SOLUBILITY OF SUBSTITUTIONAL DOPING ELEMENTS IN THERMOELECTRIC COMPOUNDS

Thermoelectric energy based on a direct conversion of heat (or thermal gradient) into electricity is one of possible solutions for the problem of affordable, environmentally clean energy. PbTe or TiNiSn half-Heusler alloys are promising materials for application in thermoelectric devices. Improving their figure of merit is usually achieved by increasing the Seebeck coefficient and/or by reduction of thermal conductivity. Due to relatively easy techniques of alloying of these materials with different elements substituting for its constituents, new ways are considered to manage their properties. Doping of these materials may change the type of conductivity and/or may lead to decomposition of alloys. Morphology of material influences the thermal conductivity presenting the additional way to manage the thermoelectric efficiency.

The aim of this presentation is to examine the influence of alloying of PbTe with Na, Ti, and Cl and TiNiSn with Cu, Fe or Sc on the type of conductivity. Also the solubility of dopants by combining Density Functional theory calculations and statistical thermodynamics is discussed.

Decomposition of TiNiSn with growing Ni contents or upon alloying with Sc as well as of PbTe alloyed by Ti is studied for T≠0. The approach bridges the gap between the quantum mechanical calculations of the phase stability in the ground state and the behavior of the alloys at elevated temperatures. It is demonstrated that existence of the miscibility gap in alloyed compounds leads to phase separation in the nano-scale and to reduction of thermal conductivity recently found in experiments.