Nontraditional temperature shift of the fundamental absorption edge for layered substances and the ferroelastic phase transition in Cs 3Bi 2I 9

Abstract

We have carried out investigations of the exciton reflection spectra of Cs 3Bi 2I 9 layered crystals as a function of temperature. For the first time for the layered substances we have found nontraditional temperature shift of the energy gap E g( T) described by the Varshni formula. We have registered a transition region in the temperature broadening of the half-width, H( T), of the exciton band with increasing of temperature in the interval between 150 and 220 K. It is shown that this region may be identified as the heterophase structure region where ferroelastic and paraelastic phases coexist. We have also found a surge in H( T) at the ferroelastic phase transition point (T c =220 K).