The optimization of thermoelectric parameters when introducing impurities with variable valence

E.I. Rogacheva1, O.N. Nashchekina1, M.S. Dresselhaus2

1National Technical University "Kharkov Polytechnic Institute",21 Frunze St., Kharkov 61002, Ukraine

2Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, USA

Despite the intensive study of new promising thermoelectric materials, there remains sustained interest in searching for principally new methods of controlling the thermoelectric properties of traditional materials. The objects of the present study are SnTe-based solid solutions in the Sn-In-Te system. SnTe is characterized by a high degree of deviation from stoichiometry, a high hole concentration and a low value of ZT. Indium is an element with variable valence. The ratio between In atoms in different charge states (In3+ and In1+) was controlled by changing the concentration of native defects (Sn vacancies). The temperature (300-800) К and concentration (1-5 at.% In) dependences of the electrical and thermal conductivity, Hall coefficient, and Seebeck coefficient were obtained. It was established that the maximum value of ZT (~1 at 800 K) can be reached when introducing In2Te3 into SnTe, which corresponds to the simultaneous introduction of indium in the In3+ charge state and cation vacancies. The experimental results were interpreted on the basis of a model for the energy spectrum of SnTe doped with In, taking into account deviation from stoichiometry in SnTe and a variable valence of indium. Thus, the introduction of impurities with variable valence creates new opportunities for controlling the thermoelectric properties.