ELECTRONIC AND THERMAL BEHAVIOR OF GERMANIUM TELLURIDE-TIN TELLURIDE SOLID SOLUTIONS NEAR THE STRUCTURAL PHASE TRANSITION

Humanity finds itself in an energy crisis, where our energy demands now and in the future are far out of balance with our ability to produce that energy safely and thoughtfully. Many energy systems today utilize the chemical energy stored in hydrocarbons in processes that create, in addition to ubiquitous carbon dioxide, extra energy in the form of heat that is usually wasted. This extra energy in the form of heat, in combustion engines and in other applications, instead of being wasted could be converted into usable electrical energy with the help of a class of materials called thermoelectrics.Thermoelectric devices are a class of materials, usually semiconductors, that convert temperature gradients into usable electrical energy. One critical drawback of thermoelectric technology today is the relatively low efficiency of thermoelectric devices. In order to evaluate a material’s efficiency researchers use the dimensionless figure-of-merit, ZT, which is a product of a number of electronic and thermal material properties. By examining the material properties which influence ZT values we can systematically develop higher efficiency thermoelectric devices. Here we present a study of the structural and transport properties of GeTe-SnTe solid solutions near the temperatures of their structural phase transition. As two well-known class IV-VI semiconducting materials GeTe and SnTe have been well studied and developed for thermoelectric applications, but there exists a relative dearth of research on their solid solutions. As a complete solid solution, Sn can replace Ge at any concentration without changing the crystal system. One aspect about GeTe that makes it interesting for thermoelectrics is that its crystal structure transforms from a low-temperature rhombohedral phase to a high-temperature cubic rocksalt phase about 670K. By contrast, SnTe undergoes a similar transformation at about 100K. Previous studies have shown that in large crystals of GeTe replacing Sn for Ge, Ge(1-x)Sn(x)Te, lowers the transition temperature as a function of Sn content. By studying the solid solution across all values of x, 0 ≤ x ≤ 1, one has available a unique crystal system with a structural phase transition spanning well above to well below room temperature. For this study, polycrystalline samples were synthesized from ingots using power metallurgy techniques and their thermoelectric properties were measured from 300-770K.We show x-ray diffraction data to show the phase purity and lattice constant of these solid solutions, as well as observe the elastic constants at room temperature. We report phase transition temperatures from observations of changes in crystal structure at elevated and room temperatures. We then show the electrical conductivity, Seebeck coefficient, and thermal conductivity as a function of temperature and Sn content. Our results show that at low Sn concentrations, Sn atoms fill Ge vacancies that cause a decrease in the electrical conductivity from a reduction of the carrier concentration. At higher Sn concentrations these filled vacancies contribute to an increase in the carrier mobility which offsets the decrease in carrier concentration while also increasing the Seebeck coefficient. The thermal behavior of the system shows strong evidence of alloy scattering with a minimum near concentrations with similar amounts of Ge and Sn. Distinct discontinuities in the total thermal conductivity also provide evidence for the determination of structural transition temperatures. Taken together, these studies enable a complete characterization of the ZT for these materials above room temperature as well as a contribution to the structural phase diagram. The highest ZT values obtained at 400 °C in Ge(1-x)Sn(x)Te were for x-values x=0.05 and x=0.60 with values of 0.36 and 0.31 respectively, which is impressive for unoptimized materials.