Yoshito Gotoh

Superconductivity in Novel BiS2-Based Layered Superconductor LaO1-xFxBiS2

Layered superconductors have provided some interesting fields in condensed matter physics owing to the low dimensionality of their electronic states. For example, the high-Tc (high transition temperature) cuprates and the Fe-based superconductors possess a layered crystal structure composed of a stacking of spacer (blocking) layers and conduction (superconducting) layers, CuO2 planes or Fe-Anion layers. The spacer layers provide carriers to the conduction layers and induce exotic superconductivity. Recently, we have reported superconductivity in the novel BiS2-based layered compound Bi4O4S3. It was found that superconductivity of Bi4O4S3 originates from the BiS2 layers. The crystal structure is composed of a stacking of BiS2 superconducting layers and the spacer layers, which resembles those of high-Tc cuprate and the Fe-based superconductors. Here we report a discovery of a new type of BiS2-based layered superconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K.

Pressure-Induced Enhancement of Superconductivity and Structural Transition in BiS2-Layered LaO1¹xFxBiS2

BiS2-based LaO1−xFxBiS2 (x = 0.5) becomes superconductive at Tc = 2.5 K. Electrical resistivity and magnetization measurements are performed under pressure to determine the pressure dependence of the superconducting transition temperature Tc up to 18 GPa. We observe that Tc abruptly increases from 2.5 to 10.7 K at a pressure of 0.7 GPa. According to high-pressure X-ray diffraction measurements of LaO1−xFxBiS2 (x = 0 and 0.5), a structural phase transition from tetragonal to monoclinic also occurs at ∼0.8 GPa. We consider that the pressure-induced enhancement of superconductivity for LaO1−xFxBiS2 is caused by the structural phase transition.

A new layered iron arsenide superconductor: (Ca,Pr)FeAs2.

A new iron-based superconductor, (Ca,Pr)FeAs2, was discovered. Plate-like crystals of the new phase were obtained, and its crystal structure was investigated by single-crystal X-ray diffraction analysis. The structure was identified as the monoclinic system with space group P2₁/m, composed of two Ca(Pr) planes, Fe2As2 layers, and As2 zigzag chain layers. Plate-like crystals of the new phase showed superconductivity, with a T(c) of ~20 K in both magnetization and resistivity measurements.

New-Structure-Type Fe-Based Superconductors: CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs)

Fe-based superconductors have attracted research interest because of their rich structural variety, which is due to their layered crystal structures. Here we report the new-structure-type Fe-based superconductors CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs), which can be regarded as hybrid phases between AeFe2As2 (Ae = Ca, Sr) and AFe2As2. Unlike solid solutions such as (Ba(1-x)K(x))Fe2As2 and (Sr(1-x)Na(x))Fe2As2, Ae and A do not occupy crystallographically equivalent sites because of the large differences between their ionic radii. Rather, the Ae and A layers are inserted alternately between the Fe2As2 layers in the c-axis direction in AeAFe4As4 (AeA1144). The ordering of the Ae and A layers causes a change in the space group from I4/mmm to P4/mmm, which is clearly apparent in powder X-ray diffraction patterns. AeA1144 is the first known structure of this type among not only Fe-based superconductors but also other materials. AeA1144 is formed as a line compound, and therefore, each AeA1144 has its own superconducting transition temperature of approximately 31-36 K.

Soft-Chemical Synthesis and Electrochemical Property of H2Ti12O25 as a Negative Electrode Material for Rechargeable Lithium-Ion Batteries

Upon heating the as-prepared H 2 Ti 3 O 7 sample, single-phase samples of H 2 Ti 6 O 13 , H 2 Ti 12 O 25 and TiO 2 (B) were prepared with different heating conditions; at 120°C for 48 h in a vacuum of 20 Pa for H 2 Ti 6 O 13 , at 260°C for 5 h in air for H 2 Ti 12 OP 25 , and at 350°C for 5 h in air for TiO 2 (B). The chemical compositions of these compounds were determined by the DTA-TG and ICP analyses. The coordination environments around hydrogen atoms in these compounds were examined by using FTIR and 1 H-MAS-NMR data. The reversible Li insertion and extraction reactions around 1.5-1.6 V with initial intercalation capacities of 283 and 236 mAh g -1 , were observed for H 2 Ti 6 O 13 and H 2 Ti 12 O 25 , respectively. The structural changes upon cycling were clarified for H 2 Ti 3 O 7 and TiO 2 (B). Among the four compounds, H 2 Ti 12 O 25 showed superior cycling performance with the discharge capacity retention rate of 96% after 50 cycles. H 2 Ti 12 O 25 is suggested to be one of the promising high-voltage oxide negative electrodes in advanced lithium-ion batteries.

Three Types of Ternary Selenides with Layered Composite Crystal Structures Formed in the Pb-Nb-Se System

Three types of ternary selenides with layered composite crystal structures, (PbSe)1.14NbSe2[A], (PbSe)1.14(NbSe2)2[B] and (PbSe)1.14(NbSe2)3[C], have been prepared and characterized by powder X-ray diffraction, electron diffraction, intercalation and electric resistivity measurement. Results suggest that each selenide consists of PbSe layers(two-atoms-thick) and NbSe2 sandwiches(three-atoms-thick) stacked alternately with the ratios of 1:1 for A, 1:2 for B and 1:3 for C, respectively. Electric resistivity measurements suggest that B and C are superconductors whose transition temperatures are 3.4 K and 4.8 K, respectively.

Crystal chemistry of CuBa2Can−1CunOy( n = 4, 5, 6) superconductors

Abstract The crystal structures of Cu-based superconductors CuxBa2Can−1CunOy and (Cu,C)Ba2Can−1CunOy with n = 4, 5, 6 have been investigated using intensity data collected on the single-crystal X-ray four-circle diffractometer. The Cu-site occupancy x in the deficient Cu(1)O2 sheet between two BaO layers increases from 0.62 to 0.73 accompanied by a decrease in the c-axis length in CuxBa2Ca3Cu4Oy. The thickness of the charge reservoir-layer block decreases with decreasing c-axis length from 8.421(4) A for c = 18.0016(39) A to 8.373(3) A for c= 17.9434(8) A in CuBa2Ca3Cu4Oy, and from 8.405(2) A for c = 21.1546(14) A to 8.34(1) A for c = 21.0784(15) A in CuBa2Ca4Cu5Oy, respectively, while the thickness of the superconducting-layer block unchanged in both series. From the results of the structure refinement and chemical analysis, we propose a general formula (CuxC1−x−δ)Ba2Can−1CunOy for these nonstoichiometric compounds, not only for the CuxBa2Can−1CunOy series but also for the carbonate-containing (Cu,C)Ba2Can−1CunOy compounds, where δ equals the vacancy content in the defect site.

Single crystal growth of the spinel-type LiMn2O4

Single crystals of the spinel-type LiMn 2 O 4 have been successfully grown by a solvent evaporation flux method at 1173 K. The maximum size of the octahedral-shaped single crystal is 0.09 x 0.09 x 0.09 mm 3 along the octahedral edges. The single-crystal X-ray diffraction study confirmed the cubic Fd3m space group and the lattice parameter of a = 8.2483(6) A of the as-grown single crystal at 297K. The preliminary single-crystal low temperature X-ray diffraction experiments confirmed the cubic-orthorhombic structure transition around 285 K on the cooling process. The tripled periodicity toward the a- and b-axis directions of the low temperature form has been also confirmed.

Preparation of LiTi2O4 single crystals with the spinel structure

Single crystals of the spinel-type LiTi{sub 2}O{sub 4} (Cubic, Fd3m, a = 8.4095(12) {angstrom}) have been prepared by the reaction of lithium metal and TiO{sub 2} at 1373 K in a sealed iron vessel. The structure refinement by a single crystal X-ray diffraction study confirmed a normal spinel cation distribution; the lithium cations are located at the tetrahedral 8a sites, the titanium cations occupy the octahedral 16d sites, and the oxygen parameter u = 0.2622(4) with the final R = 3.8% (R{sub w} = 2.7%) for 60 independent observed reflections. The octahedral Ti-O distance is 2.005(3) {angstrom}, and the tetrahedral Li-O distance is 1.999(3) {angstrom}. The superconductivity for the crystals was observed by the magnetization measurements.

Superconductivity in Fe-Based Compound EuAFe4As4 (A = Rb and Cs)

We report the discovery of a novel Fe-based superconductor EuAFe4As4 (A = Rb, Cs) and describe its superconducting properties. EuAFe4As4 has a tetragonal unit cell with a P4/mmm (No. 123) space group, indicating that this material is an 1144-type compound. The magnetic susceptibility and electrical resistivity indicate superconducting transitions at approximately 36 and 35 K for EuRbFe4As4 and EuCsFe4As4, respectively. Moreover, an anomalous magnetic transition appears at approximately 15 K, suggesting the coexistence of superconductivity and a magnetic ordered state formed by the Eu2+ ions. The determined upper critical magnetic fields and coherence lengths are approximately 920 kOe and 1.8 nm for EuRbFe4As4 and 875 kOe and 1.9 nm for EuCsFe4As4, respectively.