The Electrical Conductivity of Solid and Molten Cesium Hydroxide – A Contribution to Solid Proton Conductors

The electrical conductivity of solid cesium hydroxide is by several powers of ten larger, its activation energy by a factor of two to three lower than those of normal solid salts. A steep tenfold increase of the conductivity is observed at the α-β-phase transition temperature of solid cesium hydroxide (220°C) above which the OH −-ions can rotate freely (rotator phase). A further increase by three powers of ten occurs between 280°C and the melting point (335°C), above which the conductivity is that of a normal molten salt. The conductivity behaviour thus resembles that of potassium hydroxide (Stephen and Howe [10]), which also exhibits the pronounced breaks at the α-β-transition temperature (248°C) and at around 366°C. Since the second break in solid cesium hydroxide and potassium hydroxide coincides with the eutectic temperatures of the respective hydroxide-carbonate systems (alkali hydroxides hardly can be obtained free of carbonate) it has to be assumed that it is caused by the increase of ionic charge carriers due to partial melting of the eutectic within the solid framework of the hydroxide. The relatively high conductivity of cesium hydroxide and potassium hydroxide below the eutectic temperatures is due to a “Grotthus”-type proton conductivity mechanism (aided by CO- and H2O impurity defects) with OH−-ion rotation.