F Altorfer

Crystal structures and phase transitions of orthorhombic and rhombohedral RGaO3 (R=La,Pr,Nd) investigated by neutron powder diffraction

The rare-earth gallium oxide perovskites RGaO3 (R=La,Pr,Nd) are promising substrates for epitaxy of high-Tc superconductors. In this paper the crystal structures of orthorhombic and rhombohedral RGaO3 have been studied by neutron diffraction, which allows precise localization of light atoms such as oxygen, which are of importance in phase transitions. The lattice parameters, atom positions and tilt angles of the oxygen octahedra were determined in the temperature range 12-1773 K. Lanthanum gallate LaGaO3 shows a phase transition from an orthorhombic phase (Pbnm) to a rhombohedral phase (Rc) at 425 K. The results are discussed in the classification schemes developed for distorted perovskites.

Lattice dynamics and the diffuse phase transition of lithium sulphide investigated by coherent neutron scattering

Elastic neutron diffraction of Li2S, measured as a function of temperature, shows the onset of a diffuse phase transition near 900 K to a highly conducting state. Inelastic neutron scattering has been used to investigate the harmonic lattice dynamics of Li2S at 15 K. A shell model has been successfully fitted to the data. The dynamical properties are computed within the harmonic approximation and compared with experimental data.

H−-jump diffusion in barium-nitride-hydride Ba2NH

Barium-nitrade-hydride is one of the few documented H--ionic conductors. It crystallises in the [alpha]-NaFeO2-structure (space group R m) with distorted Ba-octahedra centred alternately by H- and N3-. Neutron diffraction shows that the hydrogen ions are distributed independent of temperature over lattice sites: 85% on (3a) and 15% on (6c). At T > 525 K a sharp increase of the ionic conductivity can be observed. The diffusion process takes place by uncorrelated jumps of hydrogen ions between regular lattice sites. Using quasielastic neutron scattering techniques and a planar jump diffusion model, we were able to determine the mean residence time [tau] on a H--site to be [tau]=20±2ps and the diffusion constant D to be 2.1±0.3×10-5 cm2/ /s at T=823 K.

Lithium diffusion in the superionic conductor Li2S

We have used quasielastic neutron scattering with polarization analysis to study the diffusion of Li in a single crystal of Li2S at 1170 K. Although Li2S shows a diffuse phase transition at 800 K the quasielastic scattering is purely incoherent and may be described in terms of a simple Chudley-Elliot model.

An investigation of anharmonic atomic vibrations of gamma -CuCl and gamma -CuBr by powder neutron diffraction

Powder neutron diffraction measurements were made on CuCl and CuBr solid ion conductors over large ranges of temperature in the cubic gamma phase (sphalerite structure). Anharmonic data treatment based on the Gram-Charlier expansion showed significant contributions up to fourth order for the thermal parameters of Cu. The thermal motion of Cu is strongly elongated along the tetrahedral diagonals. The corresponding probability density maps and the derived potential functions illustrate the characteristics.

Fast ionic diffusion in Li2S investigated by quasielastic neutron scattering

The Li diffusion in Li2S at high temperatures was investigated by quasielastic neutron scattering techniques on a single crystal of 7Li2S. It is shown by using polarized neutrons that the scattering is almost purely incoherent. The data were analysed in terms of an extended Chudley-Elliott jump diffusion model. The Li diffusion process takes place by hopping between regular tetrahedral and interstitial octahedral sites. The mean residence times on the regular Li sites were estimated to be 17.3 ps at T=1173 K, 6.7 ps at 1273 K and 4.3 ps at 1363 K.

Time-of-flight spectrometer FOCUS at SINQ: first results

Abstract Recently, the SINQ time-of-flight (TOF) spectrometer FOCUS became operational. FOCUS is a cold neutron hybrid TOF machine that combines a doubly focusing crystal monochromator with a Fermi chopper. The concept provides a highly flexible instrument that can be optimized for both quasielastic and inelastic neutron scattering experiments. The layout of the machine as well as first experimental results are described.

Charge order in high-Tc copper oxides probed by crystalline electric field interaction

Abstract Inelastic neutron scattering has been employed to study the transitions between the lowest levels of the J-multiplet of the Ho3+ ions in Ho1−xCaxBa2Cu3Oy ( x=0–0.3, y=6.3, 7 ) split by crystalline electric filed (CEF). The observed energy spectra exhibit a fine structure which reflects the behavior of the local hole density near the doping center. The results obtained are discussed in terms of the model of the CEF generated by the ordered planar charge structure.

Charge order in high-Tc copper oxides probed by crystal-field interaction

Abstract Inelastic neutron scattering has been employed to search for the transitions between the lowest lying levels of the J -multiplet of the Ho 3+ ions in Ho 1− x Ca x Ba 2 Cu 3 O y ( x =0–0.3; y =6.3–7) split by crystalline electric field (CEF). The observed energy spectra all exhibit a complex fine structure, which reflects the behavior of the local hole density near the doping center. We have found that the obtained results can be explained in terms of the model of the CEF generated by the ordered planar charge structure.