Acta Crystallographica Section B

Structural Science

Volume 56, Part 3 (June 2000)

research papers

Acta Cryst. (2000). B56, 377-391 [ doi:10.1107/S0108768199016195 ]

Symmetry and twins in the monophosphate tungsten bronze series (PO₂)₄(WO₃)_2m (2 m 14)

P. Roussel, P. Labbé and D. Groult

Abstract: Monophosphate tungsten bronze with pentagonal tunnels (PO₂)₄(WO₃)₂_m are low-dimensional materials with charge density wave (CDW)-type electron instabilities. Two forms of the structure can thus be expected for all the members of the series: a low-temperature form (LT) corresponding to the CDW state and a high-temperature form (HT) corresponding to a normal metallic state. The HT form is described here for m = 9 and compared with that of the m = 5 and m = 7 counterparts. It is shown that a systematic twin phenomenon must be taken into account for HT members because of two possible configurations of the tilting mode of WO₆ octahedra. The structure is also compared with that of m = 10, which exhibits the modulated CDW-LT form at room temperature. Owing to two possible polarization directions of the segments built of m WO₆ octahedra, a twin phenomenon is also encountered in the LT forms. A review of all the structures known at present (m = 2, 4, 5, 6, 7, 8, 9, 10, 12) leads us to propose a structural law based on the building mode of WO₆ octahedra in WO₃-type slabs to explain the symmetry changes observed between even and odd members of the series.

Keywords: Twin phenomena; Symmetry; Monophosphate tungsten bronze.