Discordant nature of Cd in PbSe: off-centering and core–shell nanoscale CdSe precipitates lead to high thermoelectric performance

Songting Cai; Shiqiang Hao; Zhong-Zhen Luo; Xiang Li; Ido Hadar; Trevor P. Bailey; Xiaobing Hu; Ctirad Uher; Yan-Yan Hu; Christopher Wolverton; Vinayak P. Dravid; Mercouri G. Kanatzidis

doi:10.1039/C9EE03087E

Discordant nature of Cd in PbSe: off-centering and core–shell nanoscale CdSe precipitates lead to high thermoelectric performance†

Songting Cai,

^a Shiqiang Hao,^a Zhong-Zhen Luo,^bc Xiang Li,^d Ido Hadar,

^b Trevor P. Bailey,

^e Xiaobing Hu,

^af Ctirad Uher,^e Yan-Yan Hu,

^dg Christopher Wolverton,^a Vinayak P. Dravid

*^af and Mercouri G. Kanatzidis

*^b

Author affiliations

* Corresponding authors

^a Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA
E-mail: v-dravid@northwestern.edu

^b Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA
E-mail: m-kanatzidis@northwestern.edu

^c School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue 639798, Singapore

^d Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, USA

^e Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA

^f NUANCE Center, Northwestern University, Evanston, Illinois 60208, USA

^g National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, FL 32310, USA

Abstract

We report a novel hierarchical microstructure in the PbSe–CdSe system, which collectively contributes to significant enhancement in thermoelectric performance, with ZT_ave ∼ 0.83 across the 400–923 K temperature range, the highest reported for p-type, Te-free PbSe systems. We have investigated the local atomic structure as well as the microstructure of a series of PbSe–xCdSe materials, up to x = 10%. We find that the behavior of the Cd atoms in the octahedral rock salt sites is discordant and results in off-center displacement and distortion. Such off-centered Cd in the PbSe matrix creates (1) L–Σ electronic energy band convergence, (2) a flattened L band, both contributing to higher Seebeck coefficients, and (3) enhanced phonon scattering, which leads to lower thermal conductivity. These conclusions are supported by photoemission yield spectroscopy in air (PYSA), solid state ¹¹¹Cd, ⁷⁷Se NMR spectroscopy and DFT calculations. Above the solubility limit (>6%CdSe), we also observe endotaxial CdSe nano-precipitates with core–shell architecture formed in PbSe, whose size, distribution and structure gradually change with the Cd content. The nano-precipitates exhibit a zinc blende crystal structure and a tetrahedral shape with significant local strain, but are covered with a thin wurtzite layer along the precipitate/matrix interface, creating a core–shell structure embedded in PbSe. This newly discovered architecture causes a further reduction in lattice thermal conductivity. Moreover, potassium is found to be an effective p-type dopant in the PbSe–CdSe system, leading to an enhanced power factor, a maximum ZT of ∼1.4 at 923 K for Pb_0.98K_0.02Se–6%CdSe.

Energy & Environmental Science

Discordant nature of Cd in PbSe: off-centering and core–shell nanoscale CdSe precipitates lead to high thermoelectric performance†

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Discordant nature of Cd in PbSe: off-centering and core–shell nanoscale CdSe precipitates lead to high thermoelectric performance

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