Temperature Dependence of Lattice Dynamics of Lithium 7

Phonon dispersion relations in 7Li have been measured by the coherent inelastic neutron scattering at 293 and 110 K. The frequency distributions are obtained from the experimental data using the Born-von Kármán general force model. The first-neighbor force constants at 293 K are found to be about 10% smaller than those at 100 K. Temperature dependences of selected phonons have been studied from 110 K to near the melting point. The energy shifts and phonon linewidths have been evaluated at 293, 383, and 424 K by comparing the widths and energies to those measured at 110 K. The lattice parameter is found to be 3.490 ± 0.003 Å at 110 K and 3.537 ± 0.003 Å at 424 K. The elastic constants obtained at 293 K from the model parameters are (1011 dyn/cm2) C11=1.73±0.10, C12=1.31±0.20, and C44=0.84±0.060. The temperature dependence of elastic constants is also determined.