[en] The 3d¹ system BaVS₃ undergoes a series of remarkable electronic phase transitions. We show that the metal-insulator transition at T_MI=70 K is associated with a structural transition announced by a huge regime of one-dimensional (1D) lattice fluctuations, detected up to 170 K. These 1D fluctuations correspond to a 2k_F=c^*/2 charge-density wave (CDW) instability of the d_z² electron gas. We discuss the formation below T_MI of an unconventional CDW state involving the condensation of the other V⁴⁺ 3d¹ electrons of the quasidegenerate e(t_2g) orbitals. This study stresses the role of the orbital degrees of freedom in the physics of BaVS₃ and reveals the inadequacy of current first principle band calculations to describe its electronic ground state

[en] This paper briefly retraces the career of André Guinier, awardee of the Gregori Aminoff prize in 1985, from his PhD thesis work on small angle scattering and the Guinier–Preston zone, to the study of disorder crystals and amorphous bodies, including his role in the French research system. (invited comment)

[en] The quasi-one-dimensional conductor BaVS₃ undergoes a metal-insulator transition at T_MI=70 K accompanied by a formation of superstructure doubling the chain periodicity and a magnetic transition at T_X=30 K. We present an anomalous x-ray scattering investigation of the charge-density wave (CDW) satellite reflections at the V K-edge. Our study shows that no charge disproportionation larger than 0.01 electron occurs at T_MI and T_X. We interpret this result by the stabilization of two out-of phase CDW's for the d_z² and e(t_2g) electrons, which induces an orbital ordering while reducing the intrachain Coulomb repulsions

[en] We present a room temperature high-pressure X-ray diffraction study of the layered compound 1T-TaS₂ up to 20 GPa. This material is known to exhibit a variety of structural phase transitions that are ascribed to the stabilization of charge density wave states. It has been recently shown that at pressures larger than 3 GPa and up to 25 GPa, 1T-TaS₂ becomes superconductor below about 5 K. It was suggested that this superconductivity coexists with different CDW states, an hypothesis that can be tested by X-ray diffraction. Our first results at room temperature show that at around 1.9 GPa, the nearly-commensurate (NCCDW) phase transforms into a phase similar to the high temperature incommensurate phase (ICCDW). Above 9 GPa, we show the existence of another IC phase, still discernable up to 20 GPa despite the pressure-induced crystal damage above 13 GPa. These results are consistent with resistivity measurements, but call for a complete exploration of the P–T phase diagram of 1T-TaS₂.

[en] We present a high-pressure x-ray diffuse scattering study of the ABO₃ ferroelectric perovskites BaTiO₃ and KNbO₃. The well-known diffuse lines are observed in all the phases studied. In KNbO₃, we show that the lines are present up to 21.8 GPa, with constant width and a slightly decreasing intensity. At variance, the intensity of the diffuse lines observed in the cubic phase of BaTiO₃ linearly decreases to zero at ∼11 GPa. These results are discussed with respect to x-ray absorption measurements, which leads to the conclusion that the diffuse lines are only observed when the B atom is off the center of the oxygen tetrahedron. The role of such disorder on the ferroelectric instability of perovskites is discussed

[en] This collective publication proposes a comprehensive synthesis on instruments, methodology, applications and works in which the interaction between X rays and condensed matter is implemented. This fourth release mainly focuses on the use the synchrotron radiation which is largely used by scientists to explore matter in new directions. After a presentation of X ray sources and of the issue of coherence, the chapters address some specific issues: small angle scattering of X rays, the EXAFS (Extended X-ray Absorption Fine Structure) as a local synchrotron technique, the state of art and evolution of X ray tomography in 2012, structural studies of complex ordered states in material science, studies of relationships between structures and properties in oxide ion based conducting materials, the possible explanation and prediction of recrystallisation from the characterization of the deformed state, and the applications of X rays to the study of metallic contaminants in the environment