Lattice dynamics of solid neon at 6.5 and 23.7 K

The low-frequency phonons of fcc neon have been studied along the high-symmetry directions at both 6.5 and 23.7 K (melting point, 24.6 K) using inelastic neutron scattering on a stress-free single crystal grown at a pressure near one bar. At 23.7 K the zero-sound-wave velocities are approximately 2.5% larger than the first-sound-wave velocities measured by Brillouin scattering. The sign of the result here is as expected and compares well with the expected larger difference of approximately 5% which was found in krypton at 114 K (melting point, 115.8 K). At 6.5 K with a lattice spacing of 4.466±{}0.002 {}, the phonon measurements are in excellent agreement with previous measurements on a clamped crystal with the stated lattice spacing 4.454±{}0.002 {}; the data show a frequency shift of (1.2±{}0.5)%, whereas the calculated volume difference assuming a Gr\"uneisen constant of 3 would suggest a shift of (2.4±{}0.5)% (the present measurements are the first phonon measurements done on a neon sample with the equilibrium lattice parameter). An analysis of the data giving the long-wavelength elastic properties is presented. The integrated intensities of the peaked responses were also studied at 23.7 K and are shown to behave anomalously; this behavior has been found in studies of other quantum crystals indicating the existence of substantial multiphonon scattering and interference effects.