Elastic stiffness constants of copper indium diselenide determined by neutron scattering

From the acoustic-dispersion curves determined by inelastic neutron scattering, we obtain at room temperature the elastic stiffness constants of ${\mathrm{CuInSe}}_2$: at ${10}^{10}$ ${\mathrm{Nm}}^{\mathrm{-}1}$, ${\mathit{c}}_{11}$=9.70, ${\mathit{c}}_{12}$=5.97, ${\mathit{c}}_{13}$=8.60, ${\mathit{c}}_{33}$=10.89, and ${\mathit{c}}_{44}$=3.62. A theoretical calculation based upon the sphalerite approximation gives good agreement with the values of ${\mathit{c}}_{11}$ and ${\mathit{c}}_{44}$. Using the elastic constants, the isothermal compressibility is calculated along the c axis and the ab plane. Two low-energy optical modes were also observed at frequencies near 55 ${\mathrm{cm}}^{\mathrm{-}1}$ at the Brillouin-zone center.