Potassium iodide + silver iodide phase diagram. High ionic conductivity of KAg4I5

Abstract

The phase diagram of the KI+AgI system has been redetermined. Two intermediate compounds have been identified. K₂AgI₃ is stable up to 130°C. At higher temperatures it undergoes a solid state disproportionation reaction to KAg₄I₅ and KI. KAg₄I₅ is stable between its incongruent melting point (253°C) and 38°C. At lower temperatures it disproportionates to β-AgI and K₂AgI₃. A eutectic between KAg₄I₅ and KI occurs at a nominal composition of 29.5 mole % KI and 238°C. The compound KAg₄I₅ has an exceptionally high ionic conductivity for a solid, reaching 0.31 ohm^–1 cm^–1 at the incongruent melting point. The current is carried by the Ag⁺ ions. Thermodynamic data derived from e.m.f. measurements indicate that the formation of KAg₄I₅ from K₂AgI₃ and AgI is accompanied by an entropy gain, implying an unusually high degree of disorder in KAg₄I₅.