Single-particle kinetic energies in solid neon

Inelastic-eV-neutron-scattering experiments have been performed on solid natural neon at five temperatures between 4.7 and 26.4 K and at pressures between equilibrium vapor pressure and 17.6 MPa. Measurements of atomic-recoil line broadening were made within the impulse approximation. Average single-particle kinetic energies 〈${\mathit{E}}_{\mathit{k}}$〉 have been obtained by assuming a Gaussian atomic momentum distribution. A ground-state kinetic-energy value of 49.1±2.8 K is found and the temperature variation of 〈${\mathit{E}}_{\mathit{k}}$〉 is determined. Comparisons are made to somewhat restricted deductions of 〈${\mathit{E}}_{\mathit{k}}$〉 from previous experiments on neutron-phonon scattering and heat capacity. Comparisons with existing theoretical calculations show that none of them predicts the observed ground-state kinetic energy, although cell-model variational calculations and self-consistent harmonic calculations in the literature find that the ground-state kinetic energy is substantially larger than the ground-state potential energy, in agreement with our results.