PbSe is a surprisingly good thermoelectric material due, in part, to its low thermal conductivity that had been overestimated in earlier measurements. The thermoelectric figure of merit, zT, can exceed 1 at high temperatures in both p-type and n-type PbSe, similar to that found in PbTe. While the p-type lead chalcogenides (PbSe and PbTe) benefit from the high valley degeneracy (12 or more at high temperature) of the valence band, the n-type versions are limited to a valley degeneracy of 4 in the conduction band. Yet the n-type lead chalcogenides achieve a zT nearly as high as the p-type lead chalcogenides. This effect can be attributed to the weaker electron–phonon coupling (lower deformation potential coefficient) in the conduction band as compared with that in the valence band, which leads to higher mobility of electrons compared to that of holes. This study of PbSe illustrates the importance of the deformation potential coefficient of the charge-carrying band as one of several key parameters to consider for band structure engineering and the search for high performance thermoelectric materials.

https://www.pnas.org/content/pnas/109/25/9705.full.pdf

Weak electron–phonon coupling contributing to high thermoelectric performance in n-type PbSe

It is shown that the crystal structures of the binary IV-VI compounds can be classified by parameters derived from a Pauli-force model potential, which has previously been used successfully for the octet and suboctet materials (ANBP-N with P=8 and 2<or=P<or=6). Trends in the transverse effective charges, in the heats of formation, in the energy gap between bonding and antibonding states, and in bond angles and bond lengths can also be explained with these parameters. The author discusses the origin of these correlations in terms of the band structure and atomic pseudopotentials, and demonstrates how a microscopic analysis of the crystal structure leads towards a quantitative understanding of the cubic to rhombohedral phase transition in the (Ge, Sn)Te system.

https://iopscience.iop.org/article/10.1088/0022-3719/13/26/009/pdf

The crystal structure of IV-VI compounds. I. Classification and description

Narrow-gap lead salts

In this chapter, the predictions of an enhancement in the thermoelectric figure of merit of low-dimensional material systems relative to their corresponding bulk counterparts, as well as the present state of experimental confirmation of these predictions, are reviewed. Progress with specific quantum well, quantum wire, and quantum dot materials systems, such as the lead salts, Si-Ge, and bismuth is discussed. To date most of the effort has gone into proof-of-principle studies, though actual demonstration of the highest thermoelectric figure of merit ( Z 3D T ) of any material to date has been seen in the low-dimensional system PbSe 0.98 Te 0.02 -PbTe, where the enhancement is attributed to quantum dot formation associated with the interface between the PbTe and PbSe 0.98 Te 0.02 . In this chapter, particular attention is given to a discussion of the structure and properties of bismuth quantum wires, which are still at an early state of research. Bismuth nanowires, however, offer significant promise for practical applications, because they can be self-assembled and are predicted to have desirable thermoelectric properties when they have wire diameters in the 5- to 10-nm range. Though temperature-dependent resistance measurements have been carried out for Bi nanowires in this diameter range, reliable thermoelectric measurements have not yet been reported.

Chapter 1 Quantum wells and quantum wires for potential thermoelectric applications

We report the room-temperature, cross-plane thermal conductivities, and longitudinal speeds of sound of multilayer films [(TiTe2)3(Bi2Te3)x(TiTe2)3(Sb2Te3)y]i (x=1−5,y=1−5) and misfit-layer dichalcogenide films [(PbSe)m(TSe2)n]i (T=W or Mo, m=1–5, and n=1–5) synthesized by the modulated elemental reactants method. The thermal conductivities of these nanoscale layered materials fall below the predicted minimum thermal conductivity of the component compounds: two times lower than the minimum thermal conductivity of Bi2Te3 for multilayer [(TiTe2)3(Bi2Te3)x(TiTe2)3(Sb2Te3)y]i films and five to six times lower than the minimum thermal conductivity of PbSe for misfit-layer dichalcogenides [(PbSe)m(TSe2)n]i. We attribute the low thermal conductivities to the anisotropic bonding of the layered crystals and orientational disorder in the stacking of layered crystals along the direction perpendicular to the surface.

W. Wanninger

Papers

Elastic constants of PbSe