Complex role for thallium in PbTe:Tl from local probe studies

When PbTe, a good thermoelectric material, is doped with a few percent Tl, the figure of merit ZT=TS2/(ρκ) [S is the Seebeck coefficient, ρ the electrical resistivity, and κ the thermal conductivity] is dramatically improved. The maximum value of ZT occurs for approximately 2% Tl, but the factors limiting ZT are as yet poorly understood. From a detailed local structure study of PbTe:Tl using the extended x-ray absorption fine structure (EXAFS) technique, we find that Tl substitutes primarily as Tl(+1) on the Pb site, with no evidence for any solubility issue, any significant fraction of Tl(+3), or any Tl interstitials. However it is not a simple substitution as there is evidence for increasing Te vacancies on neighboring sites with increasing Tl concentration, x. In addition there is also increasing disorder with x - Tl-Te bond length disorder as well as the vacancy defects - that will scatter the hole carriers and begin to increase the electrical resistivity in spite of an increase in hole concentration. This increased disorder is likely an important factor limiting ZT.