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International audience; Hydrothermal crystal growth can be described as a low temperature process under pressure using an aqueous solution as the solvent. The temperature gradient controls the transport phenomenon from the nutrient to the seeds. The solvothermal term is generally used when the solvent is a non-aqueous solution. Different aspects will be particularly discussed: (a) The principle of hydrothermal/solvothermal crystal growth processes. (b) A brief history of the development of such processes. (c) The main factors involved during the crystal growth and their impact on the quality of the resulting macroscopic single crystals. (d) How these physical and chemical factors can be optimized to improve the quality of the crystals. The conclusions focus on the future developments of hydrothermal or solvothermal crystal growth processes.

International audience; An in-situ investigation from high-resolution neutron powder diffraction data on the structural evolution of TmNiO3 and YbNiO3 perovskites across the metal-insulator transition, with TMI = 596 K and 598 K, respectively, allowed the charge disproportionation effect that these perovskites experience upon electronic localization below TMI to be followed. In the insulating (semiconducting) regime, the perovskites are monoclinic, space group P21/n, containing two inequivalent Ni1(3+σ)+ and Ni2(3‑σ)+ cations; above TMI, the samples become orthorhombic, space group Pbnm, with a single site for Ni3+. The 57Fe Mössbauer spectra of iron-doped (at 1.5%) RNiO3 (R = Tm, Yb) samples recorded below TMI exhibit for Fe1 and Fe2 (replacing Ni1 and Ni2 sites) hyperfine parameters corresponding to large (Ni1O6) and small (Ni2O6) octahedra. The remarkable difference between the quadrupole splittings (Δ1 ≈ 0.3 mm/s and Δ2 ≈ 0.07 mm/s) of Fe1 and Fe2 sites in RNi0.985Fe0.015O3 is analyzed. We calculate the lattice contribution to the electric field gradient (EFG) at 57Fe ions, and estimate, by using the experimental Δ1 and Δ2 values, the contributions of the 3d, 3p, and 2p electrons (overlap distortion and covalence effects). Above TMI, a unique state for iron atoms is observed, upon metallization of the sample.

A well-isolated Zhang-Rice (ZR) singlet as a ground state of the Cu3+ center in hole-doped cuprates is a leading paradigm in modern theories of high-temperature superconductivity. However, a dramatic temperature evolution of the Li-6,Li-7 NMR signal in La2Li0.5Cu0.5O4, a system with a regular lattice of well-isolated Cu3+ centers, reveals significant magnetic fluctuations and suggests a quasidegeneracy to be a generic property of their ground state at variance with the simple ZR model. We argue for a competition of the ZR state with nearby states formed by a "doped" hole occupying purely oxygen nonbonding a(2g)(pi) and e(u)(pi) orbitals rather than a conventional b(1g)(d(x2-y2))Cu 3d-O 2p hybrid. The temperature variation of the Li-6,Li-7 NMR line shape and spin-lattice relaxation rate point to a gradual slowing down of some magnetic order parameter's fluctuations without distinct signatures of a phase transition down to T = 2 K. This behavior agrees with a stripelike ferrodistortive fluctuating Ammm order in a two-dimensional structure of the (CuLi)O-2 planes accompanied by unconventional oxygen orbital antiferromagnetic fluctuations. DOI: 10.1103/PhysRevB.86.241107

International audience; The double manganite CaCu3Mn4O12 doped with 119Sn atoms (∼1 at.% with respect to manganese atoms) was studied by use of Mössbauer spectroscopy. Formally tetravalent Sn4+ ions substitute for isovalent manganese ions in the octahedral (Mn4+O6) polyhedra. The covalency effects on the magnetic interactions like superexchange in Cu2+-O-Mn4+ and Mn4+-O-Mn4+ bonds and supertransferred hyperfine interactions of the 119Sn probe atoms in the manganite structure are discussed. Using a semiquantitative nearest-neighbor cluster model relating the hyperfine magnetic field on the 119Sn nuclei (HSn = 105 kOe at T = 77 K) to covalency parameters and angle characterizing the Sn-O-M (M = Cu, Mn) bonds, it has been shown how such an analysis of supertransferred hyperfine interactions of tin probe ions can get fruitful information about strength and sign of the superexchange interactions between Mn4+ and Cu2+ magnetic ions. A consistent description of the results in the framework of the Weiss molecular field model considering the specific local environment of tin atoms has made it possible to estimate exchange integrals: JCuMn = −51.1 ± 0.3 K and JMnMn = −0.6 ± 0.2 K.

International audience; Solvothermal processes are defined, and the different domains of applications are described. The main physico-chemical factors playing a key role in such processes are then analyzed. The trends characterizing the current development of solvothermal processes are outlined.

International audience; alpha-quartz-type Si1−xGexO2 solid solutions (x=0.06, 0.11, and 0.24) and pure SiO2 were investigated by Raman spectroscopy. Coupled and decoupled vibrational modes were identified at room temperature as a function of composition. Tetrahedral tilting librational modes involved in the displacive alphabeta-quartz phase transition are decoupled. The wave number of the coupled A1 mode located at 464 cm−1 for pure alpha-quartz was found to vary quite linearly with germanium content. Raman spectra were recorded up to 1473 K. Substitution of germanium in the quartz lattice clearly improves the thermal stability of the alpha phase. The alphabeta-quartz phase transition temperature increases from 846±1 K for x=0 to about 1300±50 K for x=0.24. At the same time, the dynamic disorder observed in quartz well below the transition is reduced in the solid solutions.