P4W20O68: A complex charge-density-wave modulated structure with an antiferroelectric-like lattice distortion

Abstract

The charge-density-wave (CDW) structure of the monophosphate tungsten bronze ${\mathrm{P}}_{4}{\mathrm{W}}_{20}{\mathrm{O}}_{68},$ the $m=10$ member of the ${(\mathrm{P}\mathrm{O}}_{2}{)}_{4}{(\mathrm{W}\mathrm{O}}_{3}{)}_{2m}$ series has been solved at room temperature from single-crystal x-ray-diffraction data. In agreement with previous x-ray diffuse scattering experiments, intense first-order satellite reflections at $\ifmmode\pm\else\textpm\fi{}\frac{3}{7}{\mathbf{a}}^{*}$ reduced wave vectors as well as second-order satellite reflections at $\ifmmode\pm\else\textpm\fi{}\frac{1}{7}{\mathbf{a}}^{*}$ were observed. The CDW structure was refined in a four-dimensional (4D) formalism and found to have the superspace group ${P2}_{1} (\ensuremath{\alpha}00) 0.$ The modulation, of the displacive type, involves mainly the tungsten atoms inside the different ${\mathrm{WO}}_{6}$ octahedra of the ${\mathrm{WO}}_{3}$-type slab. The W displacements, which are mostly oriented in the direction of the segment of 10 ${\mathrm{WO}}_{6}$ octahedra building the slab, tends to be oriented in opposite directions between neighboring segments. It is suggested that for the large m members these features represent a good compromise between the CDW instability of the metallic bronzes, related to their quasi-1D electronic structure, and the incipient antiferroelectric lattice distortion of the insulating oxide ${\mathrm{WO}}_{3},$ which corresponds to the limit $\stackrel{\ensuremath{\rightarrow}}{m}\ensuremath{\infty}$ of this series. These structural features play a crucial role in setting the coupling between the differently oriented 1D portions of the Fermi surface required for the hidden nesting mechanism at the basis of the stabilization of a CDW ground state in the ${\mathrm{ReO}}_{3}$-type metallic W and Mo bronzes and oxides.