Excitonic absorption spectra and phase transitions in thin films of the ferroelastics Cs2CdI4 and Rb2CdI4

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The absorption spectrum of thin films of the ferroelastics M2CdI4 (M=Cs, Rb) with a structure of the β-K2SO4 type is investigated in the energy range 3–6 eV and temperature range 90–420 K. It is found that both compounds are direct-gap insulators, and the low-frequency electronic and excitonic excitations are localized in the CdI42− structural elements of their crystal lattice. From the temperature dependence of the spectral absorption and the half-widths of the low-frequency excitonic bands in Rb2CdI4 it is found that there is a phase transition at 380 K (paraphase→incommensurate), a first-order phase transition at 320 K (incommensurate→first ferroelastic phase), and a second-order phase transition at 210 K (first→second ferroelastic phase). Similar, but less pronounced phase transitions are found in Cs2CdI4 at lower temperatures. In the ferroelastic phases of these compounds an additional broadening of the bands appears, apparently due to the scattering of excitons on strain fluctuations in the domain-wall regions.