Shinichi Kikkawa

Superconductivity in the infinite-layer compound Sr1-xLaxCuO2 prepared under high pressure

Abstract A new superconductor Sr 1- x La x CuO 2 ( x =0.1) was prepared applying high pressure synthesis. It has an infinite-layer structure and its T c equals 43 K. It is probably an electron-doped superconductor because the reductive atmosphere was critical for its preparation under high pressure.

High- and low-temperature phases of lithium boron nitride, Li3BN2: Preparation, phase relation, crystal structure, and ionic conductivity

Abstract Low- (α) and high- (β) temperature phases of Li 3 BN 2 were prepared from mixtures of Li 3 N BN = 1.1 − 1.0 in molar ratio at 1070 and 1170 K, respectively. Phase relation between these phases was studied by annealing the products at various temperatures and conducting DTA in a stream of nitrogen. The phase transition temperature is at about 1135 K. The melting point of β-Li 3 BN 2 is around 1189 K. α-Li 3 BN 2 crystallizes directly from the undercooled liquid at 1160 K. The structure of α-Li 3 BN 2 , which is analyzed in the present study for the first time, has tetragonal symmetry, P 4 2 2 1 2, a = b = 4.6435(2), c = 5.2592(5), A, Z = 2, D calc = 1.747 Mg m −3 , μ = 0.082 mm −1 . The structure was determined by 208 unique X-ray reflections with F o > 3 σ ( F o ) and refined up to R = 0.042 by a full-matrix least-squares method. The lattice is composed of Li(1), Li(2), and linear (NBN) 3− ions [ r (BN) = 1.339(2), A]. The Li(1) ion is also linearly coordinated by two nitrogen atoms [ r (Li(1)N = 1.945(8), A]. The Li(2) ion is at the center of a tetrahedron of N atoms [ r (Li(2)N) = 2.125(18)A, δ(NLiN) = 103.6(2) and 112.5(9)°]. Lithium ion conductivity of 3 × 10 −5 S m −1 was measured at 400 K on a polycrystalline α-Li 3 BN 2 specimen with an activation energy of 78 kJ/mole.

Theoretical calculations on the structures, electronic and magnetic properties of binary 3d transition metal nitrides

The electronic structures of a number of binary 3d transition metal and iron nitrides, some of which still need to be synthesized, have been investigated by means of spin-polarized first principles band structure calculations (TB-LMTO-ASA). The chemical bonding in all compounds has been clarified in detail through the analysis of total and local densities-of-states (DOS) and crystal orbital Hamilton populations (COHP). The binary transition metal nitride set includes ScN, TiN, VN, CrN, MnN, FeN, CoN and NiN, both in the sodium chloride as well as in the zinc blende structure type. Antibonding metal-metal interactions for higher electron counts are significantly weaker in the zinc blende type, thus favoring this structural alternative for the later transition metal nitrides.

Formation and properties of n-alkylammonium complexes with layered tri- and tetra-titanates

Abstract The formation of n-alkylammonium complexes was studied using Na2Ti3O7 and K2Ti4O9 and the results for both compounds were compared. Alkylammonium complexes could be obtained from H2Ti3O7 and H2Ti4O9·H2O, which were prepared by HCl treatment of Na2Ti3O7 and K2Ti4O9 respectivel The complexes were formed by exchange of H+ with alkylammonium ions. Molecular intercalation of alkylamine was also possible with H2Ti4O9·H2O. However, alkylammonium complexes were not formed directly from Na2Ti3O7 and from K2Ti4O9. Orientations of alkylammonium ions in the interlayer are also discussed in relation to the structure of the titanate layers.

Preparation of lithium silicon nitrides and their lithium ion conductivity

Abstract Six single phases were prepared by reactions of Li3N and Si3N4 at 1075 K and 1475 K in nitrogen stream. Chemical analysis showed that they were ideally formulated as LiSi2N3, Li2SiN3, Li18Si3N10, Li21Si3N11 and Li8SiN4. Their X-ray diffraction patterns could be indexed with the following lattice parameters except phase II, Li2SiN2: phase I, LiSi2N3; orthorhombic, a=9.198(3), b=5.307(2), c=4.779(2); phase III, Li5SiN3; cubic, a=4.7240(3); phase IV, Li18Si3N10; tetragonal, =14.168(4), c=14.353(8); phase V, Li21Si3N11; tetragonal, a=9.470(3), c=9.530(8); phase VI, Li8SiN4; tetragonal a=10.217(2), c=9.536(3) A. The unit cell dimensions of phases III to VI were related to each other. All phases are lithium ion conductors. A new phase, Li8SiN4, has the highest lithium ion conductivity (5×10−2 Sm−1 at 400K) and the lowest activation energy (46kJ/mol) among the present products.

Energetics of binary iron nitrides

Abstract High-temperature solution calorimetry in molten sodium molybdate 3Na 2 O·4MoO 3 was used to determine the energetics of formation of a series of binary iron nitrides: γ′-Fe 4 N, e-Fe 3 N 1+ y ( y =0, 0.10, 0.22, 0.30, 0.33), ζ-Fe 2 N and γ′′-FeN 0.91 . The linear relation Δ H ° f (FeN x )=−65.23 x +13.48 kJ mol −1 was found between the enthalpies of formation from the elements at 298 K of iron nitrides FeN x and their nitrogen content x . Using this linear approximation, the enthalpy of formation of α′′-Fe 16 N 2 has been estimated to Δ H ° f (Fe 16 N 2 )=85.2±46.8 kJ mol −1 .

Deintercalated NaCoO2 and LicoO2

Abstract Na0.5CoO2 and Na0.6CoO2 were obtained by chemical deintercalations of NaCoO2 using bromine and iodine as oxidizing agents, respectively. The magnetic susceptibility of Na0.6CoO2 obeyed a Curie-Weiss law with Tθ = −550 K, μeff = 1.9 μB. This compound had a resistivity of about 1 Ωcm at room temperature with an activation energy of 0.01 eV. Na0.5CoO2 revealed an almost temperature-independent paramagnetism above 270 K. The susceptibility anomalously decreased below 270 K. Electro-chemically prepared Li0.5CoO2 showed similar behavior in its magnetic property. Na0.5CoO2 had a resistivity of about 10−1 Ωcm at room temperature and an activation energy less than 0.01 eV.

Topochemical reactions of LixNbO2

Intercalation and deintercalation of lithium in LixNbO2 were carried out by electrochemical and chemical methods. The hexagonal system for the compounds was retained over the whole range of x observed (0.51 < x < 0.93). The a parameter was increased with decrease of x, while the c parameter remained almost unchanged. This variation of the crystallographic parameters was explained by the deformation of NbO6 trigonal prisms caused by weakening of NbNb bonding in the (NbO2)n sheets. Temperature-independent paramagnetism on the order of 10−5 emu/mole and semiconductive behavior were observed for LixNbO2.

Structure of a new polymorph of lithium boron nitride, Li3BN2

Abstract A new polymorph of lithium boron nitride, Li3BN2, was synthesized by slow-cooling of a mixture of Li3N and BN from 1200 K. It has monoclinic symmetry, P2 1 c , a = 5.1502(2), b = 7.0824(2), c = 6.7908(2) A, β = 112.956(2)°, Z = 4, Dm = 1.74, Dcalcd = 1.737 g cm−3, μ = 0.082 mm−1. The structure was determined from 1352 unique X-ray reflections from a single crystal and refined to R = 0.023 by full-matrix least-squares method. Two kinds of layers alternate parallel to (100) in the structure. One layer includes Li and B atoms, and the other is composed of only N atoms. N(1) and N(2) are coordinated by six Li atoms and one B atom. Each Li atom is in a distorted tetrahedron of N atoms. Boron is linearly coordinated by two N atoms. The N(1)BN(2) bond angle is 179.12(4)°. The bond lengths of N(1)B and N(2)B are 1.3393(5) and 1.3361(5) A, respectively. Bonding electrons between boron and nitrogen atoms are clearly observed. Lithium ion conductivity of 6 × 10−5 Sm−1 was measured at 400 K on a polycrystalline sample and an activation energy was 64 kJ/mole.

Effect of intercalated alkylammonium on cation exchange properties of H2Ti3O7

Cation exchange was studied on alkylammonium intercalated H2Ti3O7 in comparison with H2Ti3O7 itself. Alkylammonium exchanged for proton in advance made it possible for alkali and alkaline earth metal ions to be taken up into H2Ti3O7 in chloride solution. Only alkylammonium was exchanged with hydrated alkali and alkaline earth metal ions. In hydroxide solution, proton was exchanged with alkali metal ions as well as alkylammonium ion. Dependence on pH and preference on ion exchange were affected by the previous uptake of alkylammonium into H2Ti3O7.