Phase equilibria in the system ZnCl₂ + KI

Abstract

Phase equilibria in the ZnCl₂+KI system have been investigated by differential thermal analysis (DTA), and by examination by X-ray powder diffraction of solidified melts. The system evidently behaves as a simple binary, and three congruently melting compounds are formed, viz. 2KI.ZnCl₂, 2KI.3ZnCl₂ and KI.4ZnC1₂. Supercooled mixtures from c. 60-90 mole % ZnCl₂ visibly phase separate at temperatures for the most part about 30K below the liquidus temperatures, and the phase separation temperatures have been determined by DTA. The composition limits of the metastable immiscibility dome have not been established, but the region of phase separation extends at least to mixtures containing as little as 53 mole % ZnCl₂. The critical composition is approximately 80 mole % ZnCl₂, and the critical temperature 470 K. Quenching of melts containing 34-100 mole % ZnCl₂, in liquid nitrogen, yields glasses. In the region 34-43 mole % ZnCl₂ the glasses are partly crystallized. Glass transition temperatures for the wholly glassy mixtures have been evaluated from DTA traces, with heating rates in the range 0.11-0.25 K s^-1. Two glass transitions have been observed for mixtures of composition 53-75 mole % ZnCl₂.