The roles of Yb-substitution on thermoelectric properties of In4−xYbxSe3

Abstract

Lead-free In4Se3 is a newly established promising middle temperature n-type thermoelectric (TE) material candidate. Series of Yb-substituted In4−xYbxSe3 (x=0.01–0.09) samples are prepared herein and their TE properties and the structure–property relationship are investigated. Magnetic molar susceptibility measurements indicate that Yb is nonmagnetic. However, the Yb doped samples behave abnormally in transport properties. The unit cell volume variations and the carrier concentration (ne) changes reveal that Yb substitutes the In4+ site when x<0.05 in the unitcell of In4Se3 and behaves in transport property as an +1 ion leading to a slow ne decrease; differently, Yb substitutes the In35+ sites in the unitcell of In4Se3 when x>0.05, and behaves as an acceptor leading to a sharp ne decline. These are considered to be attributed to the Fermi level pinning of the f and d orbitals of Yb. Such Yb substitution preference is also supported by our primary formation energy calculation. Besides, the carriers that on the pinning level are highly localized, thus contribute very little to the electrical conductivity, but contribute to some degree to the thermal conductivity as shown by property-temperature plots. In addition, the Yb atoms produce resonate states around EF that enhance the Seebeck coefficient. As a heavy atom, Yb effectively scatter phonons resulting in the thermal conductivity reduction. All these factors together give In3.95Yb0.05Se3 a significant 30% ZT at 703K relevant to that of the pure In4Se3.