Investigation of the vibrational spectrum of SbSeI crystals in harmonic and in the anharmonic approximations

The reflectivity spectrum R(ω)R(ω) of SbSeI crystals was experimentally studied in the spectral range of 10–300cm-1 over a wide range of temperatures (10–297 K) with light polarization E∥cE∥c and E⊥cE⊥c . The spectra of optical constants and optical functions were calculated using the Kramers–Kronig and optical parameter fitting methods. The dependence of the frequencies ωTωT and ωLωL of the low-frequency B1uB1u mode (for E∥cE∥c) on temperature was obtained from the experiment. From spectra ɛll(ω)ɛll(ω) and Im(ɛ-1)(ω)Im(ɛ-1)(ω) the frequencies ωLωL and ωTωT of B1uB1u normal modes were found at temperatures 10–297 K. The frequencies of normal modes and amplitudes of normal coordinates were calculated by diagonalization of the dynamical matrix in harmonic approximation. The properties of the low-frequency B1uB1u mode are explained in anharmonic approximation by employing the average potential energy function V(z)V(z). The latter is strongly anharmonic and thus causes the frequency of this mode to show a quite strong temperature dependence. The interaction between phonons creates the anharmonicity of low-frequency B1uB1u vibrational mode.