Optical properties of monoclinic SnI<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow/><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>from relativistic first-principles theory

Within the local-density approximation, using the relativistic full-potential linear muffin-tin orbital method, the electronic structure is calculated for the anisotropic, layered material SnI${}_{2}$. The direct interband transitions are calculated using the full electric-dipole matrix elements between the Kohn-Sham eigenvalues in the ground state of the system. The inclusion of spin-orbit coupling was found to change the optical properties of this material considerably. Polarized absorption and reflection spectra are calculated and compared with recent experimental results. The experimentally suggested cationic excitation for the lowest-energy transition is confirmed. From the site and angular momentum decomposed electronic structure studies and the detailed analysis of the optical spectra it is found that the lowest-energy transition is taking place between Sn $5s$ (atom type 2a) $\ensuremath{\rightarrow}$ Sn $5p$ (atom type 4i) states. The ground state calculation was repeated using the tight-binding linear muffin-tin orbital--atomic sphere approximation method, and the resulting band structure agrees very well with the one calculated with the full-potential method. In contrast to recent experimental expectations, our calculations show an indirect band gap, which is in agreement with earlier semiempirical tight-binding calculations as well as with absorption and reflection spectra.

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