J. Phys. I France
Volume 6, Numéro 8, August 1996
Page(s) 1059 - 1083
J. Phys. I France 6 (1996) 1059-1083
DOI: 10.1051/jp1:1996116

Evidence of High Critical Temperature Charge Density Wave Transitions in the (PO $\mathsf{_2}$) $\mathsf{_4}$(WO $\mathsf{_3}$) $\mathsf{_{2m}}$ Family of Low Dimensional Conductors for m ${\bf\geq}$ 8

Alberto Ottolenghi and Jean-Paul Pouget

Laboratoire de Physique des Solides CNRS-URA 02, Bâtiment 510, Université Paris-Sud, 91405 Orsay Cedex, France

Received 22 March 1996, accepted 2 May 1996

The monophosphate tungsten bronzes with pentagonal tunnels (MPTBp) of general formula (PO2)4(WO3)2m form a family of layered conductors where the average number of conduction electrons per tungsten atom 2/m can be changed while keeping the same structural array. Previous investigation of the low m=4,6 and 7 members of this family have shown that this series is subject to several successive incommensurate charge density wave (CDW) long range orders below room temperature (RT). Here we present an X-ray scattering study of the m=8, 9, 10, 11, 12, 13 and 14 members of this family. A very rich and diverse structural phase diagram is observed. The m=8 member shows only short range order below RT at two different incommensurate wave vectors while commensurate and/or incommensurate long range order is observed above RT for $m\geq 9$. Incommensurate modulations are observed for the m=11 and 13 members and commensurate ones for the other members. In most of the $7\geq m\geq 13$ members the observation of several harmonics suggests that the CDW modulation is non-sinusoidal, which could be the fingerprint of electron localization phenomena due either to strong electron-phonon or electron-electron interactions. These CDW instabilities have been tentatively attributed to chains a and ${\bf a\pm b}$ of WO6 octahedra into which the ReO3 type layers of (PO2)4(WO3)2m can be decomposed. In addition, the observation for $m\geq 9$ of a commensurate (1/2,0,0) or (1/2, 0, 1/2) modulation or (1/2, 0, l) diffuse scattering suggests the occurrence of an incipient WO3 type antiferroelectric instability which interacts with the CDW for $m\geq 13$. We further discuss the division between high m value and low m value members in relation to the x dependence of the physical properties of some MxWO3 tungsten bronzes families and finally propose a dielectric to magnetic duality between CDW tungsten oxides and superconducting copper oxides.

© Les Editions de Physique 1996