Pressure effect on ionic conductivity in NaCl

Ionic conductivity in polycrystalline NaCl was measured using a dc technique to 35 kbar in a modified Bridgeman anvil device at ambient temperatures. An analysis was made of the pressure effect on the vacancy-impurity complexes contribution to ionic conductivity using the Pierce-Lazarus conductivity expression with an extension of the Lidiard theory of vacancy-impurity association to include pressure effects. Using a value of 5 cm³/mole for the activation volume of motion of free Na vacahcies inferred from this measurement and the strain energy model of Keyes, this analysis obtained a good fit to the existing data in the "extrinsic" region obtaining values of 0.43 eV/atom for the free energy of association of complexes and 14 cm³/mole for the activation volume of reaction. Thermal Schottky disorder was found to be negligible below 600^∘{}K, whereas "extrinsic" vacancy effects are not negligible even at the melting point according to this model. Although this model suggests a large increase in apparent activation volume near the melting point using Etzel and Maurer's early results for the concentration and an activation volume of 28 cm³/mole of Schottky defects, the agreement of this calculation with previously measured results is poor. This suggests that additional conduction mechanisms may be operable above 900^∘{}K in agreement with the conclusions of Martin et al.