NMR and X-ray spectroscopy of sodium-silicon clathrates
NMR and X-ray spectroscopy of sodium-silicon clathrates
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DOI：
10.1021/jp010255e
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发表时间：
2001-05-03
影响因子：
3.3
通讯作者：
Tse, JS
中科院分区：
文献类型：
--
作者：
He, JL;Klug, DD;Tse, JS
本文详细研究了钠-硅包合物材料Na_8 Si_（46）和Na_xSi_（136）（1 < x < 24）以及母体Zintl相NaSi的Na ~（23）和Si ~（29）NMR谱，以期消除文献中的一些歧义，并确定这些潜在热电材料的性质。只有当笼形笼接近完全被Na占据时才能获得尖锐的光谱，从而实现晶体学对称性。Na在两种结构的小笼和大笼中的信号已明确分配。伪球形笼给出各向同性Na-23线，而其他笼产生一阶四极线形状。电场梯度从这些光谱，从头计算是在显着的协议。这两种核的大Knight位移都具有不寻常的温度依赖性，反映了导带中未成对电子密度分布的变化。由于Na含量的NaxSi 136是减少有急剧加宽的Na-23和Si-29光谱由于一个随机分布的空笼，因此环境，和Si光谱转移到较低的频率，表明在Si上的传导s-电子密度的减少。强度数据表明，从大笼子的钠的优先损失。XPS结果显示在包合物的表面上有二氧化硅的保护涂层。NMR、XPS和XANES结果都表明电子从Na转移到Si骨架，如通过计算预测的。XANES示出了相对于结晶Si的两种笼形物的吸收边的降低，并且因此导带的降低。
The Na-23 and Si-29 NMR spectra of the sodium-silicon clathrate materials Na8Si46 and NaxSi136 (1 < x < 24) and the parent Zintl phase NaSi have been studied in detail with a view to ironing out a number of ambiguities in the published literature and to determining properties of these potential thermoelectric materials. Sharp spectra are obtained only when the clathrate cages are close to fully occupied by Na, so that crystallographic symmetry is achieved. Signals from Na in the small and large cages of both structures have been unequivocally assigned. The pseudospherical cages give isotropic Na-23 lines, whereas the other cages produce first-order quadrupolar line shapes. Electric field gradients derived from these spectra, and ab initio calculations are in remarkable agreement. The large Knight shifts of both types of nuclei have rather unusual temperature dependences, which reflect the changing distributions of unpaired electron density in the conduction bands. As the Na content of NaxSi136 is reduced there is drastic broadening of the Na-23 and Si-29 spectra due to a random distribution of vacant cages and hence of environments, and the Si spectra shift to lower frequencies, indicating a reduction in conduction s-electron density on the Si. Intensity data suggest a preferential loss of Na from the large cages. XPS results show a protective coating of silica on the surface of the clathrates. NMR,XPS, and XANES results all indicate transfer of electrons from Na to the Si framework, as predicted by calculations. XANES shows a lowering of the absorption edge, and hence of the conduction band, of both clathrates relative to crystalline Si.