW instability of the $({\mathrm{PO}}_{2}{)}_{4}({\mathrm{WO}}_{3}{)}_{10}$ member formed by the alternate stacking of layers built with segments of $m=4$ and $m=6$ ${\mathrm{WO}}_{6}$ octahedra. Its ab initio electronic structure calculation shows that the FS of this member exhibits large flat portions which can be extremely well nested. Its best nesting wave vector accounts for the modulation wave vector stabilized by the CDW transition which occurs at 156 K. Because of the regular stacking of layers of different m values the FS is slightly split. The unusual thermal dependence of the x-ray satellite intensity provides evidence that the two types of layers become modulated at different temperature. This also leads to a slight thermal sliding of the CDW-nesting modulation wave vector, which can be accounted for within the framework of a Landau-Ginzburg theory. In addition, the observation of a global hysteresis in the thermal cycling of the satellite intensity, as well as the degradation of the interlayer order upon cooling, suggest the formation of a disordered lattice of dilute solitons. Such solitons allow to accomodate the charge transferred between the two types of layer. Finally the relevance of local charge transfers, at intergrowth defects, for example, to create pinned discommensurations that break the CDW coherence is emphasized in this whole family of bronzes.