Measurement of ionic thermocurrents in sodium iodide

Ionic thermocurrents (ITC) are measured in the 'pure' Harshaw NaI crystal. Although the atomic absorption technique shows that the concentration of the divalent cation inclusions is lower than 2 ppm, the ITC bands are intense. A comparison of the DC conductivity between the samples used and those doped with Ca 2+ indicates that the 'pure' NaI contains small amounts of divalent cations; the ionic thermocurrents observed must therefore be attributed to the reorientation process of the dipoles of the form: 'divalent cation-cation bound vacancy'. By analysing the more intense ITC band the migration enthalpy and entropy for the bound-cation vacancy are found to be 0.59 eV and 1.7 k respectively. The enthalpy value of 0.59 eV is comparable to the values 0.58-0.60 eV obtained by the DC conductivity technique for the free-cation vacancy motion. On the other hand the entropy value of 1.7 k is appreciably smaller than the values 3.02-3.23 k which correspond to free-cation motion.