Phonon Spectra of Chain TlSe and TlInSe2: Density Functional Theory Based Study

Abstract

The density functional theory (DFT) based ab initio calculations of the phonon spectra of the chain compounds, TlSe and TlInSe2, with analogous crystal structures have been carried out. For both materials, the derived frequencies and symmetry of the phonon modes in the center of Brillouin zone agree well with the experimental data on Raman scattering and infrared absorption. For TlSe, the available phonon dispersion and density-of-states data obtained by neutron scattering are also reproduced well by the results of calculations. In both compounds, the acoustic and low-frequency optic branches greatly overlap, which is a characteristic feature of structurally low-dimensional materials. For both compounds, the calculated and measured heat capacities are in a good agreement up to the temperatures limiting the validity of the used harmonic approximation. Strong anharmonisity of TlInSe2 as compared to TlSe, as well as the probable connection of this anharmonicity with incommensurate phase of TlInSe2 at temperatures above 100 K is figured out.