B. N. Brockhouse’s research while affiliated with McMaster University and other places

The crystal dynamics of an alloy with a large amount of mass disorder (Ni and Pd have masses 58.7 and 106.5 amu, respectively) have been investigated by inelastic neutron scattering. A triple-axis spectrometer was used to measure the phonon dispersion curves and widths in a crystal of the fcc random alloy Ni55Pd45. Significant broadening was observed in each branch for neutron groups corresponding to phonons of large wave vector. For transverse modes, many of the widths are large and indicate behavior characteristic of resonance modes. Neutron groups for [111] transverse vibrations at the zone boundary had the largest widths (2.1 THz, compared with a mean frequency of 3.3 THz). The widths of the longitudinal modes are generally smaller than those of transverse modes, and no definite indications of resonance behavior are apparent. The results are compared with the predictions of several theories, notably the coherent-potential approximation with some qualitative success. The form of the discrepancy between theory and experiment, and the similarity of the results to those of an experiment on crystals containing relatively dilute concentrations of Au in Cu, suggest the need to consider force-constant changes as well as mass disorder. Some new measurements for Cu(Au) are also reported.

Translational and librational modes in ND4Cl at the high-symmetry points Γ, X, M, R in the Brillouin zone were studied over a temperature range from 82 to 300 °K by coherent inelastic scattering of thermal neutrons utilizing the McMaster triple-axis crystal spectrometer at Chalk River. The temperature dependence of the frequencies and widths were obtained from the measurements. Several modes were found to undergo an abrupt decrease in frequency in the vicinity of the order-disorder transition (Tλ=249.5 °K), while others are insensitive to the transition. The widths of the neutron groups for some of the modes increased rapidly at temperatures beyond 200 °K. The dynamics of those lattice modes associated with the phase transition is briefly discussed. In addition some acoustic modes at small waves vectors were measured, and the associated elastic constants (c11 and c44) calculated, as functions of temperature. An interesting effect was noted. The constant c11 as measured for ND4Cl with neutrons (and thus appropriate to very high frequencies) shows much less effect of the transition than does c11 measured for NH4Cl at ultrasonic frequencies. The effect is believed to arise from the kinetics of the ordering process.

Dispersion curves for the [00ζ] (Δ), [ζζ0] (Σ), [ζζζ] (Λ), and [1/21/2ζ] (T) directions in ND4Cl were measured at 85 °K by inelastic scattering of slow neutrons using the McMaster triple-axis crystal spectrometer at Chalk River. The single crystals were grown from solution. A torsional (librational) branch, flat within about 2%, with an average frequency of 8.3 × 1012 cps, was found. Results are compared with a modified rigid-ion-model calculation based on Parlinski's formulation. Symmetry points and branches in the dispersion curves are classified according to their symmetry species. The Lyddane-Sachs-Teller relation as generalized by Cochran holds within experimental accuracy.

Complete dispersion curves for the [001] and [110] directions in large single crystals of ND4Cl were measured at 85°K. A flat librational branch (frequency 8.3 × 1012 cps) was found. Results are compared with model calculations for NH4Cl by Parlinski.

The complete phonon dispersion relation for the symmetry directions in silver at room temperature has been measured by triple-axis neutron spectrometry. There is some disagreement with the time-of-flight results of Drexel et al.

DOI:https://doi.org/10.1103/PhysRevLett.20.798

The dispersion relations of the lattice vibrations in the paramagnetic and ferromagnetic phases of nickel have been measured along the major symmetry directions by neutron spectroscopy at 400°C, above the Curie temperature of 358°C, at 300°C, and at room temperature. The latter results are in substantial agreement with those of Birgeneau et al. The average of the ratios of the frequencies at equivalent positions in reciprocal space for different temperatures are: , with a standard deviation of a single ratio from the mean of 0.023. Some branches, such as the Λ(0ξ1) and T(ξξξ) exhibit larger average frequency shifts. Miiller et al. have observed an average frequency ratio of 0.970±0.004 for similar measurements on palladium, an element which resembles nickel in many ways. This fact suggests strongly that the frequency shifts arise largely from anharmonicity, and not from magnetic effects. Broadening of the neutron groups at 300°C and 400°C was observed; the average widths are greater than at room temperature by roughly 45%. For palladium no significant broadening was observed. Thus the broadening may be of “magnetic” origin.

The atomic vibrations in a single crystal of Cu-3 at.% Au have been studied using inelastic neutron scattering. The line shapes and widths of the neutron groups, and the shifts from the corresponding frequencies for the host (pure copper), agree substantially with calculations based on the theory of Elliott and Maradudin, but appear to show slight deviations which are in the same direction as, but much less marked than, those exhibited by the results of previous measurements on single crystals of Cu-9.3 at.% Au.