L. Passell

NEUTRON CRYSTAL-FIELD SPECTROSCOPY IN RARE-EARTH METALLIC COMPOUNDS.

The technique of inelastic neutron scattering has been applied to the problem of crystal fields in rare‐earth metallic compounds. It is shown that in compounds where the Stark energies are somewhat greater than exchange energies, the complete energy‐level diagram can be readily obtained from the inelastic neutron spectra and hence the corresponding crystal‐field parameters may be deduced. In this paper we illustrate this by a series of measurements which we have performed on the praseodymium monopnictides, PrBi, PrSb, PrAs, PrP, and monochalcogenides PrTe, PrSe, PrS. It is found that the spectra can be reproduced in detail using ordinary crystal‐field theory together with a simple approximation to the instrumental resolution function. In addition, all of the crystal‐field levels in all seven compounds studied can be quantitatively accounted for using a nearest‐neighbor point charge model with an effective charge of −2. This is in spite of marked variations in covalency, carrier concentration and, indeed, ...

Rotational tunneling of methane on MgO surfaces : a neutron scattering study

High‐resolution inelastic neutron scattering was used to investigate the rotational tunneling of methane molecules in a ■×■R45° commensurate, square‐lattice–solid monolayer adsorbed on MgO (100) surfaces. Good matches to the observed transition energies were obtained using potential functions with C2v symmetry, suggesting that the preferred orientation of the molecule is the dipod‐down configuration with two opposite edges of the H‐atom tetrahedron parallel to the surface plane.

Spectroscopic Determination of Crystal‐Field Levels of Pr3+ in the Paramagnetic Metallic Compound PrBi

The Pr3+ 4f2 3H4 manifold is split by an octahedral crystal field into four levels Γ1, Γ3, Γ4, and Γ5. Transitions between these levels in PrBi have been measured using neutron time‐of‐flight spectrometry. Peaks in the observed spectra corresponding to crystal‐field transitions are identified both by their temperature dependence and by comparison with corresponding data from (nonmagnetic) LaBi. Well resolved crystal‐field transitions are observed at 11.7 and 5.8±0.6 meV at 90°K. Additional scattering observed at room temperature is consistent with unresolved transitions at 7.6 and 4.1 meV. As it is known from magnetic measurements that the Γ1 singlet lies lowest, these four transitions may be identified as Γ5 to Γ4, Γ4 to Γ1, Γ5 to Γ3 and Γ3 to Γ4 respectively, thus accounting for all transitions allowed by magnetic‐dipole selection rules. These data give the parameters A40 〈r4〉=6.89±0.30 meV and A60 〈r6〉=0.24±0.10 meV.

An X-ray diffraction study of krypton adsorbed on MgO(100) surfaces

Krypton in a triangular 2D solid phase was observed to form on the (100) surfaces of MgO crystallites. This system is unique because the physisorbed inert gas overlayer is not only incommensurate, but of a symmetry entirely unrelated to that of the substrate. Melting of the adsorbate occurred in the neighborhood of 70K for all coverages studied.

Diffraction Studies of Layering and Wetting Transitions

Thermodynamic arguments [22.1] show that when liquid and vapor phases co-exist in the presence of a surface several different types of behavior are possible: depending on the interfacial tensions between the three components, either the liquid can form droplets which contact the surface at a finite angle or it can spread out over the surface forming a uniform, macroscopically thick film interposed between the surface and the vapor phase. In the former case the liquid is said to partially wet the surface, in the latter it is said to wet it completely. A third possibility is that it will not wet the surface at all or, viewed another way, that the vapor phase will be interposed between the liquid and the surface. Co-existing solid and liquid or solid and vapor phases on a surface are also expected to show the same basic pattern of behavior Only recently has it been recognized that the temperature and/or concentration dependence of the relevant interfacial tensions can be quite different and that changes in these quantities can shift the system from one wetting state to another. A change from a partial to a non-wetting state is described as a “drying transition”; when the change is from a partial to a complete wetting state, it is referred to as a “wetting transition”.

Study of Spin Dynamics in Iron with Slow Neutrons

Inelastic scattering of slow neutrons in iron has been studied over a wide temperature range, from temperatures well below Tc, where spin wave modes are well defined, through the critical region in the neighborhood of Tc to temperatures above Tc where diffusive modes are dominant. Below Tc, in the temperature range 0.005<(1−T/Tc)<0.2, the spin‐wave energies vary as (1−T/Tc)0.37±0.03. Slightly below Tc the spin waves become over‐critically damped. There is no indication of a peak corresponding to a diffusive mode. Above Tc a well defined hydrodynamic region exists in which the scattering is accurately described by diffusion theory. Both the static susceptibility and the spin‐diffusion constant are found to vary as (1−Tc/T)ρ over the temperature range 0.008(1−Tc/T)<0.05, For the static susceptibility the value of ρ is 1.30±0.06, in reasonable agreement with theory; however, for the spin‐diffusion constant it is 0.14±0.04 which is considerably smaller than expectations.