Na-Si Clathrates Are High-Pressure Phases: A Melt-Based Route to Control Stoichiometry and Properties

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• 作者：Oleksandr O. Kurakevych ; Timothy A. Strobel ; Duck Young Kim ; Takaki Muramatsu ; Viktor V. Struzhkin
• 刊名：Crystal Growth & Design
• 出版年：2013
• 出版时间：January 2, 2013
• 年：2013
• 卷：13
• 期：1
• 页码：303-307
• 全文大小：298K
• 年卷期：v.13,no.1(January 2, 2013)
• ISSN：1528-7505

文摘

Three different sodium-silicon clathrate compounds鈥揘a₈Si₄₆ (sI), Na₂₄Si₁₃₆ (sII), and a new structure, NaSi₆鈥搘ere obtained for the first time using high-pressure techniques. Experimental and theoretical results unambiguously indicate that Na-intercalated clathrates are only thermodynamically stable under high-pressure conditions. The sI clathrate can be synthesized directly from the elements at pressures from 2 to 6 GPa in the 900鈥?100 K range. Over the range of conditions studied, sII clathrate only forms as an intermediate compound prior to the crystallization of sI. At higher pressures, we observed the formation of a new intercalated compound, metallic NaSi₆, which crystallizes in the orthorhombic Eu₄Ga₈Ge₁₆ structure. High-pressure crystallization from Na-Si melts provides significant improvements in the electrical properties of bulk clathrate materials (residual resistance ratio RRR = 24 for sI and > 13 for NaSi₆), compared to the typical characteristics achieved for single crystals obtained by conventional routes (RRR < 6). Since the Na-Si clathrates are stable only above 2 GPa, previous reports of their synthesis may be viewed as nonequilibrium, precursor-based routes to high-pressure phases at low-pressure conditions.