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Title: Pseudogap and anharmonic phonon behavior in Ba{sub 8}Ga{sub 16}Ge{sub 30}: An NMR study

Abstract

We have performed {sup 69}Ga, {sup 71}Ga, and {sup 137}Ba NMR on Ba{sub 8}Ga{sub 16}Ge{sub 30}, a clathrate semiconductor which has been of considerable interest due to its large figure of merit for thermoelectric applications. In measurements from 4 K to 450 K, we used measurements on the two Ga nuclei to separate the magnetic and electric quadrupole hyperfine contributions and thereby gain information about the metallic and phonon behavior. The results show the presence of a pseudogap in the Ga electronic states within the conduction band, superposed upon a large Ba contribution to the conduction band. Meanwhile the phonon contributions to the Ga relaxation rates are large and increase more rapidly with temperature than typical semiconductors. These results provide evidence for enhanced anharmonicity of the propagative phonon modes over a wide range, providing experimental evidence for enhanced phonon-phonon scattering as a mechanism for the reduced thermal conductivity.

Authors:
 [1];  [1];  [2]
  1. Department of Physics and Astronomy, Texas A&M University, College Station, Texas 77843 (United States)
  2. (United States)
Publication Date:
OSTI Identifier:
22679033
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 145; Journal Issue: 5; Other Information: (c) 2016 Author(s); Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; BARIUM 137; EXPERIMENTAL DATA; GALLIUM 71; NUCLEAR MAGNETIC RESONANCE; PHONONS; THERMAL CONDUCTIVITY

Citation Formats

Sirusi, Ali A., E-mail: alisirusi@tamu.edu, Ross, Joseph H., E-mail: jhross@tamu.edu, and Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843. Pseudogap and anharmonic phonon behavior in Ba{sub 8}Ga{sub 16}Ge{sub 30}: An NMR study. United States: N. p., 2016. Web. doi:10.1063/1.4960054.
Sirusi, Ali A., E-mail: alisirusi@tamu.edu, Ross, Joseph H., E-mail: jhross@tamu.edu, & Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843. Pseudogap and anharmonic phonon behavior in Ba{sub 8}Ga{sub 16}Ge{sub 30}: An NMR study. United States. doi:10.1063/1.4960054.
Sirusi, Ali A., E-mail: alisirusi@tamu.edu, Ross, Joseph H., E-mail: jhross@tamu.edu, and Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843. Sun . "Pseudogap and anharmonic phonon behavior in Ba{sub 8}Ga{sub 16}Ge{sub 30}: An NMR study". United States. doi:10.1063/1.4960054.
@article{osti_22679033,
title = {Pseudogap and anharmonic phonon behavior in Ba{sub 8}Ga{sub 16}Ge{sub 30}: An NMR study},
author = {Sirusi, Ali A., E-mail: alisirusi@tamu.edu and Ross, Joseph H., E-mail: jhross@tamu.edu and Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843},
abstractNote = {We have performed {sup 69}Ga, {sup 71}Ga, and {sup 137}Ba NMR on Ba{sub 8}Ga{sub 16}Ge{sub 30}, a clathrate semiconductor which has been of considerable interest due to its large figure of merit for thermoelectric applications. In measurements from 4 K to 450 K, we used measurements on the two Ga nuclei to separate the magnetic and electric quadrupole hyperfine contributions and thereby gain information about the metallic and phonon behavior. The results show the presence of a pseudogap in the Ga electronic states within the conduction band, superposed upon a large Ba contribution to the conduction band. Meanwhile the phonon contributions to the Ga relaxation rates are large and increase more rapidly with temperature than typical semiconductors. These results provide evidence for enhanced anharmonicity of the propagative phonon modes over a wide range, providing experimental evidence for enhanced phonon-phonon scattering as a mechanism for the reduced thermal conductivity.},
doi = {10.1063/1.4960054},
journal = {Journal of Chemical Physics},
number = 5,
volume = 145,
place = {United States},
year = {Sun Aug 07 00:00:00 EDT 2016},
month = {Sun Aug 07 00:00:00 EDT 2016}
}
  • Temperature dependent synchrotron powder diffraction and single crystal neutron diffraction data are used for probing the vibrational states and disorder in type I clathrates Ba{sub 8}Ga{sub 16}Si{sub 30}, Ba{sub 8}Ga{sub 16}Ge{sub 30}, Ba{sub 8}In{sub 16}Ge{sub 30}, and Sr{sub 8}Ga{sub 16}Ge{sub 30}. If an empirical disorder term is included, the temperature dependence of the atomic displacement factors (ADPs) of the framework and guest atoms can be described by a Debye and Einstein model, respectively. None of the guest atoms in the large cages are located in the center and the vibrational frequencies ({theta}{sub E}) are of the order 80 K ormore » larger for all structures, in good agreement with theoretical predictions. Even though the Sr ADPs are larger than the Ba ADPs in all the clathrates, the data show that {theta}{sub E} of Sr in Sr{sub 8}Ga{sub 16}Ge{sub 30} is larger than for the Ba atoms. This is due to stronger guest-host chemical bonding in Sr{sub 8}Ga{sub 16}Ge{sub 30}. Since {theta}{sub E} of Sr has been reported to be much smaller in the literature we have also measured the specific heat of Sr{sub 8}Ga{sub 16}Ge{sub 30} with Ba{sub 8}Ga{sub 16}Ge{sub 30} as a reference. It is found that localized excitations with a characteristic energy of approximately 35 K exist in both compounds, however, the total number of states is too low to be associated with either tunneling states or localized vibration of each of the guest atoms.« less
  • We present a detailed extended x-ray absorption fine structure (EXAFS) analysis of the thermoelectric clathrates Eu{sub 8}Ga{sub 16}Ge{sub 30} and Sr{sub 8}Ga{sub 16}Ge{sub 30}, both of which have an unusually low thermal conductivity attributed to a 'rattler' motion of the filler atoms Eu and Sr. The EXAFS results show that the Ga/Ge lattice is quite stiff with a high correlated Debye temperature {approx}400 K. Eu is on-center in the site 1 cage but off-center (0.445{+-}0.020 A) in the large cage called the Eu2 site. The results for Sr are similar, but {approx}75% are off-center 0.40{+-}0.05 A and {approx}25% are on-centermore » in the Sr2 site. Both results are in reasonable agreement with diffraction results. The temperature dependence of the nearest neighbor pair distribution widths yield low Einstein temperatures (80{+-}10 and 100{+-}10 K for Eu1 and Sr1, respectively, and 95{+-}10 and 125{+-}10 K for the shortest Eu2-Ga/Ge and Sr2-Ga/Ge pairs). In contrast, the more distant Eu2/Sr2-Ga/Ge pair distributions within the Eu2/Sr2 cage are strongly disordered even at low T, indicating considerable local disorder. This indicates that the off-center Eu or Sr atom is bonded to the side of the site 2 cage. This has two important implications for the thermal conductivity: it increases the coupling between the 'rattler' vibrations and the lattice phonons and it introduces a symmetry-breaking large mass defect.« less
  • No abstract prepared.
  • Type-I clathrates Yb{sub x}Ba{sub 8-x}Ga{sub 16}Ge{sub 30} (x=0-1.3) filled by ytterbium and barium were synthesized by melting reaction method combined with spark plasma sintering method. The structure and thermoelectric properties of double-atoms-filled clathrate compounds are investigated. X-ray diffraction patterns and Rietveld analysis reveal that the compounds prepared by this method are type-I clathrates. The filling atoms exhibit atomic displacement parameters that are larger than that of framework atoms. All specimens show n-type conduction and the room temperature carrier concentration (N{sub p}) increases with the increasing Yb content. The electrical conductivity of the compounds increases at first and then decreases withmore » the increasing Yb content. When x=0.7, it reaches the maximum. The Seebeck coefficient decreases gradually with the increasing x. The substituting of Yb atoms for Ba leads to significant influence on the lattice thermal conductivity of the compounds. The lattice thermal conductivity decreases gradually with the increasing x. Of all the Yb{sub x}Ba{sub 8-x}Ga{sub 16}Ge{sub 30} compounds, Yb{sub 0.5}Ba{sub 7.5}Ga{sub 16}Ge{sub 30} compound has the greatest ZT value and its maximal ZT value reaches 1.1 at about 950 K. Compared with the Ba{sub 8}Ga{sub 16}Ge{sub 30} sample, it increases by 90% at the same temperature.« less