Molecular-dynamics studies on solid phase epitaxy of guest-free silicon clathrates
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Molecular-dynamics studies on solid phase epitaxy of guest-free silicon clathrates Shinji Munetoh, Koji Moriguchi, Teruaki Motooka1, and Kazuhito Kamei Electronics Engineering Laboratories, Sumitomo Metal Industries, Ltd., 1-8 Fusocho, Amagasaki, Hyogo 660-0891, Japan 1
Department of Materials Science and Engineering, Kyushu University,
Hakozaki, Fukuoka 812-8581, Japan ABSTRACT Dynamical phenomena during the solid phase epitaxy (SPE) of guest-free Si clathrates (Si34 and Si46) via molecular-dynamics (MD) simulations using the Tersoff potential have been reported. The activation energy of SPE for Si34 has been found to correspond with the experimental value for the cubic diamond phase (c-Si; approximately 2.7eV), while the SPE rates of Si46 are much lower than that of c-Si. The structural transition from Si46 (type-I) to Si34 (type-II) can be also observed during the Si46 [001] SPE. The present results suggest that it is worthwhile to intensify experimental studies concerning crystal growth techniques of clathrate materials and these interesting Si forms may open up a new field in "silicon technologies". INTRODUCTION Silicon clathrates are fullerene-like solids made of large silicon cages that can encapsulate guest atoms, but all of the cages are tightly joined and their constituent atoms are tetrahedrally bonded. "Silicon technologies" have been a classic undifferentiated commodity - "industrial rice," to use the Japanese phrase. In such "silicon technologies", these clathrate forms have also attracted considerable attention theoretically and experimentally. While two forms of clathrates such as Si46 and Si34 (often referred to as Si136) have been known for many years [1], two experimental phenomena recently confirmed in the clathrates have triggered intensive investigation. One is the discovery of superconductivity in metal-doped silicon clathrates, BaxNaySi46 [2], and the other is the potential of clathrates for thermoelectric applications, where the figure of merit can be made arbitrarily large with the "phonon glass and electron crystal (PGEC)" mechanism [3]. In addition, the pioneering theoretical study by Adams et al. [4] suggests that guest-free Si46 and Si34 could be technologically important opto-electronic materials if experimentally realized. G14.5.1
Clathrate materials are classified as "functional crystals", in which each physical stage for the functions essentially originates in the crystal structure and the chemical composition. Focusing on the electrical transporting properties, the electronic band modification of the framework states by the guest atoms plays a crucial role for the carrier properties both in thermoelectrics [5] and superconductivity [6, 7]. Thermal conduction also occurs via framework atoms in the form of lattice vibrations, however, comparatively large motions of the guest atoms in the endohedral sites, or "rattling" motions, interfere with the framework vibrations and result in anomalous glassy behaviors of certain compositions [8, 9]. Since the concept of "PGEC" materials sug
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