Tetsuro Nakamura

Dielectric properties of a-site deficient perovskite-type lanthanum-calcium-titanium oxide solid solution system [(1 − x)La23TiO3 − xCaTiO3 (0.1 ≤ x ≤ 0.96)]

Solid solution system of the double perovskite La23TiO3 and the GdFeO3-type perovskite CaTiO3, (1 − x)CaTiO3 — xLa23TiO3, has been prepared to investigate the crystal structure and the dielectric properties. In this system, the structure changed from pseudo-cubic perovskite to tetragonal double perovskite at x ≈ 0.7, and to orthorhombic double perovskite at x ≈ 0.9. Microwave dielectric measurement showed a decrease in dielectric constant and temperature coefficient of the resonant frequency and an increase in Q value with increasing x. The sample with x = 0.96 showed excellent microwave characteristics, especially high Q(2700) along with high er(90) at 10 GHz. In the low frequency dielectric measurement, dielectric relaxation could be observed in the composition range x ≤ 0.3. The dielectric relaxation showed strikingly different behaviors depending on the annealing process in oxygen after sintering.

Lithium ion conductivity in the perovskite-type LiTaO3-SrTiO3 solid solution

The lithium ion conductivity in the perovskite-type xLiTa0 3 -(1 - x)SrTiO 3 solid solution (0.30 ≤ x ≤ 0.50) has been investigated. The highest ionic conductivity of 5.5 X 10 -4 S cm -1 at 300 K appears at x = 0.50. The ion conductivity decreases with a decrease in x and then rapidly decreases in the vicinity of x = 0.30. This threshold for the ion conductivity is the same as the site percolation threshold for a simple cubic lattice. This indicates that the lithium ion conduction occurs three-dimensionally among the A-sites of the perovskite.

Determination of ionic valency pairs via lattice constants in ordered perovskites (ALa)(Mn2+Mo5+)O6 (A = Ba, Sr, Ca) with applications to (ALa)(Fe3+Mo4+)O6, Ba2(Bi3+Bi5+)O6 and Ba2(Bi3+Sb5+)O6

Assuming the lattice constants of perovskite-type compounds (AA′)(BB′)O6 where (AA′ = BaLa, SrLa, CaLa) to be a linear function of the ionic radii of B and B′ ions, the ordered valency pair (Mn2+Mo5+) in (AA′)(MnMo)O6 was verified from the lattice constants of a series of compounds (AA′)(BB′)O6 with known valency pairs (B,B′) = (Mn2+Ta5+), (Ta5+Mg2+), and (Mg2+Mo5+). This method further elucidated the ordered pairs (Bi3+Bi5+) in BaBiO3 and (Bi3+Sb5+) in Ba2(BiSb)O6, and a disordered pair (Fe3+Mo4+) in (AA′)(FeMo)O6 where (AA′ = BaLa, SrLa). n nFerrimagnetic behavior and a chemical shift of the fluorescent X-ray Mn-Kβ line supported the valency pair (Mn2+Mo5+) in (SrLa)(MnMo)O6. The isomer shifts of Fe in Mossbauer absorption spectra and a random distribution of Fe and Mo from X-ray powder diffraction patterns supported the valency pair (Fe3+Mo4+) in (AA′)(FeMo)O6 where (AA′ = BaLa, SrLa). Eleven compounds were newly synthesized for the present work.

Electrical conductivity and metal-nonmetal transition in the perovskite-related layered system Can+1TinO3n+1−δ (n = 2, 3,and∞)

Abstract A series of dielectric compounds having perovskite-related structures Ca 3 Ti 2 O 7 , Ca 4 Ti 3 O 10 , and CaTiO 3 with the general formulaCa n +1 Ti n O 3 n +1 ( n = 2, 3,and∞) showed a metallic conductivity when electron carriers were doped by reduction under H 2 atmosphere. The temperature dependence of the metallic resistivity up to 300 K was found to be a linear function of T 2 in a wide temperature region. The system showed a metal-nonmetal transition as a function of temperature below 120 K. An activation type hopping conduction was observed down to a few tens kelvin, and a variable range hopping conduction below that temperature was observed. The electrical transport phenomena of the system showed a strong dependence on the oxygen deficiency δ, but little dependence of the stacking number of the perovskite slab n .

Oxide cathode with perovskite structure for rechargeable lithium batteries

Lithium ions were electrochemically inserted into perovskite-type oxides SrVO3−δ and La0.50Li0.37TiO2.94 using galvanic cell: Li|1 M LiClO4 in PC|SrVO3−δ or La0.50Li0.37TiO2.94. It is found that the lattice parameters of SrVO3−δ will be increased and the discharge capacity of SrVO3−δ will be decreased as δ increased. At the composition where all of A-site vacancies in La0.50Li0.37TiO2.94 are just occupied by the lithium ions, the lattice parameter shows a sudden increase. The chemical diffusion coefficients of lithium ions in SrVO3−δ and La0.50Li0.37TiO2.94 were determined to be the values in the range from 10−8 to 10−12 cm2 s−1. Both oxides can be considered as a useful cathode of rechargeable lithium batteries. The charge/discharge characteristic can be improved by mixing of a small amount of carbon powder with the oxide.

Metallic conductivity in perovskite-type compounds AMoO3 (A = Ba, Sr, Ca) down to 2.5K

Abstract The resistivities of perovskite-type compounds AMoO 3 (A = Ba, Sr, Ca) have been measured over the temperature range from room temperature to 2.5K. The metallic conductivity have been observed in these compounds. The order of resistivities of AMoO 3 is discussed from the viewpoint of overlap intergrals.

Calorimetric and electrical studies on the positional disorder of lithium ions in lithium lanthanum titanate

Abstract Heat capacities and electrical moduli of high lithium ion conductor Li0.35La0.51 TiO2.94 were measured below 300 K. A glass transition was observed at Tg = (102 ± 2) K as due to the freezing-in of positional disorder of lithium ions. The associated heat capacity jump was found to be (0.03−0.01+0.03)JK−1 mol−1 at 125 K. The energy of the excited state as referred to the ground state was evaluated from the jump to be ca. 9 kJ mol−1. The calorimetric and electrical relaxation times for the rearrangement of lithium ions were well fitted by a straight line on an Arrhenius plot, and the activation energy of the process was derived to be (32.0 ± 0.1) kJ mol−1. A potential close relation was suggested to exist in general between the high ionic conductivity and the positional disorder of mobile ions.

Superconductivity and magnetism in Co substituted YBCO compounds

Abstract Superconducting transition temperature Tc and magnetic susceptibility χ were measured on Co substituted YBCO system, YBa2[Cu1−xCox]3Oy. The observed characteristic dependence of Tc on the substitution concentration x w investigated taking the magnetic data into consideration. The operating mechanism of the Co ion at the Cu(I) site on the superconducting state in the [Cu(II)ue5f8O] net-planes are inquired from the two viewpoints: (1) changes in electronic state and (2) magnetic moment induction at the Cu(II) site. The possibility of both being induced by displacement of the oxygen atom O(1) between Cu(I) and Cu(II) is considered.

Cooperative interaction of oxygen octahedra for dielectric properties in the perovskite-related layered compounds Srn+1TinO3n+1,Can+1TinO3n+1 and Srn+1(Ti0.5Sn0.5)nO3n+1 (n = 1, 2, 3 and ∞)

Abstract The role of oxygen octahedra on the dielectric property of the Ruddlesdon-Popper type layered compound series Srn+1TinO3n+1,Can+1TinO3n+1 and Srn+1(Ti0.5Sn0.5)nO3n+1 constants, and calculated from an equivalent circuit model for the layered structure. As the number of layers of corner-sharing oxygen octahedra in each perovskite layer, n, increases, the measured dielectric constant increases, while the calculated dielectric constant linearly increases. This suggests that the n dependence of the dielectric constant is caused by the cooperative dielectric interaction of BO6 octahedra.

Preparation and characterization of new ruthenium compounds with perovskite structure

Abstract New perovskite compounds Sr(Li 1 4 Ru 3 4 ) O 3 , Sr(Na 1 4 Ru 3 4 ) O 3 , ( Sr 1 2 La 1 2 )( Mg 1 4 Ru 3 4 ) O 3 and ( Sr 7 8 La 1 8 )( Mg 1 4 Ru 3 4 ) O 3 have been prepared. The crystal structures of these compounds were determined by powder X-ray Rietveld analysis. The crystal structure of these compounds was orthorhombic with space group Pnma . Electrical resistivity and magnetic susceptibility were measured for the samples with the nominal ruthenium valences from 4.0+ to 5.0+. In this study, the crystal structure, electrical conductivity and magnetic properties of the new ruthenium perovskite compounds are discussed.