Abstract
A deficiency of Ga in wide band-gap AgGa1- xTe2 semiconductors (1.2 eV) can be used to optimize the electrical transport properties and reduce the thermal conductivity to achieve ZT > 1 at 873 K. First-principles density functional theory calculations and a Boson peak observed in the low temperature heat capacity data indicate the presence of strong coupling between optical phonons with low frequency and heat carrying acoustical phonons, resulting in a depressed maximum of Debye frequency in the first Brillouin zone and low phonon velocities. Moreover, the AgTe bond lengths and TeAgTe bond angles increase with rising temperature, leading to a significant distortion of the [AgTe4]7− tetrahedra, but an almost unmodified [GaTe4]5− tetrahedra. This behavior results in lattice expansion in the ab-plane and contraction along the c-axis, corresponding to the positive and negative Gruneisen parameters in the phonon spectral calculations. This effect gives rise to the large anharmonic behavior of the lattice. These factors together with the low frequency vibrations of Ag and Te atoms in the structure lead to an ultralow thermal conductivity of 0.18 W m−1 K−1 at 873 K.
Original language | English (US) |
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Article number | 1806534 |
Journal | Advanced Functional Materials |
Volume | 29 |
Issue number | 6 |
DOIs | |
State | Published - Feb 8 2019 |
Funding
The authors wish to acknowledge support from the Natural Science Foundation of China (Grant Nos. 51521001 and 51632006). This work was supported primarily by a grant from the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-SC0014520 (Northwestern University to X.S., S.H., C.W., and M.G.K., thermoelectric property measurements and band structure and phonon calculations). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357.
Keywords
- directional negative thermal expansion
- intrinsically low thermal conductivity
- phonon spectrum
- thermoelectric properties
- vacancy
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- General Chemistry
- Biomaterials
- General Materials Science
- Condensed Matter Physics
- Electrochemistry