Energy Bands in CsI

Relativistic energy bands have been calculated for CsI by means of the Green's function method. The value of the potential outside the muffin-tin spheres is adjusted so as to duplicate the observed value for the band gap. d -like conduction bands originating mainly in the d -states in the alkali ion are found to be overlapping with the s -like band more strongly than in the potassium halides. The complex structure in the intrinsic optical spectra can be explained by the excitons associated with the s -like and d -like conduction-band states at the point \(\varGamma\). Spectra of the other Cs halides can be understood by analogy with CsI. Discussion is given on the multiplet structure of the F -centre absorption band of the Cs halides.