Change of exciton–phonon interaction in layered ferroelastic crystals (Cs3Bi2I9)

The half-width of exciton absorption band (n=1) of Cs3Bi2I9 layered ferroelastic crystals was studied carefully as function of temperature in the range from 5 to 300 K. For the first time, we have found a new physical effect: change of exciton–phonon interaction (from weak to strong) in the same sample as temperature increases. It was established that the temperature value T*=150 K may be considered as characteristic one, below which a crystal loses the nature of layered substance. The effect is explained using a model that takes into account the reconstruction of the crystal lattice from non-layered to layered one.