Electro-deposited copper in bismuth telluride

Observations have been made of the electrical conductivity as a function of time in single-crystal Bi 2 Te 3 during copper plating. The diffusion coefficient along the basal planes is so high that the copper is deposited internally. It has been found that there is a saturation effect which depends on the plating current density, as well as a change of conductivity when the plating current is reduced to zero. These effects can be explained in terms of an outward migration of copper into the electrolyte that is proportional to the copper concentration in the Bi 2 Te 3 . A moderately good fit to the conductivity-time curves can be obtained assuming that each copper atom in Bi 2 Te 3 donates 0·65 electrons. A rather better fit might be obtained if it were supposed that each copper atom donated one electron, but it would also be necessary to assume that the diffusion coefficient has only half the accepted value.