Analysis of Ce- and Yb-Doped TAGS-85 Materials with Enhanced Thermoelectric Figure of Merit

Doping of TAGS-85 with 1 at% Ce or Yb forms a dilute magnetic semiconductor system with non-interacting localized magnetic moments that obey the Curie law. X-ray diffraction patterns and slight broadening in Te-125 NMR, attributed to paramagnetic effects, suggest that Ce and Yb atoms are incorporated into the lattice. Te-125 NMR spin-lattice relaxation and Hall effect show similar hole concentrations of approximate to 10(21) cm(-3). At 700 K, the electric conductivity of the Ce- and Yb-doped samples is similar to that of neat TAGS-85, while the thermal conductivity and the Seebeck coefficient are larger by 6% and 16%, respectively. Possible mechanisms responsible for the observed increase in thermopower may include i) formation of resonance states near the Fermi level and ii) carrier scattering by lattice distortions and/or by paramagnetic ions. Due to the increase in the Seebeck coefficient up to 205 mu V K-1, the thermoelectric power factor of Ce- and Yb-doped samples reaches 36 mu W cm(-1) K-2, which is larger than that measured for neat TAGS-85, 27 mu W cm(-1) K-2. The increase in the Seebeck coefficient overcomes the increase in the thermal conductivity, resulting in a total increase of the figure of merit by approximate to 25% at 700 K compared to that observed for neat TAGS-85.