Na-Si Clathrates Are High-Pressure Phases: A Melt-Based Route to Control Stoichiometry and Properties
Na-Si Clathrates Are High-Pressure Phases: A Melt-Based Route to Control Stoichiometry and Properties
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DOI：
10.1021/cg3017084
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发表时间：
2013-01-01
影响因子：
3.8
通讯作者：
Struzhkin, Viktor V.
中科院分区：
文献类型：
--
作者：
Kurakevych, Oleksandr O.;Strobel, Timothy A.;Struzhkin, Viktor V.
利用高压技术首次获得了三种不同的钠硅笼形化合物Na_8Si_46（sI）、Na_24Si_136（sII）和一种新的结构NaSi_6。实验和理论结果明确地表明，钠插层笼形物仅在高压条件下是热稳定的。在900 - 1100 K范围内，在2 - 6 GPa的压力下，可由元素直接合成sI包合物。在所研究的条件范围内，sII包合物仅在sI结晶之前作为中间化合物形成。在较高的压力下，我们观察到形成一个新的插层化合物，金属NaSi6，它在正交晶系Eu4Ga8Ge 16结构中结晶。与通过常规途径获得的单晶的典型特性（RRR <6）相比，从Na-Si熔体的高压结晶提供了本体笼形物材料的电性能的显著改善（对于sI，剩余电阻比RRR = 24，对于NaSi6，剩余电阻比RRR> 13）。由于Na-Si包合物仅在2GPa以上才稳定，因此先前关于其合成的报道可被视为在低压条件下以非平衡、基于质子交换剂的途径合成高压相。
Three different sodium-silicon clathrate compounds-Na8Si46 (sI), Na24Si136 (sII), and a new structure, NaSi6-were 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 900-1100 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.