The lattice parameters of members of the solid solution series KxRb1-xFeS2 (0 < x < 1) were found to vary linearly with composition in accordance with Vegard's law. When KFeS2 was heated with RbI in a 1:3 molar ratio, ion exchange reactions were observed to proceed at temperatures well below the melting point of the mixture, measured at 611 degrees C. Ion exchange reactions during a 2 h heating cycle at four different reaction temperatures resulted in the products K0.63Rb0.39FeS2 at 350 degrees C, K0.44Rb0.56FeS2 at 400 degrees C, K0.33Rb0.67FeS2 at 450 degrees C, and K0.27Rb0.73FeS2 at 520 degrees C. The product obtained at 520 degrees C is nearly identical to the ideal product K0.25Rb0.75FeS2, and reactions at lower temperatures resulted in incomplete ion exchange. The lattice constants of the products were consistent with their formulas with the exception of the product obtained at 350 degrees C, which displayed an anomalously low b lattice parameter. (c) 2008 Elsevier B.V. All rights reserved.