Pressure Dependence of Ionic Conductivity in KCl, NaCl, KBr, and NaBr

Abstract

The pressure dependence of the ionic conductivity of KCl, NaCl, KBr, and NaBr has been measured to 6 kbar in the intrinsic and extrinsic temperature regions. In the intrinsic region the conductivity deviates markedly from a simple exponential decrease with pressure. A detailed analysis, including contributions from anions and extrinsic cation vacancies, shows that pressure can suppress the intrinsic vacancies sufficiently to cause such a deviation. The analysis also shows that the uncertainties in the estimated activation volumes are greater than those determined in previous work. For KCl, NaCl, KBr, and NaBr crystals, respectively, the estimated activation volumes for motion of cation vacancies are 8 ± 1, 7 ± 1, 11 ± 1, and 8 ± 1 ${\mathrm{cm}}^3$/mole, and the activation volumes for formation of Schottky defects are 61 ± 9, 55 ± 9, 54 ± 9, and 44 ± 9 ${\mathrm{cm}}^3$/mole. The values for activation volumes of motion agree with predictions of the strain-energy model of Keyes. Schottky-defect-formation volumes are substantially larger than the molar volumes in KCl and NaCl, but in KBr and NaBr they are approximately equal.