The influence of replacing Se by Te on electronic structure and optical properties of Tl4PbX3 (X = Se or Te): experimental and theoretical investigations

Abstract

The energy band structure, electronic charge density and optical features of Tl₄PbSe₃ and Tl₄PbTe₃ single crystals were studied within a framework of the recently modified Becke–Johnson potential (mBJ). The earlier experimentally defined atomic positions were additionally optimized by minimizing the forces acting on the atoms using a generalized gradient approximation (PBE–GGA) approach. The performed band structure calculations have shown that the conduction band minimum (CBM) and the valence band maximum (VBM) are located at the Γ point of the first Brillouin zone, resulting in a direct energy band gap equal to about 0.21 eV (PBE–GGA) and 0.32 eV (mBJ) for Tl₄PbSe₃ in comparison to the experimental value (0.34 eV), while for Tl₄PbTe₃, the band gap is equal to 0.10 eV (PBE–GGA) and 0.18 eV (mBJ) compared with the experimental value (0.19 eV). We have established that the mBJ approach succeeds by a large amount in bringing the calculated energy gaps into close agreement with the measured one. The angular momentum projected density of states explores the existence of weak hybridization between the states, defining the degree of covalent bonding. The calculated valence band electronic charge density space distribution confirms the prevailing covalent origin of the chemical bond. The calculated optical dispersion for the principal optical constants shows that these materials exhibit negative uniaxial anisotropy. We have measured the dispersion of the imaginary part of the dielectric susceptibility ε₂ (ω) and evaluated the experimental optical band gaps. The measured ε₂ (ω) dispersion confirms our theoretical evaluations that the substitution of Se by Te causes a band gap reduction.