OPTICAL-PROPERTIES OF MONOCLINIC SNI2 FROM RELATIVISTIC FIRST-PRINCIPLES THEORY

Within the local-density approximation, using the relativistic full-po tential linear muffin-tin orbital method, the electronic structure is calculated for the anisotropic, layered material SnI2. The direct inte rband 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 experi mental results. The experimentally suggested cationic excitation for t he lowest-energy transition is confirmed. From the site and angular mo mentum decomposed electronic structure studies and the detailed analys is of the optical spectra it is found that the lowest-energy transitio n is taking place between Sn 5s (atom type 2a) --> Sn 5p (atom type 4i ) states. The ground state calculation was repeated using the tight-bi nding linear muffin-tin orbital-atomic sphere approximation method, an d the resulting band structure agrees very well with the one calculate d with the full-potential method. In contrast to recent experimental e xpectations, our calculations show an indirect band gap, which is in a greement with earlier semiempirical tight-binding calculations as well as with absorption and reflection spectra.