The MonoPhosphate Tungsten Bronzes (MPTB) family, (PO2)4(WO3)2m, can be described by a regular intergrowth of (PO4) tetrahedra layers and of slabs constituted by corner-sharing-(WO6) octahedra, with a thickness depending on the m parameter. These low-dimensional oxides are known to exhibit successive transitions toward Charge Density Wave (CDW) states. These transitions are associated to lattice distortions leading to the appearance of incommensurate or commensurate structural modulations [1]. In this family, the electronic anisotropy and the density of carriers of the system can be tuned by modifying the thickness of the WO3 slabs i.e. changing m. MPTB family is thus a relevant system to analyse the effect of the dimensionality on the CDW electronic instabilities. Temperature-dependant X-Ray Diffraction (XRD) [1] and transport measurements reported in the literature, for different terms of the family, reveal a significant change of behaviour between the terms with a low and high value of m, m < 7 and m > 7 respectively. Classical CDW transitions are reported for the low terms, characterized by a smooth resistivity jump and by the formation of clusters of tungsten in the centre of the WO3 slabs [2]. For the high terms [3], a structural transition is observed in XRD but the electronic transport studies do not show the usual signature attributed to a CDW. Moreover, the only structural study performed on a high term in the modulated state (m=10) [3] evidences anti-ferroelectric-type (AFE) atomic displacements for the tungsten atoms without reporting of the formation of clusters of tungsten.
We will present both the transport properties and the analysis of the structural modulations for the m=8 term. Three first-order transitions associated with large thermic hysteresis were identified. The analysis of the structural modulations characterizing the different states, via the use of the super-space formalism, reveals the existence of AFE-type displacements and the formation of clusters for the tungsten atoms. These signatures can be assigned to the coexistence of AFE and CDW properties in the material. These two properties are a priori incompatible, but an extensive study of the transport properties versus temperature supports this hypothesis. This result enlightens the very interesting position of P4W16O56 (m =8) in the border area between the low and the high m values in the MPTB family to discuss the competition regime between CDW and ferroelectric instabilities.