Electronic band structure and optical properties of Li2In2GeSe6 crystal

We present results of a complex experimental and theoretical study of peculiarities of the electronic structure and optical constants of Li2In2GeSe6 compound crystallizing in monoclinic non-centrosymmetric space group Cc. In particular, we have measured the XPS spectra of core-level and valence electrons of atoms composing a high optical quality Li2In2GeSe6 crystal for both pristine and treated with middle-energy Ar+ ions surfaces. Further-more, the XPS spectrum as measured for the valence-band range of the Li2In2GeSe6 crystal is compared with the total density of states (DOS) theoretically calculated within a density functional theory (DFT) framework as implemented in the augmented plane wave plus local orbitals (APW+lo) method. The present theoretical APW+lo results reveal a very good agreement regarding the shapes and energy positions of main features of the theoretical total DOS curve and the experimental XPS spectrum when the first-principles DFT calculations are made using the modified Becke-Johnson (mBJ) functional in the form of Tran-Blaha and involving the Hubbard correction parameter U treated for highly correlated In 4d electrons (TB-mBJ+U approach). This technique gives also band gap value of 2.26 eV, which is in good agreement with that experimentally measured for this com-pound. The present TB-mBJ calculations indicate that, at the top and in the upper region of the valence band, Se 4p states make the biggest contributions; the central region is formed mainly by Ge 4p states, while In 5s states dominate at the bottom and in the lower portion of the Li2In2GeSe6 valence band. With respect to peculiarities of filling the valence band by Se(Ge) 4p states, the present calculations are verified experimentally. Furthermore, the optical properties of Li2In2GeSe6 are studied theoretically in detail employing the TB-mBJ+U technique.