Optical Properties and Electronic Structure of Polymorphous TICI and TIBr

TICI and TIBr can be forced to crystallize in the NaCl structure by evaporation on alkali-halide substrates, whereas they usually grow in the CsCl structure. Optical measurements on fcc layers of TlCl and TlBr are described for the first time. They are in good agreement with relativistic band-structure calculations, which are also reported in this Letter. An order-of-magnitude enhancement of exciton binding energies is explained as a consequence of a change in the type of chemical bond.