[en] The structure of the Y-phase in the Mg-Ni-Sn system with Mg_74.5Ni_14.5Sn₁₁ composition was solved by applying the direct methods refined by a standard least square procedure. The space group is N deg. 101 (P4₂ cm) and the magnitude of the cell parameters are a=b=9.851(2) A and c=6.868(2) A. The final reliability factor is R=0.04 for 1575 independent reflections. The structure can be described in terms of an ideal structure containing two chains of cubes of Mg aligned along the c-direction. In each chain, Sn (resp. Ni) atoms occupy different opposite faces for adjacent cubes forming a stacking of tetrahedra along the c-direction. An alternative description in terms of two distorted polyhedral units is also given and the structure is compared with related inter-metallic compounds

[en] The previous studies of the superconducting properties of the hexagonal tungsten bronzes have repeatedly come up against the lack of reproducibility of the data and, at the first place, of the data relating to the stability of superconducting state. We revisited this problem and identified the main causes of these contradictory data in Rb_xWO₃, among which the major one is the ordering of the alkali atoms. Our study has emerged onto a determination of the x dependence of the order-disorder transition, of the lattice parameters and of the superconducting transition temperature T_c. We have also clarified the crystal structure and examined the mechanisms involved in the oxidation and reduction of this compound. Finally, we have assembled our data to draw a plausible Rb-W-O phase diagram

[en] A sol-gel method was successfully applied to synthesize RuSr₂GdCu₂O₈ (Ru-1212) as a single phase. The crystallization of Ru-1212 arises at 950 deg. C under O₂ flow with a small amount of secondary phase. The pure Ru-1212 phase is achieved after heating at 1020 deg. C in O₂ flow. The X-ray diffraction (XRD) pattern is refined (Rietveld) in the tetragonal space group P4/mmm with lattice parameters a=3.83904(9) Angst and c=11.5678(4) Angst. In situ high-temperature XRD and differential thermal analysis coupled with thermal-weight measurement show a structural decomposition of the phase at T_d=1050 deg. C followed by a partial melting at T_m=1118 deg. C. The decomposition produces crystallized Sr₂GdRuO₆, SrRuO₃ phases and a mixture rich in copper containing 'Sr, Gd, Cu, O' that does not diffract X-ray. This phase reduces to Cu⁺ at T_m with an important weight loss and a significant amount of liquid. The grain size and/or inappropriate grain boundaries of the pure phase treated below T_m does not permit to detect in sol-gel samples superconductivity otherwise observed in compounds prepared by solid-state reaction

[en] Phase relations at 1050 deg. C have been determined for M-phase solid solutions in the LiO_0.5-NbO_2.5-TiO₂ ternary phase system by the quench method. Rietveld analysis has been used to help determine phase boundaries and to study structure composition relations. The M-phases have trigonal structures based on intergrowth of corundum-like layers, [Ti₂O₃]²⁺, with slabs of (N-1) layers of LiNbO₃-type parallel to (0001). Ideal compositions are defined along the pseudobinary join LiNbO₃-Li₄Ti₅O₁₂ by the homologous series formula Li_NNb_N-4Ti₅O_3N, N≥4. Homologues with N≤10 lie to the low-lithia side of the LiNbO₃-Li₄Ti₅O₁₂ join and show extended single-phase solid solution ranges separated by two-phase regions. The composition variations along the solid solutions are controlled by a major substitution mechanism, Li⁺+3Nb⁵⁺↔4Ti⁴⁺, coupled with a minor substitution 4Li⁺↔Ti⁴⁺+3□, where □=vacancy. The latter substitution results in increasing deviations from the stoichiometric compositions A_2N+1O_3N with increasing Ti substitution. The non-stoichiometry can be reduced by re-equilibration at lower temperatures. Expressions have been developed to describe the compositional changes along the solid solutions

[en] Single crystals of a new compound of formula MnVSbO₆ were grown by slow cooling from a V₂O₅-B₂O3₃ flux at 900 deg. C. The compound crystallizes in the orthorhombic space group Pbcn (No. 60), with cell parameters (in the Pcnb setting) a=4.6604(3) A, b=4.9603(3) A, c=17.1433(9)A, Z=4. The crystal structure was solved from 1188 independent reflections to R_w=3.20% and goodness-of-fit 1.5 for 44 refined parameters. The structure can be described as a superstructure of the α-PbO₂ type with a cation ordering similar to that found in Fe₂WO₆. Cations occupy octahedral sites in the PbO₂-like layers. Zigzag chains of edge-sharing MnO₆ octahedra alternate with mixed Sb/V chains following a -Mn-Sb/V-Sb/V- sequence. The magnetic susceptibility of MnVSbO₆ follows the Curie-Weiss law down to ca. 15 K, where it orders antiferromagnetically. The bond lengths and Curie constant are consistent with the expected charge distribution Mn²⁺V⁵⁺Sb⁵⁺O₆

[en] We present a comparison of the crystal-chemical properties of the Bi-, Tl-, and Hg-based superconducting cuprates, in relation to their physical properties. Despite the global similarity of their layered structures, the different crystal-chemical nature of these three cations leads to quite different structural properties for the three series. The present knowledge of the relationships between crystal-chemical and superconducting properties in these systems is presented, with emphasis on the different doping mechanisms observed. (orig.)

[en] The perovskite-related phase Ca₃Nb₂O₈, when grown as single crystals from a calcium vanadate flux, incorporates a small amount of vanadium from the flux to form the composition Ca₃Nb_2-xV_xO 8 with x=0.025. The crystals have pseudo-cubic symmetry with a=6xa c(perovskite). The actual symmetry is rhombohedral, space group R3, with a h=16.910(1) A, c h=41.500(2) A. The structure was solved using a combination of single-crystal methods together with constrained refinements of powder X-ray and neutron powder data. The unit-cell composition is [Ca 1 3 8□ 2 4] A [Ca₄₂Nb₁₁₇V₃] B[O 4 8 0□ 6], with vacancies in both the anion sites and A-cation sites. The Ca and Nb atoms are fully ordered in the B-sites such that (001) layers containing only Nb-centered octahedra alternate with layers containing both Nb-centered and Ca-centered octahedra. At the origin B-site, ordered oxygen vacancies result in the octahedron being transformed to a tetrahedron, which, in the single crystals, is occupied by vanadium. The structure displays a new type of octahedral tilt system in which 3x3x3 blocks of (a⁺a⁺a⁺) tilts are periodically twinned on the pseudo-cubic {1 0 0} c planes

[en] Neutron diffraction and magnetic measurements on La_0.5-xCe_xSr_0.5MnO₃ (x≤0.4) reveal that both the Mn-moments (for all x) and the Ce-moments (x≥0.2) order ferromagnetically at different temperatures. Since Ce- and Mn-moments are coupled antiferromagnetically below T∼100 K, they form a ferrimagnetic ground state. Further, for x=0.3 and 0.4, the temperature dependence of resistivity shows a maximum and a minimum, in contrast to a metallic behavior expected from double exchange ferromagnetism and large band width W. These results suggest an antiferromagnetic Kondo interaction between the localized Ce : 4f and polarized conduction (Mn : e_g) electrons in a ferromagnetic Kondo lattice formed by the local Mn : t_2g and itinerant e_g spins

[en] Single crystals of MgB₂ have been grown at high pressure via the peritectic decomposition of MgNB₉. The crystals are of a size up to 1.5x0.9x0.2 mm³ with a weight up to 230 μg, and typically have transition temperatures between 37 and 39 K with a width of 0.3-0.5 K. Investigations of the P-T phase diagram prove that the MgB₂ phase is stable at least up to 2190 deg. C at high hydrostatic pressure in the presence of Mg vapor under high pressure. Small variations of T_c are caused by doping with metal elements from the precursor or annealing of defects during the crystal growth process

[en] The pair distribution analysis method is a fast spreading structural analysis method allowing to go beyond classical crystallographic analysis by providing quantitative information about local as well as meso-structure. It based on powder diffraction data fourier transformed to direct space. We will present here the main characteristics of the method, and its domain of application. (author)