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Title: Intermediate bands in type-II silicon clathrate with Cu and Ag guest atoms

Authors:
; ; ;
Publication Date:
Sponsoring Org.:
USDOE
OSTI Identifier:
1357078
Grant/Contract Number:
SC0006433
Resource Type:
Journal Article: Publisher's Accepted Manuscript
Journal Name:
Physical Review B
Additional Journal Information:
Journal Volume: 95; Journal Issue: 19; Related Information: CHORUS Timestamp: 2017-05-15 22:09:16; Journal ID: ISSN 2469-9950
Publisher:
American Physical Society (APS)
Country of Publication:
United States
Language:
English

Citation Formats

Huang, Zhaohui, Li, Huashan, Lusk, Mark T., and Wu, Zhigang. Intermediate bands in type-II silicon clathrate with Cu and Ag guest atoms. United States: N. p., 2017. Web. doi:10.1103/PhysRevB.95.195208.
Huang, Zhaohui, Li, Huashan, Lusk, Mark T., & Wu, Zhigang. Intermediate bands in type-II silicon clathrate with Cu and Ag guest atoms. United States. doi:10.1103/PhysRevB.95.195208.
Huang, Zhaohui, Li, Huashan, Lusk, Mark T., and Wu, Zhigang. Mon . "Intermediate bands in type-II silicon clathrate with Cu and Ag guest atoms". United States. doi:10.1103/PhysRevB.95.195208.
@article{osti_1357078,
title = {Intermediate bands in type-II silicon clathrate with Cu and Ag guest atoms},
author = {Huang, Zhaohui and Li, Huashan and Lusk, Mark T. and Wu, Zhigang},
abstractNote = {},
doi = {10.1103/PhysRevB.95.195208},
journal = {Physical Review B},
number = 19,
volume = 95,
place = {United States},
year = {Mon May 15 00:00:00 EDT 2017},
month = {Mon May 15 00:00:00 EDT 2017}
}

Journal Article:
Free Publicly Available Full Text
Publisher's Version of Record at 10.1103/PhysRevB.95.195208

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  • The effects of guest atomic species in Si clathrates on the lattice thermal conductivity were studied using classical molecular dynamics calculations. The interaction between a host atom and a guest atom was described by the Morse potential function while that between host atoms was described by the Tersoff potential. The parameters of the potentials were newly determined for this study such that the potential curves obtained from first-principles calculations for the insertion of a guest atom into a Si cage were successfully reproduced. The lattice thermal conductivities were calculated by using the Green-Kubo method. The experimental lattice thermal conductivity ofmore » Ba{sub 8}Ga{sub 16}Si{sub 30} can be successfully reproduced using the method. As a result, the lattice thermal conductivities of type-I Si clathrates, M{sub 8}Si{sub 46} (M = Na, Mg, K, Ca Rb, Sr, Cs, or Ba), were obtained. It is found that the lattice thermal conductivities of M{sub 8}Si{sub 46}, where M is IIA elements (i.e., M = Mg, Ca, Sr, or Ba) tend to be lower than those of M{sub 8}Si{sub 46}, where M is IA elements (i.e., M = Na, K, Rb, or Cs). Those of {sup m}M{sub 8}Si{sub 46}, where m was artificially modified atomic weight were also obtained. The obtained lattice thermal conductivity can be regarded as a function of a characteristic frequency, f{sub c}. That indicates minimum values around f{sub c}=2-4 THz, which corresponds to the center of the frequencies of the transverse acoustic phonon modes associated with Si cages.« less
  • We have used X-ray absorption spectroscopy (XAS) to investigate the local structure of Cu and Ge in the Cs{sub 8}Na{sub 16}Cu{sub 5}Ge{sub 131} type II clathrate. We show that the local structure parameters for Ge (coordination number and distances) are consistent with those derived on the basis of XRD investigation of Cs{sub 8}Na{sub 16}Ge{sub 136}. The EXAFS data suggest that Cu either randomly substitutes for Ge on the clathrate framework or preferentially on the 96g site but not preferentially on the 32e or 8a sites (Wyckoff notation). Furthermore, we find that the Cu-Ge distance is smaller than the Ge-Ge distancemore » by 0.13 A, indicating a local distortion around the Cu atoms. The estimated degrees of disorder for Cu-Ge and Ge-Ge interactions indicate the Cu-Ge clathrate framework to be relatively stiff, while those for Na-Ge and Cs-Ge interactions corroborate previous observations of strong thermal disorder of the alkali guests in these materials. Our XAS results offer insight into the site substitution of Cu in this material, information unattainable from X-ray diffraction due to the lack of scattering contrast between Cu and Ge. - Graphical abstract: Comparison of Fourier transforms of Cu K-edge and Ge K-edge EXAFS spectra for Cs{sub 8}Na{sub 16}Cu{sub 5}Ge{sub 131} collected near the temperature of liquid nitrogen (LN). The data clearly show that the Cu-Ge distance is smaller than the Ge-Ge distance, indicating the presence of a local distortion around Cu.« less
  • The origin of the “glass-like” plateau in thermal conductivity of inorganic type I clathrates has been debated for more than a decade. Here, it is demonstrated that the low temperature thermal conductivity of Sr{sub 8}Ga{sub 16}Ge{sub 30} can be controlled by the synthesis method: A flux-grown sample has a “glass-like” plateau in thermal conductivity at low temperature, while a zone-melted sample instead has a crystalline peak. A combination of flux-growth and zone-melting produces an intermediate thermal conductivity. In a comprehensive study of three single crystal samples, it is shown by neutron diffraction that the transition from crystalline peak to “glass-like”more » plateau is related to an increase in Sr guest atom off-centering distance from 0.24 Å to 0.43 Å. By modifying ab initio calculated force constants for the guest atom to an isotropic model, we reproduce both measured heat capacity and inelastic neutron scattering data. The transition from peak to plateau in the thermal conductivity can be modeled by a combined increase of Rayleigh and disorder scattering. Measurement of heat capacity refutes simple models for tunneling of Sr between off-center sites. Furthermore, the electronic properties of the same samples are characterized by Hall carrier density, Seebeck coefficient, and resistivity. The present comprehensive analysis excludes tunneling and charge carrier scattering as dominant contributors to the “glass-like” plateau. The increased guest atom off-centering distance controlled by synthesis provides a possible microscopic mechanism for reducing the low temperature thermal conductivity of clathrates.« less
  • We have used X-ray absorption spectroscopy (XAS) to investigate the local structure of Cu and Ge in the Cs8Na16Cu5Ge131 type II clathrate. We show that the local structure parameters for Ge (coordination number and distances) are consistent with those derived on the basis of XRD investigation of Cs8Na16Ge136. The EXAFS data suggest that Cu either randomly substitutes for Ge on the clathrate framework or preferentially on the 96g site but not preferentially on the 32e or 8a sites (Wyckoff notation). Furthermore, we find that the Cu-Ge distance is smaller than the Ge-Ge distance by 0.13 Angstroms, indicating a local distortionmore » around the Cu atoms. The estimated degrees of disorder for Cu-Ge and Ge-Ge interactions indicate the Cu-Ge clathrate framework to be relatively stiff, while those for Na-Ge and Cs-Ge interactions corroborate previous observations of strong thermal disorder of the alkali guests in these materials. Our XAS results offer insight into the site substitution of Cu in this material, information unattainable from X-ray diffraction due to the lack of scattering contrast between Cu and Ge.« less
  • The vibrational bands of acetone in solid argon and in the structure II 2acetylene/acetone/17water clathrates are presented. The results show that the band frequencies for enclathrated acetone are not greatly different from the unperturbed acetone frequencies. The acetone does not rotate on the IR time scale, and little evidence for site effects was observed (except for possibly lifting the degeneracy of an antisymmetric stretch of CH/sub 3/). Both of the C-C stretches of acetone undergo small blue shifts, though the reason is unclear.