Phonon Properties of CuInS2 Crystals: Experiment and First-Principles Calculations

Phonon properties of CuInS2 crystals are studied experimentally by the Raman scattering method and also theoretically from first principles using the density functional theory (DFT). From the Raman scattering studies, the frequencies of Raman-active modes have been determined. From the first principles, the phonon mode dispersion, the origin of energy states, partial densities of states (PDOS) projected on atoms, and the optical phonon frequencies have been calculated. In addition, six elastic constants с11, с12, с13, с33, с44, and с66 are calculated and the bulk modulus, the shear modulus, the Young’s modulus, and the Poisson ratio are determined. The Grüneisen parameter and the Debye temperature are estimated. The velocity of propagation of longitudinal and transverse acoustic waves in CuInS2 crystals in various crystallographic directions and also temperature dependences of the thermodynamic functions (free energy, phonon part of the internal energy, entropy, and specific heat) are also calculated from the first principles.