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Materials 2016, 9(8), 691; doi:10.3390/ma9080691

Effect of Guest Atom Composition on the Structural and Vibrational Properties of the Type II Clathrate-Based Materials AxSi136, AxGe136 and AxSn136 (A = Na, K, Rb, Cs; 0 ≤ x ≤ 24)

Department of Physics, Texas Tech University, Lubbock, TX 79409-1051, USA
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Author to whom correspondence should be addressed.
Academic Editors: Matt Beekman and Yuri Grin
Received: 1 July 2016 / Revised: 3 August 2016 / Accepted: 5 August 2016 / Published: 11 August 2016
(This article belongs to the Special Issue Inorganic Clathrate Materials)
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Abstract

Type II clathrates are interesting due to their potential thermoelectric applications. Powdered X-ray diffraction (XRD) data and density functional calculations for NaxSi136 found a lattice contraction as x increases for 0 < x < 8 and an expansion as x increases for x > 8. This is explained by XRD data that shows that as x increases, the Si28 cages are filled first for x < 8 and the Si20 cages are then filled for x > 8. Motivated by this work, here we report the results of first-principles calculations of the structural and vibrational properties of the Type II clathrate compounds AxSi136, AxGe136, and AxSn136. We present results for the variation of the lattice constants, bulk moduli, and other structural parameters with x. These are contrasted for the Si, Ge, and Sn compounds and for guests A = Na, K, Rb, and Cs. We also present calculated results of phonon dispersion relations for Na4Si136, Na4Ge136, and Na4Sn136 and we compare these for the three materials. Finally, we present calculated results for the elastic constants in NaxSi136, NaxGe136, and NaxSn136 for x = 4 and 8. These are compared for the three hosts, as well as for the two compositions. View Full-Text
Keywords: clathrates; type II-structure; guests; alkali atoms; Group 14; first principles theory; VASP; phonon; elastic constants; PACS; 63.20dk; 63.20Pw; 61.50Ah; 61.66Fn clathrates; type II-structure; guests; alkali atoms; Group 14; first principles theory; VASP; phonon; elastic constants; PACS; 63.20dk; 63.20Pw; 61.50Ah; 61.66Fn
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Xue, D.; Myles, C.W.; Higgins, C. Effect of Guest Atom Composition on the Structural and Vibrational Properties of the Type II Clathrate-Based Materials AxSi136, AxGe136 and AxSn136 (A = Na, K, Rb, Cs; 0 ≤ x ≤ 24). Materials 2016, 9, 691.

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