Elektrische Eigenschaften von festen Stoffen mit gemischter Elektronen- und Ionenleitung z.B. Ag₂S

Abstract

The electrical properties e.g. the partial electronic or ionic conductivity of binary solid compounds, e.g. Ag₂S or NiS have well defined values only when, for a given pressure or given temperature, a further degree of freedom, e.g. the thermodynamic activity or the chemical potential of one component is fixed. Instead of the thermodynamic activity one can fix the exact deviation from the ideal stoichiometric constitution. The chemical potential or the thermodynamic activity of one component may be evaluated or fixed by the constitution of the surrounding phase, yet in many cases it is more advantageous to use a solid-state galvanic cell. In case of silver sulfide the solid-state galvanic cell Ag/AgJ/Ag₂S may be used, the EMF of the cell being related to the chemical potential of the silver in silver-sulfide. In this cell AgJ is a practically pure ionic conductor for silver ions. A flow of current through the cell is an equivalent for the mass of silver which is brought or withdrawn from the Ag₂S. With this type of cell C. Wagner by an “Electrochemical Titration” has determinded the relation between the chemical potential of the silver and the deviation from ideal stoichiometry. Between 200 and 400°C the stoichiometric point of Ag₂S is near the equilibrium with liquid sulphur. Because of the high disorder of the silver ions a change of the chemical potential of silver by a change of the stoichiometric constitution is only given by a change of concentration of the quasi-free electrons, respectively defect electrons. The thermodynamic behaviour of the electrons shows the transition from ideal gaseous behaviour up to beginning of degeneracy. From the temperature dependance of the EMF of the cell the partial molar enthalpies and entropies for the transition from silver to silver sulfide may be calculated. The behaviour of these values can also be explained by the thermodynamic behaviour of the electrons in Ag₂S.

The partial ionic- and electronic conductivities can be measured independently in siutable galvanic cells. As expected from the type of disorder in Ag₂S the partial conductivity of silver ions is independent of the chemical potential of the silver. A comparision between the partial conductivity in Ag₂S and the self-diffusion-coefficient shows that the Nernst-Einstein-Relation is satisfied within the order of magnitude only. The deviation may be interpreted by correlation effects associated with selfdiffusion. The partial electronic conductivity varies according to the concentration of the electrons.

Quite another behaviour is shown by NiS at 400°C. While the partial conductivity of the electrons is independent of deviations from the stoichiometric constitution, that of the ions will change in proportion to the activity of the sulphur. It can be assumed that at 400°C a high intrinsic disorder of electrons exists in NiS independent of the stoichiometric constitution. The partial ionic conductivity of the nickel ions is caused by nickel ion vacancies by the simultaneous deficiency of metal. The thermodynamic behaviour of NiS is determind by the changes of concentration of the nickel ion vacancies. These conclusions can be drawn from kinetic measurements on the formation of NiS, which can also be carried out with solid state galvanic cells.

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