The three adiabatic second-order elastic stiffness coefficients of single-crystal iron are reported from room temperature to 500°C. The coefficients vary in a linear manner with temperature, as opposed to previous results where a departure from linearity took place above 300°C. It seems likely now that this previous behavior was due to the silicone fluid bond between the quartz transducer and the iron specimen, and not the specimen itself. The present work shows that vacuum-evaporated thin-film transducers of zinc sulfide can be used reliably at higher temperatures, in both the longitudinal and transverse modes. However, certain difficulties remain in making transducers to generate all the desired modes for elastic-constant measurements.

In this paper we calculate the two-phonon efficiency for light scattering from rare gas crystals at all temperatures. We use a Lennard-Jones potential and.the Lorentz-Lorenz model following previous theoretical work and use the firstorder self-consistent phonon theory, supplemented by a simple model giving the phonon anharmonic widths and shifts. Our results, though larger than the experimental results, agree well with earlier calculations at 0 K and related molecular dynamics work. The calculated spectra show a rich structure that varies with temperature. We suggest that further experiments could lead to significant new insights into the nature of excitations in rare gas solids as a fimction of temperature.

The two-phonon Raman spectrum of solid argon at 19 K has been calculated using the point-dipole model, with phonon frequencies derived from a model fitted to neutron scattering experiments. The positions of peaks in the calculated spectra correlate well with peaks in the measured results. In order to obtain correctly the sharpness of the peaks it is necessary to include a small linewidth in the phonon spectral[unctions. Some discrepancies remain, which may show the inadequacy of the point-dipole matrix element, assuming that the correction of the data for background, etc. is reliable.

The two-phonon Raman spectra of solid argon at 60 K, 22.1 cm 3/mole and at I53.5K, 22.3 cm3/mole are calculated, with phonon frequencies obtained from a calculation in which cubic anharmonicity is included self-consistently and short-range correlations are incorporated. The results are compared with existing molecular dynamics calculations and with experimental measurements. Certain general trends, such as the presence of a two-phonon cutoff and a smoothing out of the features of the spectra, are correctly reproduced, but the calculations predict too low intensity at frequencies around 40 cm-1, between the sum and difference bands.