Silicon Clathrate Compounds Research Articles

NMR in the silicon clathrate compounds Na xBa ySi 46 and Na xSi 136

29Si, 23Na, 137Ba, and 135Ba NMR experiments were carried out in the superconducting silicon clathrate compound Na xBa ySi 46 to study the electronic states above the superconducting transition temperature. At all the atomic sites, considerable Knight shift and the Korringa relation in NMR relaxation were found, which indicates metallic electronic structure with moderate electronic correlation. The conduction-electron spin/charge density was found to be strongly site-dependent. The Na atoms are partially ionized as well as the Ba atoms, which shows noticeable difference from metal-doped fullerides. The results of comparative NMR experiments with non-superconducting clathrate compound Na xSi 136 suggested that the existence of the Si site with remarkably large Knight shift in Na xBa ySi 46 has some relevance to the superconductivity.

Structures and Properties of Semiconductor Microclusters. Syntheses, Structures and Properties of Silicon Clathrate Compounds and Layer Structured Silicon.

Silicon clathrate compounds are Si analogs of carbon cluster C60; those consist of polyhedral cages of Si20 and Si24, which are linked three dimensionally by sharing faces. The Barium containing silicon clathrate compound, NaxBaySi46, shows superconductivity below 4 K. This is the first superconductor with a sp3-Si network. Silicon clathrate compounds are prepared from Zintl phases containing [Si4]4- cluster anions. Another kind of Zintl phase, CaSi2 contains corrugated Si layers with Ca atoms between them. A layer structured polysilane (SiH)n was derived from CaSi2 by removing Ca in a concentrated HCl solution at -30°C. The polysilane is a semiconductor with a band ga of about 2.5 eV.

Preparation and Superconductivity of Silicon Clathrate Compounds

Preparation and Superconductivity of Silicon Clathrate Compounds

Composition dependence of the superconducting transition temperature of silicon clathrate compound Na xBa 6Si 46

The barium containing silicon clathrate superconductors Na x Ba 6Si 46 with various Na contents (0.2 < x < 1.5) were prepared by evaporation of Na from Na 2Ba 6Si 46 at elevated temperatures. All of the samples showed superconductivity. The transition temperature increased with the decrease of Na content in a temperature range of 4.8–2.6 K.

NMR study in the superconducting silicon clathrate compound NaxBaySi46.

Si, Na, Ba, and Ba NMR experiments were carried out in the superconducting silicon clathrate compound NaxBaySi46 to study the electronic states above the superconducting transition temperature. We observed three distinct Si signals with different Knight shift ~2036, 862, and 720 ppm at 90 K with a small temperature dependence of ;10%!, which were ascribed to inequivalent Si sites in the Si46 structure. The results indicate that the conduction-electron spin and/or charge density is strongly site-dependent. At all the atomic sites, the Korringa relation in NMR relaxation was observed, indicative of metallic electronic structure with moderate electronic correlation. The Na and Ba atoms are partially ionized in NaxBaySi46 , which shows the noticeable difference from the system of alkali-metal-doped fullerides. The comparison with band calculations is discussed. @S0163-1829~96!06042-0#

NMR studies of Na atoms in silicon clathrate compounds.

We observed the NMR spectrum of $^{23}\mathrm{Na}$ in the semimetallic clathrate (Na,Ba${)}_{\mathit{x}}$${\mathrm{Si}}_{46}$, which becomes superconducting at low temperatures (4 K). We also obtained spectra for isolated sodium atoms encapsulated in cages in the insulating clathrate ${\mathrm{Na}}_{\mathit{x}}$@${\mathrm{Si}}_{136}$. The observed NMR shifts in this phase are almost two times larger than the ordinary Knight shift in metallic sodium. We attribute this large paramagnetic shift to the fact that sodium atoms inside silicon clathrate cages are not ionized. Due to the interaction of unpaired sodium 3s electrons with silicon nuclei, we also observed very large shifts in the NMR spectrum of $^{29}\mathrm{Si}$. \textcopyright{} 1996 The American Physical Society.

ChemInform Abstract: Preparation and Superconductivity of Barium Containing Silicon Clathrate Compound, (Ba, K)xSi46.

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An ESR study on the thermal electron excitation of a sodium atom incorporated in a silicon clathrate compound

An ESR study was carried out for sodium-containing silicon clathrate compounds, Na x Si 136 with 3 ⩽ × ⩽10. For Na 3Si 136 a quartet with an isotropic hf splitting of 13.3 mT was observed together with the weak septet of 6.7 mT at 4 K; the former and the latter being attributed to the Na atom and the dimer cation Na 2 +, respectively. The temperature-dependent ESR spectral intensity of the Na atom was observed over a wide temperature range from 4 to 200 K. A deviation from the paramagnetic Curie law was observed in the higher temperature region and is discussed in terms of a thermal electron excitation from the Na impurity level to the conduction band of the silicon clathrate compound.

Superconductivity in the Silicon Clathrate Compound (Na,Ba)xSi46

A new silicon clathrate compound containing barium, (Na,Ba) ${}_{x}$S${\mathrm{i}}_{46}$, becomes a type-II superconductor with a critical temperature $({T}_{\mathrm{c}})$ of about 4 K. In the compound, the silicon atoms are bonded tetrahedrally with about the same bond distances as in ordinary cubic diamond Si, but form 12- and 14-hedral cages which are linked by shared faces. The metal atoms are located in the center of the polyhedra. This is the first superconductor consisting of a Si ${\mathrm{sp}}^{3}$ covalent network.

Preparation of Barium-Containing Silicon Clathrate Compound

ABSTRACT A new silicon clathrate compound containing Ba was prepared by the thermal degradation of a solid solution (Na2Ba)Si4 at 500°C in vacuum. The compound has the Na8Si46 clathrate type structure, which consists of silicon tetrahedral networks having two types of cages, pentagonal dodecahedra and 14-face polyhedra. The Ba atoms are placed only in the larger cages.

Preapration of barium-containing silicon clathrate compound

Preapration of barium-containing silicon clathrate compound