The concept of hidden Fermi surface nesting was introduced to explain the general observation that certain low-dimensional metals with several partially filled bands exhibit charge density wave (CDW) instabilities, although their individual Fermi surfaces do not reveal the observed nesting vectors. This concept was explored by considering the Fermi surfaces of the purple bronze AMo ₆ O ₁₇ (A = sodium or potassium) and then observing the CDW spatial fluctuations expected from its hidden nesting on the basis of diffuse x-ray scattering experiments. The concept of hidden Fermi surface nesting is essential for understanding the electronic instabilities of low-dimensional metals.

The 2:1 family of organic salts (o-DMTTF)2X (X~=~Cl, Br, I) exhibits regular stacks and a high-symmetry structure, which provide a perfect three-quarter-filling-band system allowing a rich phase diagram in the presence of strong electronic correlations. In this paper, we present a detailed study of this series combining complementary experimental techniques such as resistivity, thermopower, electron spin resonance, static magnetic measurements, and x-ray diffraction. In particular, we show that at ambient pressure (o-DMTTF)2X with X~=~Br and Cl undergoes two successive phase transitions setting successively a 4kF charge density and bond order wave order, then a spin-Peierls (SP) ground state. We discuss the symmetry of these phases and its relationship with the transport and magnetic properties. These phases are also followed under pressure by transport experiments, allowing the establishment of a generic phase diagram for this series of salts, where, with the onset of a one-dimensional to three-dimensional deconfinement transition, the 4kF order vanishes and the SP ground state transforms into a Peierls one. Interestingly, this phase diagram differs significantly from the one previously reported in other three-quarter-filled systems such as (TMTTF)2X and $\delta${}-(EDT-TTF-CONMe2)2X.

We present here the first accurate determination of the exact structure of κ-(BEDT-TTF)₂Cu₂(CN)₃. Not only did we show that the room temperature structure used over the last twenty years was incorrect, but we were also able to correctly and precisely determine it. The results of our work provide evidence that the structure presents a triclinic symmetry with two non-equivalent dimers in the unit cell, which implies a charge disproportionation between the dimers. However, structural refinement shows that the charge disproportionation is quite weak at room temperature.

The understanding of the superconducting phase under pressure in the famous iron-based spin ladder ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ commands a perfect understanding of the structural properties. We present an accurate single crystal x-ray analysis which demonstrates that the structure admitted until now at room temperature is totally ruled out. We show that the system has a polar structure already at 300 K and is thus ferroelectric. Its structure is modified below the Néel transition and the polarization is intensified. These features unambiguously prove the multiferroic character of ${\mathrm{BaFe}}_2{\mathrm{Se}}_3$ as proposed theoretically. This system evidences a competition between multiferroicity and superconductivity.

A mixed-valence conducting cation radical salt of the unsymmetrically substituted o-Me₂TTF donor molecule (TTF is tetrathiafulvalene) was obtained upon electrocrystallization in the presence of the non-centrosymmetric NO₃ ⁻ anion. It crystallizes at room temperature in the monoclinic P2₁/c space group, with the anion disordered on an inversion centre. The donor molecules are stacked along the a axis. A 90° rotation of the longest molecular axis of o-Me₂TTF generates a chessboard-like structure, preventing lateral S...S contacts between stacks and providing a strongly one-dimensional electronic system, as confirmed by overlap interaction energies and band structure calculations. A strong dimerization within the stacks explains the semiconducting behaviour of the salt, with σ_{room temp} = 3–5 S cm⁻¹ and E _activated = 0.12–0.14 eV. An X-ray diffuse scattering survey of reciprocal space, combined with full structure resolutions at low temperatures (250, 85 and 20 K), evidenced the succession of two structural transitions: a ferroelastic one with an anion-ordering (AO) process and the establishment of a (0, ½, ½) superstructure below 124 (±3) K, also visible via resistivity thermal dependence, followed by a stack tetramerization with the establishment of a (½, ½, ½) superstructure below 90 (±5) K. The latter ground state is driven by a spin-Peierls (SP) instability, as demonstrated by the temperature dependence of the magnetic susceptibility. Surprisingly, these two kinds of instability appear to be fully decoupled here, at variance with other tetramethyltetrathiafulvalene (TMTTF) or tetramethyltetraselenafulvalene (TMTSF) salts with such non-centrosymmetric counter-ions.