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The phonon dispersion of fcc krypton has been measured along high-symmetry directions at 10'K on a single crystal with an equilibrium lattice spacing of 5.646 + 0.002 A. The data can be fitted with a general three-neighbor-forceeonstant model of nine parameters with only slight depatures from central force axially symmetric conditions. Utilizing theoretical estimates of the weak additional forces due to the fourththrough eighth-nearest neighbors, the zero-sound elastic constants are determined to be c "=514 + 5, c "=284 + 6, and c "=268 g 3 x 10' dyn cm '-. The agreement of the measured dispersion curves with theoretical calculations by Barker et af. is excellent; the maximum deviation of =3% occurs near the zone boundary. A broadening of the neutron response for zone-boundary modes was observed that was comparable to the energy itself when the sample was near its melting point.

The neutron-inelastic-scattering technique has been M to measure the phonon dispersion relations for a fcc single crystal of xenon at 10 K. All branche's in the high-s~~etry directions were determined with special emphasis on the small-wave-vector portions. Careful consideration was given to the effects of instrumental resolution in an effort to achieve the highest possible accuracy. The corrected data were fitted to several harmonic Born-von Khrmin force-constant models. From the force constants the frequency distribution function was determined, which made possible calculation of some of the thermodynamic properties. The zero-sound elastic constants derived from these force constants agree with those from our earlier determination using only the small-wave-vector data.

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