Complex changes in the framework of endohedrally Na-doped type II Si clathrates with respect to Na content

Ban, Takayuki; Ogura, Takuya; Ohashi, Yoshitaka; Himeno, Roto; Ohashi, Fumitaka; Kume, Tetsuji; Ohya, Yutaka; Natsuhara, H.; Iida, Tamio; Habuchi, Hitoe; Nonomura, Shuichi

doi:10.1007/s10853-012-6886-0

Cited by 16 publications

(13 citation statements)

References 23 publications

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“…16,27,28 In particular, a first decreases slightly for 0 < x < 8 but then increases for 8 < x < 24, in agreement with prior work. 16,27,28 This can be attributed to selective occupation of the eight larger, oversized Si 28 cages for x < 8, whereas the sixteen smaller Si 20 cages become significantly occupied only once the larger Si 28 cages are near full occupation (x > 8) ( Table I). The non-monotonic response to filling is also reflected in the evolution of local Na-Si distances for the two cages, suggesting very different guest-framework interactions for the two cages.…”

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confidence: 90%

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Control of thermal expansion in a low-density framework modification of silicon

Beekman

Kaduk

Wong‐Ng

et al. 2018

Applied Physics Letters

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The low-density clathrate-II modification of silicon, Si 136 , contains two distinct cage-like voids large enough to accommodate various types of guest atoms which influence both the host structure and its properties. Although the linear coefficient of thermal expansion of Si 136 (293 K < T < 423 K) is only about 20% larger than that of the ground state a-Si (diamond structure), the coefficient of thermal expansion monotonically increases by more than 150% upon filling the framework cages with Na atoms in Na x Si 136 (0 < x < 24), ranging from a ¼ 2.6 Â 10 À6 K À1 (x ¼ 0) to 6.8 Â 10 À6 K À1 (extrapolated to x ¼ 24) by only varying the Na content, x. Taken together with the available heat capacity and bulk modulus data, the dramatic increase in thermal expansion can be attributed to an increase in the mode-averaged Gr€ uneisen parameter by a factor of nearly 3 from x ¼ 0 to x ¼ 24. These results highlight a potential mechanism for tuning thermal expansion, whereby guest atoms are incorporated into the voids of rigid, covalently bonded inorganic frameworks to influence the lattice dynamics.

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“…The non-monotonic response to filling is also reflected in the evolution of local Na-Si distances for the two cages, suggesting very different guest-framework interactions for the two cages. 16,27,28 The change in lattice parameter as a function of temperature for Na x Si 136 , Da T ð Þ ¼ a T ð Þ À að293 KÞ, is shown in Fig. 2(a).…”

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confidence: 99%

Control of thermal expansion in a low-density framework modification of silicon

Beekman

Kaduk

Wong‐Ng

et al. 2018

Applied Physics Letters

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“…Considerable experimental work on Na x Si 136 by Beekman and collaborators focused on detecting the isotropic, “off-center” displacement of Na in Si 136 and studying the system in significant detail [4,32,33,34,35]. An early report by them on the Na vibrations in Si 28 showed a nearly x -independent “off-center” displacement measured to be approximately 0.4–0.5 Å [32]. Our first-principles calculations correlate well with these experimental results.…”

Section: Resultsmentioning

confidence: 99%

First-Principles Study of the Electronic, Vibrational Properties and Anharmonic Effects of Some Si-Based Type-II Binary Clathrates

Xue

Myles

2019

Materials

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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 Ω(T) of the guest (Na, Rb). The predicted values of Ω(T) for Na24Si136 at 300 K are significantly higher (approximately six times larger) than the value at absolute zero, which contrasts with the case of Rb8Si136. Moreover, underestimation of the isotropic atomic displacement parameter Uiso is caused by the temperature-dependent quartic anharmonicity of Na, and this discrepancy might be offset by the square of the “off-center” displacement.

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“…Si clathrates [1][2][3] the free electrons provided by the positively ionized Na atoms [4]. However, the clathrates without guest atoms, better known as guest-free clathrates, exhibit semiconducting properties [5][6][7][8][9][10][11][12]. The guest-free type-I Si clathrate shows an indirect band structure with a gap of about 1.8 eV [13].…”

Section: Introductionmentioning

confidence: 99%

Cross-sectional transmission electron microscope observation of Si clathrate thin films grown on Si (111) substrates

et al. 2017

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By means of high-resolution transmission electron microscopy (HR-TEM) observations, and energy dispersive X-ray analyses performed with a scanning TEM (STEM-EDX), we evaluated the residual Na contents and the induced crystal strains in type-II Si clathrate films grown on a Si substrate and treated with iodine for Na elimination. Cross-sectional TEM and STEM-EDX observations verified the formation of the type-II Si clathrate thin film on the Si substrate. The guest-free (without any Na inclusion) clathrate crystal was obtained by iodine (I 2) treatment for more than 3 cycles. The resulting films were polycrystalline. No buffer layer was observed at the boundary of the Si clathrate crystal film and the substrate, suggesting a chemical bonding between them. The crystalline strains defined here by the deviation of the d value were within 3.5%. These findings might allow us to fabricate epitaxial Si clathrate films on Si substrates.

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Complex changes in the framework of endohedrally Na-doped type II Si clathrates with respect to Na content

Cited by 16 publications

References 23 publications

Control of thermal expansion in a low-density framework modification of silicon

Control of thermal expansion in a low-density framework modification of silicon

First-Principles Study of the Electronic, Vibrational Properties and Anharmonic Effects of Some Si-Based Type-II Binary Clathrates

Cross-sectional transmission electron microscope observation of Si clathrate thin films grown on Si (111) substrates

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