Gloria J. Lehr

Enhanced thermoelectric power factor in Yb1−xScxAl2 alloys using chemical pressure tuning of the Yb valence

We have investigated the influence of Sc substitution on the transport and magnetic properties of YbAl2, a well-known intermediate valence compound. Sc substitution provides a chemical pressure that decreases the lattice constant and thereby alters the Yb valence as a function of composition and temperature. We observe a strong correlation between the Seebeck coefficient and the ratio of trivalent to divalent Yb in these compounds, as determined from magnetic susceptibility measurements. This correlation indicates that the largest absolute value of the Seebeck coefficient is achieved when the average Yb valence is near 2.5 (the ratio of divalent to trivalent Yb is 1:1). It is shown that Sc concentration can be used as a means to tune both the magnitude of the maximum of the Seebeck coefficient and the temperature at which this absolute maximum occurs, improving the prospects of the use of these materials in cryogenic Peltier coolers.

Eu2+-Eu3+ valence transition in double, Eu-, and Na-doped PbSe from transport, magnetic, and electronic structure studies

The Eu atoms in Pb1-xEuxSe have long been assumed to be divalent. We show that p-type doping of this magnetic semiconductor alloy with Na can modify the effective Eu valence: a mixed, Eu2+-Eu3+ state appears in Pb1-x-yEuxNaySe at particular values of y. Magnetization, carrier concentration, resistivity, and thermopower of Pb1-x-yEuxNaySe are reported for a number of samples with different x and y. A pronounced increase in thermopower at a given carrier concentration was identified and attributed to the presence of enhanced ionized impurity scattering. A strong decrease in the hole concentration is observed in Pb1-yNaySe when Eu is added to the system, which we attribute to a Eu2+-Eu3+ self-ionization process. This is evidenced by magnetization measurements, which reveal a significant reduction of the magnetic moment of Pb1-xEuxSe upon alloying with Na. Further, a deviation of magnetization from a purely paramagnetic state, described by a Brillouin function, identifies antiferromagnetic interactions between the nearest-neighbor Eu atoms: a value of Jex/kB = -0.35 K was found for the exchange coupling parameter. The conclusion of a Eu2+-Eu3+ self-ionization process being in effect is supported further by the electronic structure calculations, which show that an instability of the 4f7 configuration of the Eu2+ ion appears with Na doping. Schematically, it was found that the Eu 4f levels form states near enough to the Fermi energy that hole doping can lower the Fermi energy and trigger a reconfiguration of a 4f electronic shell.

Interplay of chemical expansion, Yb valence, and low temperature thermoelectricity in the YbCu2Si2−xGex solid solution

YbCu2Si2 is a promising low temperature thermoelectric material because of the large broad peak in the Seebeck coefficient near 100 K combined with a low electrical resistivity. This behavior is thought to arise from fluctuating, or intermediate, valence effects due to partial occupation of Yb 4f energy states near the Fermi level. Previous studies of the magnetic properties under pressure have demonstrated that the average Yb valence is sensitive to the contraction of unit cell volume. By forming a solid solution of YbCu2Si2 with YbCu2Ge2, an isostructural compound with a larger unit cell volume, here we examine the subtle effects of lattice expansion on the transport properties and average Yb valence. We observe a shift in the peak of the Seebeck coefficient towards higher temperatures, as well as an enhanced power factor in the solid solutions. At the same time, a reduction in thermal conductivity due to alloy scattering enhances the thermoelectric figure of merit. Chemical pressure effects may thus be...

Effect of chemical pressure manipulation on the valence and thermoelectric properties of the intermediate valence compound YbCu2-xNixGe2

The thermoelectric properties of YbCu2Ge2 at low temperature are unremarkable, however it has been found to be an intermediate valence compound. Previous work has shown that manipulation of valence in such compounds by chemical pressure can have dramatic effects on the value of the Seebeck coefficient. YbCu2Ge2 has been found to exist in a nearly divalent, Yb2+, state at room temperature, which if increased towards 2.5 has the promise of increasing the Seebeck coefficient substantially. In this study a full solid solution of YbCu2Ge2 and YbNi2Ge2 was created to exert a chemical pressure on the Yb and shift the valence away from the nominally divalent. A dramatic increase in the Seebeck coefficient was found as well as a substantial decrease in the thermal conductivity. These effects resulted in a substantial increase in ZT at low temperature and further emphasized the applicability and large effect that valence manipulation can have on intermediate valence compounds.