S. Yamanaka

Preparation and electrochemical properties of layered lithium–cobalt–manganese oxides

Abstract The solid solutions Li(Co 1− x Li x /3 Mn 2 x /3 )O 2 were prepared over the whole compositional range between two kinds of layer structured compounds, LiCoO 2 and Li 2 MnO 3 . The performance of the solid solution as the cathode of the lithium cell was comparable with that of LiCoO 2 , although the discharge capacity decreased with the increase of x . The cyclic voltammetry of the Mn-substituted samples showed two anodic peaks at 3.9 and 4.4 V.

Novel synthesis of the layer structured β-ZrNCl by the direct reactions of zirconium metal or zirconium hydride with ammonium chloride

Abstract The layer structured β-ZrNCl can be synthesized in high yield by the direct reaction of zirconium metal or zirconium hydride with the vapor of ammonium chloride at temperatures ranging from 550 to 700°C. The as-prepared sample was contaminated with a small amount of oxide, and purified by chemical transport in a vacuum-sealed tube with a temperature gradient from 750 to 850°C. The sample transported to the higher temperature zone is highly crystalline and the structure is characterized by a rhombohedral stacking sequence of ZrNCl layers rather than a hexagonal random stacking sequence found in the as-prepared samples.

Chemical vapor transport of layer structured crystal β-ZrNCl

Abstract A layer structured compound β-ZrNCl is transported to a higher temperature zone with the aid of ammonium chloride as the transporting agent in the temperature range of 823–1173 K. The transport mechanism can be explained by the formation of a volatile compound (NH4)2ZrCl6: β− ZrNCl +5 NH 4 Cl →( NH 4 ) 2 ZrCl 6 +4 NH 3 . The measurements of the vapor pressure and the mass spectrum revealed that (NH4)2ZrCl6 decomposed congruently according to the equation: ( NH 4 ) 2 ZrCl 6 ( s )→ ZrCl 4 ( g )+2 NH 3 ( g )+2 HCl ( g ) The enthalpy change for the decomposition was determined to be 533 kJ/mol. By combining the above two equations, a simplified transport equation is derived: β− ZrNCl ( s )+3 HCl ( g )⇌ ZrCl 4 ( g + NH 3 ( g ) .

μSR studies of intercalated HfNCl superconductor

Abstract Muon spin relaxation measurements of the magnetic field penetration depth λ have been performed in a quasi two-dimensional (2-d) superconductor based on HfNCl intercalated with Li and organic molecules. The relaxation rate σ(T→0)∼0.5 μs −1 was obtained with transverse fields applied perpendicular to the aligned HfN planes. Using the relation σ∝λ −2 ∝n s2d /m ∗ (2-d superconducting carrier density/effective mass), we deduced the 2-d Fermi temperature TF2d. The ratio Tc/TF2d for the HfNCl-(Li) system is very close to those in two other quasi-2-d superconductors, YBa2Cu3O7 and (BEDT-TTF)2-X. Compared to the Kosterlitz–Thouless (KT) transition temperature TKT=TF2d/8 for the limit of strong coupling, Tcs of these systems are reduced by a factor of 3–4, indicating the importance of carrier overlapping and/or 3-d coupling.

Tunneling spectroscopy of MgB2 and Li0.5(THF)yHfNCl

Abstract Break-junction tunneling spectra have been measured on MgB2 and Li0.5(THF)yHfNCl to investigate the nature of the superconducting gap. The observed largest gap values at 4xa0K are 2Δ=18–20 and 11–12xa0meV for MgB2 (Tc=39xa0K) and Li0.5(THF)yHfNCl (26xa0K), respectively. These values lead to the similar ratio 2Δ/kBTc=5–6, which is extremely large in comparison with that of the conventional strong-coupling superconductors.

Electrochemical characteristics of transition-metal trichalcogenides in the secondary lithium battery

Abstract Transition-metal trichalcogenides have been investigated as cathode materials in the lithium electrochemical cell. Three lithium ions per TiS3 molecule have been found to be marginally reversible, while only one lithium is reversible in TaSe3. This difference may be related to exfoliation of these substances due to the progress of lithium intercalation.

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.

Tunneling spectroscopy of the electron-doped layered superconductor Li0.48(THF)0.3HfNCl

Abstract Tunneling measurements on the electron-doped layered superconductor Li 0.48 (THF) 0.3 HfNCl(THF;C 4 H 8 O) with T c =25.5 K have been carried out. Since the surface of this compound is very reactive, we have employed in situ break junction to obtain unaffected interface. The result shows the gap value of 2 Δ =9–12 meV, which leads to the strong-coupling gap ratio 2 Δ / k B T c ≃5. This is the largest value among non-cuprate superconductors.

Pressure effect and neutron scattering study on Ax HfNCl (A; alkali metals and organic molecules)

Abstract The transition temperature of Na x HfNCl ( T c max =20.0 K) is found to be lower than that of Li 0.5 THF γ HfNCl ( T c =24.0 K). Pressure effect on Na 0.3 HfNCl ( T c =19.4 K) is found to be small, i.e., d T c /d P =−0.013(6) K/kbar. Powder neutron inelastic scattering on Na 0.3 HfNCl shows the increase in S ( Q,E ) at anomalous high energy, i.e., about 30 meV, in comparison with 2Δ∼8 meV with decreasing temperature from 25 to 10 K.

Unusual ionic conductivities of hydrothermally prepared MZr2(PO4)3 (M = Na, K)

Abstract MZr 2 (PO 4 ) 3 (M=Na, K) were hydrothermally prepared from the aqueous mixtures of ZrO 2 , MH 2 PO 4 (M=Na, K) and H 3 PO 4 in a Teflon pressure bomb. The KZr 2 (PO 4 ) 3 sample precalcined below 1000°C showed an unusual temperature dependence of the ionic conductivity: the conductivity increased with decreasing temperature in the range of 320–220°C and then decreased again from 200–100°C. The NaZr 2 (PO 4 ) 3 sample also showed a similar behavior. The unusual temperature dependence of the conductivity can be interpreted in terms of the surface extremely sensitive to moisture and the presence of more than one kind of ionic carriers.