Phase Transition and Temperature Dependence of the A₁ Low-Frequency Exciton Band Parameters in Quaternary Compound Rb_0.5Cs_0.5Ag₄I₅ Thin Films

The absorption spectra of the A₁ low-frequency exciton band in quaternary compound Rb_0.5Cs_0.5Ag₄I₅ thin films have been measured by an UV-2100 spectrophotometer at the temperature (T) range from 78 to 289 K. The measurements at intermediate temperatures are performed within an interval of photon energies from 3.44 to 3.70 eV. The spectra have been computer-processed to separate the A₁ band from the interband absorption edges, and to determine their parameters: the spectral peak position (E_m), the half-width (G) of the exciton band, and the Gaussian shape ratio in the spectrum. The results show that at T < 107 K, dE_m/dT = -6.34 x 10^-4 eV.K^-1, dG/dT = 3.04 x 10^-4 eV.K^-1; at T > 107 K, dE_m/dT = -3.01 x 10^-4 eV.K^-1, dG/dT = 5.47 x 10^-4 eV.K^-1. It is emphasized that the phase transition γ → β in this material is carried out at temperature 107 K.