Absorption spectrum and excitons in thin films of the solid electrolyte RbCu4Cl3I2

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We have investigated the absorption spectrum of thin films of the superionic conductor RbCu4Cl3I2 synthesized on NaCl crystalline substrates. It is shown that the electron and exciton excitations in the energy interval 3–6 eV are associated with optical transitions in the CuHal sublattice, and the edge of the fundamental band is controlled by optical transitions in the Cu(II)Hal sublattice. It is found that the large band gap of this compound (Eg=3.86 eV) in comparison with those of CuCl and CuI is a result of the small number of Cu ions in the second coordination sphere. The temperature dependence of the spectral position and half-width of the low-temperature exciton band reveals features associated with the phase transitions γ→β (Tc1=170 K) and β→α (Tc2=220 K) and with disordering of the cation sublattice attendant to the transition to the superionic state.