[en] The title compounds have been prepared as polycrystalline powders by thermal treatments of stoichiometric mixtures of R₂O₃ and MoO₃ in air. The room-temperature crystal structure for all the series has been refined from high-resolution neutron powder diffraction data. All the phases are isostructural (space group C2/c, Z=8) with the polymorph α-R₂MoO₆, typified by Sm₂MoO₆. The structure contains four zigzag, one-dimensional MoO₅ polyhedral rows per unit cell, running through the RO₈ polyhedral framework along the [001] direction. MoO₅ form discrete units (i.e. do not share common oxygen), with Mo-O distances ranging from 1.77 to 2.24 A, although the oxygen coordination can be extended to distances of about 3.1 A, giving rise to strongly distorted MoO₈ scalenohedra. Thus, MoO₈ and RO₈ polyhedra are fully ordered in R₂MoO₆ compounds, which in fact can be considered as superstructures of fluorite (M₃O₆), containing 24 MO₂ fluorite units per unit cell, with unit-cell parameters related to that of cubic fluorite (a_f=5.5 A). A bond valence study demonstrates that the present crystal structure is especially stable for small rare-earth cations, and becomes more unstable when the R³⁺ size increases, thus explaining the observed preference of the large rare-earth molybdates for polymorphs β and γ with the same stoichiometry

[en] Neutron diffraction study of (La_1-yPr_y)_0.7Ca_0.3MnO₃ shows that with increase of Pr content (decrease of average A-cation radius), the low temperature state of the compounds is changing from homogeneous FM to quasi-homogeneous with canted magnetic structure, passing the stage of the two-phase state with FM-metallic and AFM-insulating regions of mesoscopic (>1000 A) dimensions. Different atomic structure of the appearing phases and developing field of lattice deformations are the factors stabilizing the two-phase state. (author)

[en] Layered brownmillerite-type manganese oxides, Sr₂GaMnO_5+x with mixed Mn³⁺/ Mn⁴⁺ valence, are studied by neutron powder diffraction. The type of magnetic order is controlled both by the structure of the non-magnetic GaO_1+x buffer layer and by the orbital configuration of Mn. In Sr₂GaMn(O,F)₆, the Mn-valence can be varied while all anion positions in the buffer layer are completely filled. We present the crystal and magnetic structures for the end members of the family (x=0 and 0.5), for an intermediate composition (x=0.41), and for Sr₂GaMnO_4.8F_1.2 with Mn-valence +3.8

[en] We propose the Markovian random walk approach for calculation of the depolarization function of the polarized neutron beam transmitted through a magnetic medium. This approach allows one to obtain exact analytical results for the depolarization function P(λ) which is valid for any wavelength λ. Two magnetic configurations were considered in the present work: random ferromagnetic domains, and staggered ferromagnetic domains. (author). 10 refs, 1 fig

[en] The layered brownmillerite-type manganese oxide system Sr₂GaMnO_5+x with mixed Mn³⁺/ Mn⁴⁺ valence has been studied by neutron powder diffraction. The type of magnetic order is controlled by both the structure of the nonmagnetic GaO_1+x buffer layer and orbital configuration of Mn. In Sr₂GaMn(O,F)₆ the Mn-valence can be varied while all the anion positions in the buffer layer are kept completely filled. We present the results of our studies of the crystal and magnetic structures in the end members of this family (x=0, 0.52) and for intermediate compositions (x=0.41) and Sr₂GaMnO_4.8F_1.2 with Mn valence +3.8

[en] We present high pressure-high temperature neutron diffraction data of α-iron in the 0-8 GPa and 300-550 K range. We find the isochors of α-iron strictly parallel to those of ε-iron, except at high P/T when the α-γ-ε triple point to the non-magnetic γ- and ε-phases is approached. This gives evidence for magnetoelastic effects in α-iron at high P/T. The measurements have been made possible by the use of a cell which enables neutron diffraction by external heating in the 0-10 GPa range and to at least 550 K. The technique should have numerous applications in research on condensed matter using neutrons.

[en] The new neutron powder diffractometer at the Swiss continuous spallation neutron source SINQ is designed as a flexible instrument for high resolution [best values δd/d: (<1, ∼ 2.5) x 10^-3 with d = lattice spacing in the high resolution or high intensity modes, respectively]. It uses large scattering angles 2Θ_M = 120 deg or 90 deg of the monochromator, a 28 cm high, vertically focusing wafer type Ge(hkk) monochromator and a position-sensitive ³He detector(3.6 bar) produced by Cerca at Romans, France. It has 1600 (25x64) detectors with an angular separation of 0.1 deg and includes modern electronics developed by E. Berruyer, Cerca and PSI. The SICS software of PSI controls the instrument with a server running on an unix workstation and clients written in Java through the TCP/IP network. The design principles and first experiences are presented. The interdisciplinary applications of HRPT will permit high-resolution refinement of chemical and magnetic structures as well as phase analysis including the detection of defects and internal microstrain. In particular real-time investigations of chemical or structural changes and of magnetic phase transitions in crystalline, quasicrystalline, amorphous and liquid samples including technically interesting new materials are possible. (author)

[en] We present a method which allows high-quality powder neutron diffraction patterns to be obtained under pressure by angle-dispersive diffraction to at least 10 GPa. This technique uses a new type of Paris-Edinburgh press in conjunction with sintered boron nitride anvils. As an example, we show NiO diffraction patterns obtained under purely hydrostatic pressures up to 10 GPa. These data were collected within a few hours, and are free from any contaminating signal from the pressure cell. High-resolution nuclear and magnetic structural information can be readily extracted by Rietveld refinements, without additional data correction. This technique will allow powder neutron diffraction at elevated pressures to become a standard tool on continuous neutron facilities

[en] The crystal structure of the layered cobaltite HoBaCo₂O_5.5 (substituted with both ¹⁶O and ¹⁸O) was studied across an insulator to metal (MI) transition at T_MI=305 K employing high-resolution neutron diffraction. We have found that the transition at T_MI is of the first-order type, accompanied by an abrupt negative change of the unit cell volume (∼0.15%) and melting of the orbital order in the pyramids. The existence of an isotope effect on T_MI suggests that the structure changes are caused by the electron delocalization above the transition

[en] Full text: The Fe isotope effect (Fe-IE) on the superconducting transition temperature T_c and the crystal structure was studied in the Fe chalcogenide superconductor FeSe_1-x by means of magnetization and neutron powder diffraction (NPD). The substitution of natural Fe (containing ∼92% of ⁵⁶Fe) by its lighter ⁵⁴Fe isotope leads to a shift in T_c of 0.22(5) K corresponding to an Fe-IE exponent of α=0.81(15). Simultaneously, a small structural change with isotope substitution is observed by NPD. This may help to clarify the currently existing controversial results of the Fe-IE. Upon correcting the isotope effect exponent for these structural effects, an almost unique value of α∼0.35-0.4 is observed for at least three different families of Fe-based HTS. (author)