TY - JOUR
T1 - First-principles study of the electronic, vibrational properties and anharmonic effects of some Si-based type-II binary clathrates
AU - Xue, Dong
AU - Myles, Charley W.
N1 - Funding Information:
We would like to acknowledge grateful discussions with Professor M. Sanati (Texas Tech University) about the use of VASP.We also appreciate many hours of computing time at the High Performance Computing Center of Texas Tech University. This research received no external funding.
Publisher Copyright:
© 2019 by the authors.
PY - 2019/2/11
Y1 - 2019/2/11
N2 - Electronic, vibrational, and anharmonic studies on some binary clathrate A x Si 136 (A = Na, K, Rb, Cs; 0 < x ≤ 24) are theoretically presented. The Fermi energy lies in the range of 1.1 eV to 1.4 eV for Na x Si 136 and increases as stoichiometry (x) is tuned from 8 to 12 to 16. The determined isotropic "Mexican-hat" shape of the guest-host potential describing Na motion in the Si 28 cage indicates the "off-center" position when the temperature is elevated beyond zero. Accordingly, the calculated Na "off-center" displacements correlate well with the X-Ray Diffraction (XRD) data (0.4 Å-0.5 Å) for a similar composition range (0 < x < 24). The lack of first-principles analysis on quartic anharmonicity motivates us to initiate a self-consistent model to examine the temperature-dependent rattling frequency Ω(T) of the guest (Na, Rb). The predicted values of Ω(T) for Na 24 Si 136 at 300 K are significantly higher (approximately six times larger) than the value at absolute zero, which contrasts with the case of Rb 8 Si 136 . Moreover, underestimation of the isotropic atomic displacement parameter U iso is caused by the temperature-dependent quartic anharmonicity of Na, and this discrepancy might be offset by the square of the "off-center" displacement.
AB - Electronic, vibrational, and anharmonic studies on some binary clathrate A x Si 136 (A = Na, K, Rb, Cs; 0 < x ≤ 24) are theoretically presented. The Fermi energy lies in the range of 1.1 eV to 1.4 eV for Na x Si 136 and increases as stoichiometry (x) is tuned from 8 to 12 to 16. The determined isotropic "Mexican-hat" shape of the guest-host potential describing Na motion in the Si 28 cage indicates the "off-center" position when the temperature is elevated beyond zero. Accordingly, the calculated Na "off-center" displacements correlate well with the X-Ray Diffraction (XRD) data (0.4 Å-0.5 Å) for a similar composition range (0 < x < 24). The lack of first-principles analysis on quartic anharmonicity motivates us to initiate a self-consistent model to examine the temperature-dependent rattling frequency Ω(T) of the guest (Na, Rb). The predicted values of Ω(T) for Na 24 Si 136 at 300 K are significantly higher (approximately six times larger) than the value at absolute zero, which contrasts with the case of Rb 8 Si 136 . Moreover, underestimation of the isotropic atomic displacement parameter U iso is caused by the temperature-dependent quartic anharmonicity of Na, and this discrepancy might be offset by the square of the "off-center" displacement.
KW - Off-center displacement
KW - Quartic anharmonicity
KW - Self-consistent model
UR - http://www.scopus.com/inward/record.url?scp=85061496992&partnerID=8YFLogxK
U2 - 10.3390/ma12030536
DO - 10.3390/ma12030536
M3 - Article
AN - SCOPUS:85061496992
VL - 12
JO - Materials
JF - Materials
SN - 1996-1944
IS - 3
M1 - 536
ER -