Advanced Physical Research

The phonon properties of ZnIn₂Se₄  crystals were investigated experimentally by Raman scattering and infrared reflection, and theoretically from the first principles using the density functional theory (DFT). The frequencies of optical modes were determined from the Raman scattering and infrared reflection studies. The dispersion of phonon modes, the origin of energy states, the partial densities of states (PDOS) projected onto atoms, and the frequencies of optical phonons were calculated from first principles. The values of six elastic constants c₁₁, c₁₂, c₁₃, c₃₃, c₄₄ and c₆₆ of ZnIn₂Se₄ were also calculated, the bulk modulus, shear and Young's moduli, and Poisson's ratio were determined. The velocities of longitudinal and transverse acoustic waves in ZnIn₂Se₄ crystals in different crystallographic directions, as well as the temperature dependences of thermodynamic functions – free energy, phonon part of internal energy, entropy, and heat capacity – were calculated from first principles. It is shown that theoretically calculated values of heat capacity of ZnIn₂Se₄ crystals are in good agreement with known data of experimental measurements.