Hitoshi Kawaji

Superconductivity at 25.5 K in electron-doped layered hafnium nitride

The electronic properties of crystals with a layered structure can be radically altered by the intercalation, between the layers, of guest species that act as electron donors or acceptors. Such studies have been performed extensively on graphite, transition-metal dichalcogenides and oxide bronzes. Interest in redox intercalation reactions has increased recently because the high-transition-temperature (high-Tc) superconductors based on copper oxide also have layered structures, the superconductivity occurring within two-dimensional CuO2 planes separated by charge-reservoir oxide layers. Similarly, in metal-doped fullerenes, which show relatively high transition temperatures, the electron donor atoms sit in the interstitial sites between adjacent fullerene balls. In a previous study, we described a layered nitride, β-ZrNCl, consisting of Zr–N double layers sandwiched between two close-packed chlorine layers. On lithium intercalation, the crystal changed from a semiconductor to a metal, and became a superconductor at 13u2009K. Here we report the properties of the isostructural compound β-HfNCl. After electron-doping the crystal by lithium intercalation, we observe superconductivity with a Tc of up to 25.5u2009K. This transition temperature is higher than that observed in any intermetallic compound, and suggests that layered nitride structures may offer transition temperatures comparable to those observed in layered copper oxide structures.

Substitution and cointercalation effects on superconducting electron-doped layer structured metal nitride halides

Abstract The effects of substitution of halogens and metals in β-MNX (M = Zr, Hf; X = Cl, Br, I) on the superconductivity of the electron-doped compounds were studied. The onset temperature ( Tc ) was mainly determined by the type of nitride layers (ZrN or HfN), and varied linearly with the compositions between the Tc s of the end members. The post-cointercalation of propylene carbonate molecules with Na greatly expanded the spacing between the nitride layers without changing the doping level and the two-dimensional structure. The cointercalation phase had irreversible temperature in the magnetization curves, and showed an anisotropic effect on Tc and the magnetic susceptibility.

Composition dependence of the superconducting transition temperature of silicon clathrate compound NaxBa6Si46

The barium containing silicon clathrate superconductors NaxBa6Si46 with various Na contents (0.2 < x < 1.5) were prepared by evaporation of Na from Na2Ba6Si46 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 in the silicon clathrate compounds NaxBaySi46 and NaxSi136

29Si, 23 Na, 137 Ba, and 135 Ba NMR experiments were carried out in the superconducting silicon clathrate compound Na x B y Si 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 Be atoms, which shows noticeable difference from metal-doped fullerides. The results of comparative NMR experiments with non-superconducting clathrate compound Na x Si 136 suggested that the existence of the Si site with remarkably large Knight shift in Na x B y Si 46 has some relevance to the superconductivity.

1H-NMR STUDY OF HYDROGEN INTERCALATED IN β-ZIRCONIUM NITRIDE CHLORIDE

Abstract Hydrogen is taken up by a layer structured compound β-ZrNCl during its chemical vapor transport with NH4Cl in a sealed silica tube. The transported sample can be further hydrogenated by hydrogen spillover, and dehydrogenated by oxidation with Cl2 and KMnO4. The infrared absorption spectra indicate that the hydrogen is taken up in the double ZrN layer of β-ZrNCl, coordinating to a nitrogen atom. The second moments of 1H-NMR spectra suggest the formation of hydrogen pairs in β-ZrNCl. A motional line narrowing of the 1H-NMR spectrum was observed at temperatures around 200 K. The activation energy for the jumping motion of the hydrogen was determined to be 10.6 kJ/mol from the temperature dependence of the spin-lattice relaxation time. A possible structural model for the location of hydrogen was proposed.

NMR Studies of Silicon Clathrate Compounds

Abstract NMR spectra of 29Si and 137Ba in silicon clathrate compounds, NaxBaySi46, NaxSi136 and Ba8 T xSi46 (where T = Au, Ag, and Cu) are reported. We found that the local density of states at the Fermi level increases with rising superconducting transition temperature. The silicon Sl site was suggested to hybridize with Ba-orbital in the superconducting clathrate and has a large local density of states.

Superconductivity in Electron-doped Layered Metal Nitrides

New high-transition temperature (high-Tc) superconductors based on layered metal nitrides have been developed. Layer structured crystals s-MNCl (M = Zr, Hf) have the SmSI type structure; the M-N double layers occupy the SmS positions and are sandwiched between the close-packed chlorine layers. The layers with a sequence of [Cl-M-N-N-M-Cl] are stacked with each other by a weak van der Waals interaction. The crystals are characterized as semiconductors with a band gap of larger than 3 eV. The nitride layers can be electron-doped by intercalation of lithium between the chloride layers. On intercalation the layers gave a metallic behavior and became superconductors with Tc as high as 13 and 25.5 K for s-ZrNCl and s-HfNCl, respectively.

Preparation and Superconductivity of New Silicon Clathrate Compounds

New silicon clathrate compounds containing barium with alkali metal, (Ba, M)xSi46 (M = Na, K) have been prepared from ternary Zintl phases (M2, Ba)Si4 consisting of isolated [Si4]4- cluster anions. On removal of a part of the alkali metal by evaporation at elevated temperatures, the clusters are linked three-dimensionally and the Zintl phase is segregated into a mixture of BaSi2 and a Na8Si46 type clathrate phase. BaSi2 can be removed by dissolving with water. The chemical analysis data showed that the compositions of the clathrates were close to the ideal composition of Ba6M2Si46 (M = Na, K). The silicon clathrate compound is composed of a Si-sp 3 open network having two types of cages, Si20 pentagonal dodecahedra and Si24 tetrakaidecahedra; the two types of polyhedra are linked by shared faces. The X-ray Rietveld analysis revealed that the alkali metal atoms mainly occupied the former cages and the barium atoms were in the latter cages. The barium-containing clathrate compounds became type II superconductors with Tc of about 4 K. Diamagnetism due to superconductivity was observed in good agreement with the resistivity measurements. This is the first superconductor found for a Si-sp 3 covalent network.