The charge-density-wave (CDW) structure of the monophosphate tungsten bronze ${\mathrm{P}}_4{\mathrm{W}}_{20}{\mathrm{O}}_{68},$ the $m=10\mathrm{}$ 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 $\pm \frac{3}{7}{\mathbf{a}}^{*}$ reduced wave vectors as well as second-order satellite reflections at $\pm \frac{1}{7}{\mathbf{a}}^{*}$ were observed. The CDW structure was refined in a four-dimensional (4D) formalism and found to have the superspace group ${P2\mathrm{}}_1\hspace{0.5em}\left(\alpha 00\right)\hspace{0.5em}0.$ 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 $\overrightarrow{m}\infty$ 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.

• Received 29 July 1999

DOI:https://doi.org/10.1103/PhysRevB.62.176