The influence of phase transitions in the Rb2CdI4 ferroelastic on the exciton absorption spectrum

The parameters of the long-wavelength exciton band for Rb2CdI4 films are investigated in the temperature range 90-410 K. It is found that the Rb2CdI4 films undergo a sequence of phase transitions at temperatures T-c1 = 380 K (paraphase --> incommensurate phase), T-c2 = 290 K (incommensurate phase --> ferroelastic phase I), and T-c3 = 210 K (ferroelastic phase I --> ferroelastic phase II). The parameters of the exciton band (such as the spectral position and the half-width) measured during heating and cooling of the Rb2CdI4 film differ significantly. This is especially true for the incommensurate phase. Upon heating of the incommensurate phase, the domain boundaries become frozen, whereas the cooling of this phase is accompanied by the generation of solitons and their pinning, which, in turn, results in a first-order phase transition at the temperature T-c2. It is revealed that the oscillator strength of the exciton band anomalously increases in the range of existence of commensurate phase I (T-c3 less than or equal to T less than or equal to T-c2) due to ordering of the Rb2CdI4 crystal lattice. (C) 2004 MAIK "Nauka/Interperiodica".