Influence of cation substitution on optical constants of (Cu1-xAgx)7SiS5I mixed crystals - I. P. Studenyak; S. M. Bereznyuk; M. M. Pop; V. I. Studenyak; A. I. Pogodin; O. P. Kokhan; B. Grancic; P. Kúš (Semiconductor Physics, Quantum Electronics and Optoelectronics, 2020)