Abstract
Electronic, vibrational, and anharmonic studies on some binary clathrate AxSi136 (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 NaxSi136 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 Si28 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 W(T) of the guest (Na, Rb). The predicted values of W(T) for Na24Si136 at 300 K are
significantly higher (approximately six times larger) than the value at absolute zero, which contrasts
with th
Original language | English |
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Pages (from-to) | 536-548 |
Journal | Materials |
State | Published - Feb 11 2019 |