Transport Properties of Type II Sodium-Silicon Clathrates
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Transport Properties of Type II Sodium-Silicon Clathrates M. Beekman,1 J. Gryko,2 and G. S. Nolas1 1 Department of Physics, University of South Florida, Tampa, FL 33620 2 Department of Physical and Earth Sciences, Jacksonville State University, Jacksonville, AL 36265 ABSTRACT We have synthesized the type II silicon clathrates Na1Si136 and Na8Si136, and report on the electrical and thermal transport in these materials. The crystal structure consists of a covalently bonded silicon framework in which sodium guest atoms are encapsulated inside the silicon host framework. Differential scanning calorimetry measurements show the compounds decompose above 600oC to diamond-structure silicon. Temperature dependant electrical resistivity measurements show the specimens to have an insulating character, with magnitudes that decrease with increasing sodium content. For the first time, thermal conductivity measurements on type II sodium-silicon clathrates are presented. The thermal conductivity is very low for both specimens, and for Na8Si136 exhibits a clear dip in the range from 50 to 70 K. These data suggest that the “rattling” behavior observed in type I clathrates may also be present in type II clathrates.
INTRODUCTION Clathrate materials are currently attracting attention in fields as diverse as thermoelectrics, [1] superconductivity, [2] and photovoltaics. The general characteristic of these materials is a covalently bonded framework in which “guest” atoms are inside polyhedra formed by the framework. This unique structural arrangement results in a wide range of interesting physical properties, including a glasslike thermal conductivity that clathrates such as Sr8Ga16Ge30 and Eu8Ga16Ge30 possess. [3, 4] In these materials, the localized vibrational modes of the guest atoms can resonantly scatter the framework heat carrying acoustic phonons, resulting in very low thermal conductivity. Coupled with the good semiconducting properties that some clathrates possess, [5] these materials continue to attract attention as potential thermoelectric materials. Silicon-based clathrates such as Si136 possess wide optical band gaps [6, 7] and offer new direction in the search for novel optoelectronic and/or photovoltaic materials. The present work is part of an ongoing investigation into the potential that type II clathrates may possess for technological applications. The type II clathrates crystallize with the space group Fd 3 m , and consist of a covalently bonded framework of typically Group IV atoms. The 136 framework atoms per conventional unit cell reside at three crystallographically distinct sites, 6a, 32e, and 96g. This framework forms polyhedral atomic “cages,” eight larger hexakaidecahedra and 16 smaller dodecahedra per conventional unit cell. “Guest” atoms, typically alkali species, can then be encaged inside these polyhedra at the 8b and 16c sites. Thus type II clathrates may be depicted by a general chemical formula of A8B16E136, where A and B represent the guest atoms in the larger and smaller cages, respecti
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