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

The reflectivity spectrum R ( ω ) of SbSeI crystals was experimentally studied in the spectral range of 10 – 300 cm - 1 over a wide range of temperatures (10–297 K) with light polarization E ∥ c and E ⊥ 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 ω T and ω L of the low-frequency B 1 u mode (for E ∥ c ) on temperature was obtained from the experiment. From spectra ɛ ll ( ω ) and Im ( ɛ - 1 ) ( ω ) the frequencies ω L and ω T of B 1 u 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 B 1 u mode are explained in anharmonic approximation by employing the average potential energy function 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 B 1 u vibrational mode.