Electronic absorption spectrum of Cs2ZnI4 thin films

The absorption spectrum of Cs2ZnI4 thin films in the energy range 3-6 eV at temperatures from 90 to 340 K has been investigated. It is established that this compound belongs to direct-gap insulators. Low-frequency exciton excitations are localized in ZnI4 structural elements of the lattice. Phase transitions at 280 K (paraelectric phase <-> incommensurate phase), 135 K (incommensurate phase <-> monoclinic ferroelastic phase), and 96 K (monoclinic phase <-> triclinic ferroelastic phase) have been found from the temperature dependences of the spectral position and halfwidth of the low-frequency exciton band. Additional broadening of the exciton band is observed for ferroelastic phases; it is likely to be due to exciton scattering from strain fluctuations near domain walls.