W. Bührer

Low-temperature effects in magnetic spectral response of CeAl3-based systems

Magnetic excitation spectra of Ce ions in CeAl3 and in the substituted systems Ce0.9Y0.1Al3 and Ce0.9Gd0.1Al3 were studied by inelastic neutron scattering in the temperature range 0.03 K < T < 100 K. Below ≈ 60 K peak positions are strongly temperature dependent. The effect is enhanced due to chemical pressure and looks like a pure single ion feature given by the interaction of the Ce ion with the conduction electrons. Additional investigations of the spectra of the paramagnetic impurity ion Pr3+ in CeAl3 and in the reference system LaAl3 showed an anomalous attenuation of the relaxation broadening for the CEF excitations in the CeAl3 matrix. All observed features are interpreted as a consequence of the Kondo ground state formation, strongly influenced by the interaction between the crystalline electric field and the local moment of the Ce Kondo ion.

Lattice dynamics of Na2S

Inelastic neutron scattering has been used to measure the phonon dispersion relation along (001), (110) and (111) directions of a single crystal of Na2S at 50K. A simple shell model has been successfully fitted to the data. Dynamic quantities such as density of states, Debye temperature, displacement-displacement correlation functions and mean square ionic displacement are computed from the model and compared with CaF2. Finally some remarks on the ionic conductivity of antifluorites are given.

Guinier-preston I zones in Al-1.75 at.% Cu single crystals

Abstract Two Al-1.75 at.% Cu single crystals aged together for 60 min at 353 K were investigated by elastic diffuse neutron scattering. A large range of the scattering vector from 0.175 to 6.9 reciprocal lattice units along 〈h00〉 and the choice of isotopically pure 63Cu and 65Cu for contrast variation allowed the variation of the extracted results for short-range order scattering due to the finite series expansion of the static atomic displacement scattering to be followed systematically. Short-range order scattering was best separated if the scattering within the first Brillouin zone was included in the evaluation. Only then the short-range order scattering is found to be nearly independent of the scattering vector which indicates a considerably lower amount of multilayer Guinier-Preston I zones than found by other authors from three-dimensional diffuse X-ray scattering measurements.

Tentative model for the incommensurate and ferroelectric phases in Pb2 CoWO6

Abstract The sequence of cubic, incommensurate and ferroelectric phases found in Pb2 CoWO6 is described in the framework of the Landau theory. The model, which involves two different order-parameters, accounts for the first-order character of the transitions, the reentrant behaviour of the incommensurate phase and its coexistence with the ferroelectric phase.

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.

Dynamics of OH and H2O groups in sodalites

Abstract Hydrated and deuterated hydroxy-sodalites have been investigated by elastic, quasielastic and inelastic neutron scattering. The dynamics can clearly be separated into vibrational and diffusional components of rather different time scales. The apparently similar compounds show a wide variation in behavior.

Guinier-Preston zones in Al-rich AlCu and AlAg single crystals

Abstract Guinier-Preston zones in Al Ag and Al Cu were investigated by diffuse X-ray and neutron scattering. In Al-3 at % Ag no differences are found between zones formed above and below 500 K (e- and η-zones, respectively). Their average Ag concentration is (80 ± 10) at %, and the Ag concentration decreases by 10–15% from shell to core of the zones. In Al-1.75 at % Cu, Guinier-Preston I zones are dominantly monolayers with a Cu fraction of (84 ± 2) at %.

Coupled quadrupole-phonon excitations: Inelastic Neutron scattering on van vleck paramagnet PrNi5

Abstract Inelastic neutron scattering experiments on a single crystal of PrNi 5 (0.4 cm 3 ) were performed at the sample temperature T = 8 K. The dispersion relations of the acoustic transverse phonons and the low-energy magnetic excitations were measured in the T and Δ-directions. An analysis of the intersection region of these excitations shows that an additional neutron intensity to the nuclear and magnetic dipolar scattering occurs due to magnetoelastic scattering. The hybridized spectra of phonons and quadrupole excitons are discussed.

Crystal fields in the valence-unstable CeNi-based compounds

Abstract The influence of a partial delocalization of cerium 4f electrons on the crystalline electric field (CEF) effects measured on Pr paramagnetic impurity ions have been investigated for the series of (Pr, Ce)x(La, Y)1−xNi compounds, which undergo the transition from the intermediate valence (IV) to Kondo state due to the chemical substitution. In the Kondo state the hybridization of cerium 4f electrons and conduction electrons results only in the simultaneous increase of all CEF parameters, while in the IV regime the partial delocalization of cerium 4f electrons is responsible for a charge redistribution that manifests itself in different scales of changes for different CEF parameters.

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.