Smolanka O. Excitons and phase transitions in Cs3Bi2I9 layered ferroelastic

Results of complex experimental studies of Cs3Bi2I9 layered ferroelastic crystals by means of low-temperature reflection and Raman scattering spectroscopy methods have been presented. The fundamental optical characteristics were found and Argand's diagrams є2 = f (є1) were determined in monoclinic and hexagonal phases. Self-consistent method was developed for unambiguous dividing of complicated spectral line-shapes into individual components using computer modeling of real є1(E) and imaginary є2(E) parts of the complex dielectric function є(E) taking into account Van Hove singularities. For the first time nontraditional temperature shift of Eg(T) for layered substances was found. It was detected that this shift is described by the Varshni formula very well and explained as follows: Cs3Bi2I9 crystals are usual semiconductors in the monoclinic phase and layered ones in the hexagonal phase. The transition region in the temperature interval 150-220 K was registered. It consists of heterophase structure region (183-220 K) and the region of residual internal strain in the monoclinic phase (150-183 K). The change of exciton-phonon interaction from weak to strong with increasing of temperature was found near 150 K. The nature of Raman lines was determined and Davydov doublets were found. It is established that the structure phase transition at Tс = 220 K belongs to that of the first order close to the second one. For the first time a new giant thermodynamical optical effect near the phase transition point in Cs3Bi2I9 layered crystal was found. The effect is appeared as periodical oscillations in time of the reflection coefficient. This phenomenon is caused by the small temperature deviations in thermodynamical system the appearance of which in the reflection spectra is strongly amplified in the ferroelastic phase transition point. The optical oscillations are explained on the base of a model that takes into account the temperature dependence of the refractive index n(T) through the order parameter (spontaneous strain Dsp(T)). The optical analogue of the transistor was suggested.