Seebeck and Figure of Merit Enhancement by Rare Earth Doping in Yb(14-x)RE(x)ZnSb(11) (x = 0.5)

Yb(14)ZnSb(11) has been of interest for its intermediate valency and possible Kondo designation. It is one of the few transition metal compounds of the Ca(14)AlSb(11) structure type that show metallic behavior. While the solid solution of Yb(14)Mn(1-x)Zn(x)Sb(11) shows an improvement in the high temperature figure of merit of about 10% over Yb(14)MnSb(11), there has been no investigation of optimization of the Zn containing phase. In an effort to expand the possible high temperature p-type thermoelectric materials with this structure type, the rare earth (RE) containing solid solution Yb(14-x)RE(x)ZnSb(11) (RE = Y, La) was investigated. The substitution of a small amount of 3+ rare earth (RE) for Yb(2+) was employed as a means of optimizing Yb(14)MnSb(11) for use as a thermoelectric material. Yb(14)ZnSb(11) is considered an intermediate valence Kondo system where some percentage of the Yb is formally 3+ and undergoes a reduction to 2+ at ~85 K. The substitution of a 3+ RE element could either replace the Yb(3+) or add to the total amount of 3+ RE and provides changes to the electronic states. RE = Y, La were chosen as they represent the two extremes in size as substitutions for Yb: a similar and much larger size RE, respectively, compared with Yb(3+). The composition x = 0.5 was chosen as that is the typical amount of RE element that can be substituted into Yb(14)MnSb(11). These two new RE containing compositions show a significant improvement in Seebeck while decreasing thermal conductivity. The addition of RE increases the melting point of Yb(14)ZnSb(11) so that the transport data from 300 K to 1275 K can be collected. The figure of merit is increased five times over that of Yb(14)ZnSb(11) and provides a zT ~0.7 at 1275 K.