H.G. Smith

Experimental study of lattice dynamics in LaB6 and YbB6

Abstract Lattice dynamics of La 11 B 6 was studied on the neutron triple axis spectrometers at HFIR. The acoustic and optical modes were measured in the high symmetry directions. The rotary motion of the B 6 molecule was measured only near the zone boundaries but shows no softening. Instead, very flat low frequency La modes were observed. The agreement with the elastic constants and the temperature dependence of specific heat is satisfactory. In YbB 6 , the elastic constants and specific heat were measured. Both results give the quite different feature from those of LaB 6 , no softening of C 12 and more softened flat low frequency Yb modes, due to the lack of the metallic bonds.

High-Pressure Cell for Neutron Scattering

We have developed a high-pressure cell for neutron scattering. The cell uses a barrel-shaped support cylinder made of sintered ceramics. Samples 6 mm in diameter and 10 mm long can be accommodated. The cell is suitable for use at low and high temperatures, after a pressure generated at room temperature is locked. The change of pressure on cooling to cryogenic temperatures is estimated by measuring the lattice parameter of NaCl. Besides the pressure locking technique, continuous pressurization can be achieved at room temperature by using a built-in mini-press while the cell is mounted on a goniometer of neutron spectrometer.

Inelastic neutron scattering from a martensitically transforming Indium-Thallium alloy

Abstract The phonon dispersion curves for an In 24 at % Tl alloy have been measured by inelastic neutron scattering and an acceptable fit to the experimental data has been obtained using Born-von Karman interactions extended to fourth nearest neighbours. In addition a model pseudopotential which also provides a reasonable fit to the data is described. The failure to observe phonon softening for the [ζ ζ 0] [ζ ζ 0] branch either in this crystal or in the temperature dependence of selected [ζ ζ 0] [ζ ζ 0] phonons in an In 27 at % Tl crystal is discussed in relation to current understanding of the structural transformation in these alloys.

Phonon anomalies and superconductivity in the hcp metals, Tc, Re, and Ru

Abstract The phonon dispersion curves have been measured for the hcp metals, Tc, Re, and Ru in the [001] direction at room temperature. A pronounced phonon anomaly has been observed for the LO mode in Tc and Re which is absent in Ru. A low temperature study of Tc revealed a strong temperature dependence of the anomaly. It is suggested that the magnitude of the phonon anomaly is correlated with the strength of the electron-phonon interaction in these metals.

The phonon dispersion relations in a disordered Ni1-xPtx system

Abstract The phonon dispersion relations of disordered alloys Ni 1- x Pt x were measured for five specimens covering the full range of atomic concentration by means of neutron inelastic scattering. At the resonant frequencies well separated double peak structures were observed in the phonon spectra. For the specimens with x ≧ 0.30 , the energy splitting of the phonon dispersion curve for the L -branch takes place at a higher frequency than that for the T -branch. The maximum phonon energy decreases with increasing Pt concentration but deviates from the theoretical prediction based on the mass defect CPA model. For x = 0.95 the localized mode cannot be observed as a well defined peak and does not seem to be split off from the main phonon band.

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.

Martensitic transformation from BCC to 9R and FCC in metallic lithium

Abstract High resolution neutron diffraction experiments have been performed on polycrystalline lithium-7 metal over a temperature and pressure range of 45 K to 300 K and 1 bar to 6.5 kbar. The partial transition from the bcc phase to the 9R (Sm-type) phase occurred at approximately 90 K at 6.5 kbar. On warming at 6.5 kbar, the 9R phase transformed to the fcc phase over a narrow temperature range near 140 K.

Phonon Spectra and Superconductivity in Some Transition‐ and Actinide‐Metal Carbides

The phonon dispersion curves have been measured by inelastic neutron scattering for the carbides of the transition metals, Ta, Hf, Nb, V, and Zr in which unusual features in some of the modes are observed for those compounds which become superconducting, (TaC, NbC), compared to those that do not. Studies immediately above and below the transition temperature in TaC did not reveal any changes in phonon frequency or width. No unusual features are observed in the phonon spectra of the actinide UC, a non‐superconductor. Although NbC.76 (not superconducting above 1.2°K) does not show the changes in some of the modes that are seen in NbC.98 some of the phonons are extremely broad and asymmetric. This is possibly related to the highly defect nature of the crystal. These results seem to corroborate McMillans suggestion for the transition metals, i.e., that the electron‐phonon coupling constant, and hence the transition temperature, is very sensitive to the phonon frequencies in the strong‐coupled superconductors.

XV congress and general assembly of the IUCr

Abstract The XV Congress and General Assembly of the IUCr met in Bordeaux, France on the campus of the University of Bordeaux, July 19-28, 1990. Nearly 2000 scientists attended with representatives from qost of the countries of the world, since crystallography covers so many diverse fields. The format of the Congress was to begin each morning with two main lectures in parallel – one devoted to biological subjects and one to some other area of crystallography. These lectures were followed by six or seven microsymposia in specific fields. The afternoons were reserved for poster sessions and viewing the commercial exhibits. There were a few evening lectures by award winners, for example Nobel Laureate R. Huber and Ewald Medal Awardee B. K. Vainshtein.

Effect of the application of uniaxial stress on precipitation in an Al(Cu) alloy

Abstract The effects of uniaxial stress applied during the aging time for precipitation growth on the formation of G.P. zones in an Al−1.8% Cu alloy have been studied by means of neutron scattering. When a a compression is applied along the [100] direction the precipitates grow preferentially on the (100) planes. Making use of this result, the effect of the formation of G.P. zones on the resonance anomaly in the [ζ00]T phonon branch previously observed in this alloy has also been studied with respect to the relative direction between the phonon wave vector q and the plane of the precipitate. The experimental result indicates that the precipitates perpendicular to q are more effective for the formation of the resonance anomaly than those parallel to q .