Temperature dependence of lattice dynamics of lithium 7

Abstract

Phonon dispersion relations in $^{7}\mathrm{Li}$ 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 K\'arm\'an 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 \ifmmode\pm\else\textpm\fi{} 0.003 \AA{} at 110 K and 3.537 \ifmmode\pm\else\textpm\fi{} 0.003 \AA{} at 424 K. The elastic constants obtained at 293 K from the model parameters are (${10}^{11}$ ${\mathrm{d}\mathrm{y}\mathrm{n}/\mathrm{c}\mathrm{m}}^{2}$) ${C}_{11}=1.73\ifmmode\pm\else\textpm\fi{}0.10$, ${C}_{12}=1.31\ifmmode\pm\else\textpm\fi{}0.20$, and ${C}_{44}=0.84\ifmmode\pm\else\textpm\fi{}0.060$. The temperature dependence of elastic constants is also determined.