Origin of anisotropy of the near band gap absorption in Tl4HgBr6 single crystals

The origin of optical absorption anisotropy in the Tl4HgBr6 single crystals was studied using the first principles density functional calculations with different exchange correlation potentials. The results of the performed benchmark calculations include the structural, electronic, and optical properties of Tl4HgBr6. To the best of the authors' knowledge, these results are reported for the first time. It was shown that Tl4HgBr6 has a direct band gap of about 1.8 eV. All obtained data were compared to the corresponding experimental data; peculiar features of the polarized experimental absorption spectra were explained. It was shown that varying properties of the Hg–Br chemical bonds along different directions in the crystal lattice are in charge of the optical anisotropy, which is clearly revealed in the experimental spectra. The first estimations of the direct band gap value, refractive index and optical birefringence are given in this paper.