DFT calculations and experimental studies of the electronic structure and optical properties of Tl4PbI6

Abstract

We present data of an experimental and theoretical study of the electronic band structure of ternary thallium lead iodide, Tl4PbI6. The XPS studies yield high quality of the Tl4PbI6 crystal, its low hygroscopicity, high stability regarding Ar+-ion-irradiation and comparatively strong degrees of covalency of the chemical Tl–I and Pb–I bonds. The latter peculiarity of the crystal under study is confirmed by first-principles calculations that are made within density functional theory (DFT). In the present DFT calculations we consider different models for exchange-correlation potential, and found that the best agreement between experiment and theory is observed when we use modified Becke-Johnson potential as elaborated by Tran-Blaha and involve also the Hubbard correcting parameter U and spin-orbit coupling (SOC) effect (the so-called TB-mBJ + U + SOC model). Employing the TB-mBJ + U + SOC calculations, we examine in detail partial and projected densities of states as well as band dispersions. The TB-mBJ + U + SOC calculations indicate that Tl4PbI6 is a direct gap semiconductor with the energy band gap value of 2.347 eV. This theoretical value is in excellent agreement with that evidenced from experimental measurements of spectral allocation of the absorption coefficient in the range of the fundamental absorption edge yielding Eg = 2.35 eV at room temperature. The present measurements of the second harmonic generation intensity of the Tl4PbI6 crystal feature high prospective of its application in NLO devices for the studied range of the fundamental beam, approximately 1 μm. We have established that the ignoring of the SOC effect in the calculations predicts a nondirect semiconductor nature in Tl4PbI6 being in conflict with the experiment. Based on the TB-mBJ + U + SOC model, we have calculated in detail the main optical constants of Tl4PbI6 allowing to draw a conclusion that it is a good semiconductor for application in optoelectronic devices.