Kinetics of solid-state reactions between iodides of silver, potassium and rubidium

Abstract

The kinetics of the formation of (K,Rb)Ag₄I₅ in solid state reactions between AgI and (K,Rb)I have been investigated in the range 110–210°C. The thickness of the product layer between the reacting polycrystalline solids was measured by direct visual observation and by electrical conductivity determination. In an initial period the product growth rate is constant. The apparent activation energy of this part of the reaction is 1.4 eV for KI+γ-AgI, 0.9₅eV for KI+α-AgI and 2.0 eV for RbI+γ-AgI. The rate controlling step is the diffusion of K⁺ or Rb⁺ ions through a region between the unreacted AgI and the (K,Rb)Ag₄I₅ product, where the structure is that of a solid solution of (K,Rb)I in AgI. The rates of reaction of mixed crystals indicate that a partial “chromatographic” separation of the K⁺ and Rb⁺ ions occurs. In later stages of reaction, diffusion across the product layer becomes rate determining.