The effect of rare earth element doping on thermoelectric properties of GeTe

GeTe has attracted extensive attention due to high thermoelectric performance. Herein, we comprehensively investigate the effect of rare earth element (Eu, Gd, Er and Tm) substitutions at Ge site of GeTe on its thermoelectric performance. It is found that the substitution limit of Eu, Gd, Er and Tm at Ge site of GeTe increases from 0.8 to 1.8 mol.% with increasing atomic number. The local magnetic moment arising from the substituted rare earth element enhances the Seebeck coefficient and in turn results in enhanced power factor at the entire measurement temperature range (from 303 to 723 K). Additionally, the increasing substitution limit with increasing atomic number of rare earth element strengthens the mass and strain-field fluctuations, which contributes to strengthened phonon scattering, and leads to reduced lattice thermal conductivity. The simultaneously enhanced power factor and reduced lattice thermal conductivity enhances the dimensionless figure of merit at the entire measurement temperature range, and correspondingly enhances the average dimensionless figure of merit from 0.38 of GeTe to 0.63 of Ge_0.98Er_0.02Te. This work indicates rare earth element can be used an effective dopant in GeTe to improve the thermoelectric properties.