Thermoelectric power and heats of transport of the alkali-halides

• Main Author: Tan, Poh Lin
• Published: Imperial College London 1968
• Subjects: 669
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.623143

In principle the most direct way of obtaining the experimental heats of transport is by measuring the crystal thermopower. This is done for pure NaC1 single crystals using platinum electrodes in the temperature range of 870°K to 1060°K. However, the results of the alkali-halides measurements have not been successfully analysed so far. This is because in the alkali-halides system irreversible electrodes (no common ion between electrodes and crystal) have to be used, and the existing theories for the irreversible electrode-crystal thermopower are unsatisfactory. The possibility that the heterogeneous thermopower might be caused by the electrons, which are transferred from the electrodes to the crystal, and which do not fall into any traps, is investigated. This model is also found to be unsatisfactory. The dynamics of the diffusion of a vacancy in a linear chain is studied. A simple relationship between the heat of transport,Q*,and the activation energy for thermal diffusion, E, is found, namely, Q* = (2+q/T)E, where q is related to the dynamics of the system. The subsequent calculations show that q is negative in agreement with the experimental data for AgBr and AgC1. In the course of calculating q to attempt to give theoretical heats of transport for the alkali-halides, various force constants operating between the ions have to be evaluated. This is first done by evaluating the Schottky -pair formation energies by lattice calculations.This calculations, which prove. very successful,show that the basic Born-Mayer form of potential chosen here is realistic,