Effect of doping Ag$_{y}$Sb$_{y}$Ge$_{50-2y}$Te$_{50}$ thermoelectric materials with rare earths

The Ag$_{y}$Sb$_{y}$Ge$_{50-2y}$Te$_{50}$ system represents some of the most efficient thermoelectrics, the so-called TAGS materials. In order to understand the effect of doping of Ag$_{6.52}$Sb$_{6.52}$Ge$_{36.96}$Te$_{50}$ (``TAGS-85'') with rare earth atoms on the Ge and Te sites, Ag$_{6.52}$Sb$_{6.52}$Ge$_{36.96-x}$R$_{x}$Te$_{50}$ and Ag$_{6.52}$Sb$_{6.52}$Ge$_{36.96}$R$_{x}$Te$_{50-x}$ materials with R = Gd and Dy (rare earth atoms with large magnetic moments) have been studied by measuring X-ray diffraction (XRD) and $^{125}$Te nuclear magnetic resonance (NMR) at 300 K, thermopower and resistivity at 300-760 K, and the magnetization at 1.8-350 K and in magnetic field 0-55 kOe. XRD and $^{125}$Te NMR show that some rare earth atoms are incorporated into the lattice and enhance the thermopower by $\sim $10{\%}. At 700 K, this yields a power factor of up to 36 $\mu $W$\cdot $cm$^{-1}\cdot $K$^{-2}$, which is $\sim $20{\%} higher than in TAGS-85. All materials studied can be considered as degenerate magnetic semiconductors with non-interacting localized magnetic moments formed by rare earth atoms, with a different effect of rare earths on the Ge and Te sites. Reasons for the thermopower enhancement due to doping with rare earths including magnetic and non-magnetic phenomena are discussed.