Absorption Spectra and Ionic Conductivity of Rb_xCs_1-xAg₄I₅ Superionic Conductors Thin Films

A series of Rb_xCs_1-xAg₄I₅ (x=0--1) thin films were grown by vacuum evaporation on NaCl crystal substrates at 350K. The absorption spectra of these films were measured at 80K in the wavelength range from 240nm to 400nm. It is shown that superionic conductor thin films of quaternary compound Rb_0.5Cs_0.5Ag₄I₅ and ternary compound RbAg₄I₅ can be obtained at x=0.5--0.6 and x=0.7--1, respectively. At x=0.65, the combined compound film of the mixture of 30mol\% RbAg₄I₅ and 70mol% Rb_0.5Cs_0.5Ag₄I₅ is presented. Then, based on the spectral positions of the A₁ and A₂ peaks, we determined that the Rb_0.5Cs_0.5Ag₄I₅ exciton coupling energy R_ex is 0.21eV, the forbidden zone width E_g is 3.82eV and the exciton radius a_ex is 0.70nm. Furthermore, the ionic conductivities of superionic conductor thin films of RbAg₄I₅ and Rb_0.5Cs_0.5Ag₄I₅ and their mixture film are investigated, respectively, in the temperature range 303K--393K.