ELECTROCHEMICAL-CELL FOR COMPOSITION DEPENDENT ELECTRONIC CONDUCTIVITY MEASUREMENTS ON MIXED CONDUCTORS AND APPLICATION TO SILVER TELLURIDES

An electrochemical cell of simple design is described which permits measurement of the electronic conductivity of mixed ionic-electronic conductors by means of the van der Pauw method, even if their ionic conductivity is of the same order as the electronic one. The cell represents a solid-state galvanic cell into which the sample is incorporated as an electrode. The composition of the latter can thus be varied by coulometric titration. On one face of the disc-shaped sample four peripheral, roughly equidistant electronic contacts are mounted, two of which are switched as current carrying contacts, the other two as potential probes. The electronic conductivity results from two steadystate measurements with an intermediate interchange of the connections. During each measurement the composition of the sample is inhomogeneous since the current is fed to it through ionically blocking contacts, thus leading to a transient ion transference at the beginning of the measurement. Apart from an error of 5 per cent at most, the measurement yields the average electronic conductivity of the sample outside certain regions around the current carrying contacts. In order to keep the inhomogeneity in this relevant part of the sample below an emf interval of DELTA-E the voltage between the potential probes has to remain smaller than DELTA-E/6. In conclusion, some experimental results for low-temperature Ag1.92Te as well as for alpha-, alpha'- and beta-Ag2Te are given.