Ionic conductivity and solid phase transitions in Na2SO4–Cs2SO4 system

The ac conductivity and DSC calorimetry data reveal interesting solid state interactions and solid phase transition behavior in the Na2SO4–Cs2SO4 binary system. The β → α solid transition of NaCsSO4 is reported for the first time. These data indicate preferential reaction between Cs2SO4 and the high temperature phase I of Na2SO4, exemplifying the Hedvall effect, to form the compound NaCsSO4 which further interacts with excess Na2SO4 or excess Cs2SO4 to form 1:1 association complexes, viz. NaCsSO4:Na2SO4 and NaCsSO4:Cs2SO4. These complexes appear to dissociate at low temperatures reminiscent of spinodal-type decomposition behavior. The reaction enthalpies of NaCsSO4, β → α transition of NaCsSO4 and complex associations are reported.The report contains two unexpected results: (i) a subdued higher ionic conductivity relative to its Rb2SO4 analogue and (ii) a conductivity drop accompanying the high temperature phase transition which is in strong contrast to all the other Na2SO4 – mixed alkali compositions.