G. Dolling’s research while affiliated with Canadian Nuclear Laboratories and other places

The frequencies of selected intermolecular modes of vibration and libration in a single crystal of deuterated ammonia (ND3) have been measured by the technique of coherent inelastic neutron scattering, at temperatures of 20 and 95 K. The results are compared with the previous optical measurements at the Γ point, and with calculations based on two different models for the intermolecular potential function. A detailed assessment of these data leads to a set of mode frequencies for the Γ, R, and M points. The elastic constants are calculated from the measured acoustic mode dispersion curves propagating along the three major symmetry directions. The existing intermolecular force models are in good qualitative agreement with experiment, but significant discrepancies remain to be resolved by future theoretical refinements.

A very simple model for the lattice dynamics of CS2 is presented. This model is fully stable for a unit cell distorted by no more than 2% from the measured cell and gives good agreement with lattice mode frequencies obtained from optical studies. Coulombic forces appear not to be important for this structure. The molecular motion in modes for all the special points in the Brillouin zone is described through the use of an explicit notation. The group theoretical work of Grout el al. is extended to give the full selection rules for coherent inelastic neutron scattering. These rules show that certain modes can be measured with minimal interference from scattering from modes of different symmetry when certain experimental conditions are adopted, and therefore these rules will be of great use in the neutron scattering studies now in progress.

Measurements have been carried out on selected phonon modes in sodium at temperatures down to 36 °K. No phonon instabilities have been found to be associated with the martensitic transformation in this metal.

The structural and dynamical properties of the orientationally disordered solid SF6 have been studied. The present understanding of this state is discussed in terms of the microscopic nature of the orientational disorder, the influence of steric repulsion, and the concept of orientational frustration. A possible mechanism is considered for the phase transition to an ordered structure.

Measurements of the dispersion curves of the normal modes of vibration, by inelastic neutron scattering techniques, have been used to find the parameters of dipole approximation models to describe the lattice dynamics of germanium, silicon, diamond and gallium arsenide. The model for diamond, which has not previously been published, shows marked differences in the dispersion curves between this material and the other three and these give rise to characteristic differences in the physical properties. The models for all four materials give good agreement with the experimental results and enable other physical properties to be calculated with reasonable confidence in the frequencies of the normal modes computed from the models. Detailed calculations are made of the frequency distribution and its moments, and of the specific heat which is compared with experimental results. The agreement obtained is quite satisfactory for diamond but less so for the other materials. The negative thermal expansion coefficient of these materials at low temperatures is found to arise easily from these models, when they are extended in a simple way to include the central force part of the anharmonicity between nearest-neighbour ions. The infra-red absorption spectra are calculated both by assuming a joint density of states approximation, and also by assuming first- and second-nearest-neighbour interactions to describe the dipole moments. While none of these calculations is able to give a good account of the experimental results, the latter calculation suggests that second-nearest-neighbour interactions are necessary to explain the measured spectra. The calculations for diamond lead to an entirely different interpretation of the critical points in the spectra from that given previously. The Raman spectra of the elements have been calculated including the effects of anharmonicity on the one-phonon part of the spectra. Since the results depend on the orientation of the specimen and the geometry of the experiment a direct comparison with experiment is not possible. Nevertheless the results do suggest that this type of calculation may well be able to provide agreement with more detailed experiments. Critical points are frequently deduced from measurements of the infra-red and Raman spectra. The difficulties both in identifying and in assigning them to the appropriate normal modes are discussed.

The contributions to the dynamic form factor S(Q, omega ) arising from interference between its one-phonon and multiphonon parts have been observed in the scattering of neutrons from potassium. Interference makes two qualitatively distinct contributions to S(Q, omega ). One is approximately proportional to the one-phonon part, S1(Q, omega ), and contributes scattering intensity largely within the one-phonon peak region. The other is asymmetric around the one-phonon frequency and for narrowly peaked phonon groups contributes intensity of opposite sign in the multiphonon region directly on each side of the one-phonon peak. Both contributions, and their dependence on wavevector Q and frequency omega , can be described by an anharmonic theory (Ambegaokar et al. (1965)) which includes interference between the one- and two-phonon parts of S(Q, omega ) via the cubic anharmonic term.

The frequencies of normal modes belonging to the Sigma 4 and G4 branches in potassium at 4.3K have been measured by means of coherent inelastic neutron scattering experiments. No evidence was found in the Sigma 4 branch of the anomaly predicted in a recent theoretical calculation based on a rather sophisticated pseudopotential model. It is likely that this anomaly arises from a particular mathematical problem in calculating the Sigma 4 normal mode frequencies, and that when this problem is avoided, the model provides an excellent description of the observed frequencies. The parameters of a widely used phenomenological model have been re-evaluated in the light of all the phonon frequency data now available for potassium.

The normal mode Gruneisen gamma parameters have been measured for several modes of vibration of potassium at 4.5K over the pressure range 0.001 to 4.1 kbar, by means of coherent inelastic neutron scattering experiments. The results are compared with six different theoretical calculations based on a variety of screened pseudopotential models for the interionic forces in this simple metal, including a very recent calculation described in the following paper by Taylor and Glyde (ibid., vol.6, no.10, p.1915 (1976)). There is overall qualitative agreement between several theories and experiment, but significant discrepancies exist for four particular normal modes. Significant differences are also found between the present results for certain low frequency transverse modes and the analogous gamma values derived from the pressure dependence of the elastic constant C44 at room temperature.

The frequencies of particular intermolecular modes of vibration and libration in single crystals of solid carbon disulphide have been measured by coherent inelastic neutron and Raman scattering techniques 148K. Measurements were made at the special points in the Brillouin zone Gamma , S, R, Z, A, Q and Y and, including earlier infrared results, 37 of the 42 distinct frequencies at these points have been identified. Six of the ten dispersion curves along Lambda (001) have been measured and three dispersion curves have been outlined along Delta (100). Values of the elastic constants C11, C33, C44 and C55 have been determined from the acoustic branches. The present results are compared with calculations based on two different models for the interatomic potentials. Both models calculate the general level of the intermolecular frequencies to be too high and the bandwidth of the frequencies to be too large. Neither model can be considered an adequate description of the present results.

The Chalk River Laboratories of AECL Research provides neutron beams for research with the NRU reactor. The NRU reactor has eight reactor loops for engineering test experiments, 30 isotope irradiation sites and beam tubes, six of which feed the neutron scattering instruments. The peak thermal flux is 3 x 1014n cm−2s−1. The neutron spectrometers are operated as national facilities for Canadian neutron scattering research. Since the research requirements for the Canadian nuclear industry are changing, and since the NRU reactor is unlikely to operate much beyond the year 2000, a new Irradiation Research Facility (IRF) is being considered for start-up in the first decade of the next century. An outline is given of this proposed new neutron source.