Thermoelectric behaviour of molten Tl[sbnd]Te alloys at compositions approaching pure thallium

Abstract

In the composition range 70 to 100 atomic % thallium, the phase diagram of Tl[sbnd]Te has a region with two immiscible phases below 600°c, corresponding to the separation of a metallic and a liquid semiconductor phase. We have made measurements of the electrical resistivity p ancl the Seebeck coefficient S at temperatures up to 850°c in the range in which the two liquid phases are mutually soluble in orcler to study the transition between metallic and semiconductor behaviour. As the concentration of tellurium is increased, p and the magnitude of S increase, and their temperature coefficients change from weakly positive values typical of metals to weakly negative values typical of liquid semiconductors with low resistivity. We find that the transport behaviour can be accounted for very well by a model which treats the alloy as a solution containing Tl₂Te molecules plus metallic TI atoms. As Te is added to Tl₂ electrons are removed from the conduction band, and scattering centres due to Tl₂Te are added. Analysis of the behaviour of S and p in terms of equations which allow for gross deviations from nearly-free-electron behaviour has led to quantitative information about the behaviour of the scattering distance λ_z, the scattering cross section of Tl₂Te, and the density of states at the Fermi energy. At compositions approaching Tl₂Te, the ‘impurity’ contribution to the scattering is consistent with the nature of scattering derived in previous work. It is found that the product of λ; and the Fermi wave vector remains above unity, but it approaches unity near the composition Tl₂Te.