II-IV-V2 and I-III-VI2 nonlinear optical crystals for mid-IR range: Schottky defects concentration

The work deals with ternary nonlinear optical crystals of I- III-VI2, and II-IV-V2-type. Theoretical thermodynamic method of quasi-chemical reactions was used. Entropy of vacancy was calculated as sum of configuration and vibration components. For the first one uncertainty in positions of new bonds formed by unpaired electrons arisen when one of consistent atoms is deleted and uncertainty in new positions of relaxing atoms were considered. Standard Boltzmann expression for configuration entropy (Delta) Sconf equals (Sigma) klnw and geometry considerations were used. It was assumed that the vibration component is caused by change in vibration frequency of atoms surrounding formed vacancy. Change in length of bonds and in bonding force took into account. Approximate expression for vibration entropy change (Delta) Svibr equals 3Nkln, geometry considerations and results of bonds lengths calculations were used.