Thermoelectric power of ionic crystals—III Thermoelectric power and ionic conductivity of potassium bromide containing barium bromide

Abstract

The thermoelectric powers of single crystals of pure potassium bromide and potassium bromide containing 1.95 × 10 −2 mole per cent barium bromide were measured as a function of temperature between 520 and 700°, using platinum metal- and graphite-electrodes. The data were analysed on the basis of the theory in the first paper of this series. The heat of transport of a cation vacancy was −1.0 eV. The conductivities of single crystals of pure potassium bromide and potassium bromide doped with different concentrations of Ba 2+ as well as of Zn 2+ and Pb 2+ were also measured as a function of temperature using platinum- and graphite-electrodes. The ionic conductivity parameters of potassium bromide were calculated from the theory describing a simple association between a divalent cation impurity and a cation vacancy. The following values were found: heat of formation of a Schottky defect, 2.40 eV; heat of motion of a cation vacancy, 0.83 eV; free energy of association of a barium ion and a cation vacancy, −0.27 ±0.1 eV; entropy of activation for migration of a cation vacancy, 4.0 k; entropy of formation of a Schottky defect, 8.7 k, where k is the Boltzmann constant. It was found that the values of conductivity of potassium bromide crystals containing lead ions were small despite of their large lead contents. Some comments and corrections are made concerning the condition of local thermal equilibrium in the equations in the Paper I, though the final conclusion related to heats of transport is unchanged.