arXiv subjects

T. Motohashi

Publications and source records attributed to T. Motohashi.

14 recordsLinked to original sources

arXiv2011

Impact of lithium composition on the thermoelectric properties of the layered cobalt oxide system LixCoO2

Thermoelectric properties of the layered cobalt oxide system LixCoO2 were investigated in a wide range of Li composition, 0.98 >= x >= 0.35. Single-phase bulk samples of LixCoO2 were successfully obtained through electrochemical deintercalation of Li from the pristine LiCoO2 phase. While LixCoO2 with x >= 0.94 is semiconductive, the highly Li-deficient phase (0.75 >= x >= 0.35) exhibits metallic conductivity. The magnitude of Seebeck coefficient at 293 K (S293K) significantly depends on the Li content (x). The S293K value is as large as +70 ~ +100 uV/K for x >= 0.94, and it rapidly decreases from +90 uV/K to +10 uV/K as x is lowered within a Li composition range of 0.75 >= x >= 0.50. This behavior is in sharp contrast to the results of x <= 0.40 for which the S293K value is small and independent of x (+10 uV/K), indicating that a discontinuous change in the thermoelectric characteristics takes place at x = 0.40 ~ 0.50. The unusually large Seebeck coefficient and metallic conductivity are found to coexist in a narrow range of Li composition at about x = 0.75. The coexistence, which leads to an enhanced thermoelectric power factor, may be attributed to unusual electronic structure of the two-dimensional CoO2 block.

cond-mat.str-el↗

arXiv2009

Electronic phase diagram of the layered cobalt oxide system, LixCoO2 (0.0 <= x <= 1.0)

Here we report the magnetic properties of the layered cobalt oxide system, LixCoO2, in the whole range of Li composition, 0 <= x <= 1. Based on dc-magnetic susceptibility data, combined with results of 59Co-NMR/NQR observations, the electronic phase diagram of LixCoO2 has been established. As in the related material NaxCoO2, a magnetic critical point is found to exist between x = 0.35 and 0.40, which separates a Pauli-paramagnetic and a Curie-Weiss metals. In the Pauli-paramagnetic regime (x <= 0.35), the antiferromagnetic spin correlations systematically increase with decreasing x. Nevertheless, CoO2, the x = 0 end member is a non-correlated metal in the whole temperature range studied. In the Curie-Weiss regime (x >= 0.40), on the other hand, various phase transitions are observed. For x = 0.40, a susceptibility hump is seen at 30 K, suggesting the onset of static AF order. A magnetic jump, which is likely to be triggered by charge ordering, is clearly observed at Tt = 175 K in samples with x = 0.50 (= 1/2) and 0.67 (= 2/3), while only a tiny kink appears at T = 210 K in the sample with an intermediate Li composition, x = 0.60. Thus, the phase diagram of the LixCoO2 system is complex, and the electronic properties are sensitively influenced by the Li content (x).

cond-mat.str-el↗

arXiv2009

Measurement of electron correlations in LixCoO2 (x=0.0 - 0.35) using 59Co nuclear magnetic resonance and nuclear quadrupole resonance techniques

CoO2 is the parent compound for the superconductor NaxCoO2\cdot1.3H2O and was widely believed to be a Mott insulator. We performed 59Co nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) studies on LixCoO2 (x = 0.35, 0.25, 0.12, and 0.0) to uncover the electronic state and spin correlations in this series of compounds which was recently obtained through electrochemical de-intercalation of Li from pristine LiCoO2. We find that although the antiferromagnetic spin correlations systematically increase with decreasing Li-content (x), the end member, CoO2 is a non-correlated metal that well satisfies the Korringa relation for a Fermi liquid. Thus, CoO2 is not simply located at the limit of x->0 for AxCoO2 (A = Li, Na) compounds. The disappearance of the electron correlations in CoO2 is due to the three dimensionality of the compound which is in contrast to the highly two dimensional structure of AxCoO2.

cond-mat.supr-con↗

arXiv2008

Electrochemical synthesis and properties of CoO2, the x = 0 phase of the AxCoO2 systems (A = Li, Na)

Single-phase bulk samples of the "exotic" CoO2, the x = 0 phase of the AxCoO2 systems (A = Li, Na), were successfully synthesized through electrochemical de-intercalation of Li from pristine LiCoO2 samples. The samples of pure CoO2 were found to be essentially oxygen stoichiometric and possess a hexagonal structure consisting of stacked triangular-lattice CoO2 layers only. The magnetism of CoO2 is featured with a temperature-independent susceptibility of the magnitude of 10-3 emu/mol Oe, being essentially identical to that of a Li-doped phase, Li0.12CoO2. It is most likely that the CoO2 phase is a Pauli-paramagnetic metal with itinerant electrons.

cond-mat.str-el↗

arXiv2007

Synthesis and properties of CoO2, the x = 0 end member of the LixCoO2 and NaxCoO2 systems

We report here the synthesis of single-phase bulk samples of CoO2, the x = 0 end member of the AxCoO2 systems (A = Li, Na), from a pristine LiCoO2 sample using an electrochemical technique to completely de-intercalate lithium. Thus, synthesized CoO2 samples were found to be oxygen-stoichiometric and possess a crystal structure consisting of stacked triangular-lattice CoO2 layers only. The magnetic susceptibility of the CoO2 sample was revealed to be relatively large in its initial value and then level off as the temperature increases, suggesting that CoO2 is a Pauli-paramagnetic metal with itinerant electrons.

cond-mat.str-el↗

arXiv2006

Enhancement of giant magnetoresistance effect in the Ruddlesden-Popper phase Sr3Fe2-xCoxO7-d: Predominant role of oxygen nonstoichiometry and magnetic phase separation

The magnetic and magnetotransport properties of the Sr3Fe2-xCoxO7-d system (0.2 <= x <= 1.0) were systematically investigated. This oxide system exhibits a giant magnetoresistance (GMR) effect at low temperatures, reaching up to 80% in 7 T at 5 K. Ac-susceptibility measurements show that there exists a strong competition between ferromagnetic (F) and spin glass states, and the balance between these two magnetic states can be controlled by varying cobalt (x) and/or oxygen contents (d). Importantly, the MR effect is closely related to the magnetic property: the development of magnetic disordering leads to enhancement in the negative MR effect. It is suggested that the compound segregates into F clusters embedded in a non-F matrix, being a naturally occurring analog of the artificial granular-GMR materials, as in the doped perovskite cobaltites, La1-xSrxCoO3 (x < 0.18).

cond-mat.str-el↗

arXiv2005

The impact of niobium doping upon the magnetotransport properties of the oxygen-deficient perovskite SrCo1-xNbxO3-d

The oxygen-deficient perovskite cobaltite SrCo1-xNbxO3-d was synthesized by direct solid-state reaction and its magnetotransport properties were investigated. This cobaltite exhibits an unusual ferromagnetic behavior with a transition temperature Tm = 130-150 K and a spin glass like behavior below Tm. Importantly, this phase reaches a large magnetoresistance (MR) value, MR = -(rH - r0) / r0 = 30% at 5 K in 7 T. The large MR effect is believed to be related to the disordered magnetic state induced by the Nb-for-Co substitution.

cond-mat.str-el↗

arXiv2005

Competition between ferromagnetism and spin glass: the key for large magnetoresistance in oxygen deficient perovskites SrCo1-xMxO3-d (M = Nb, Ru)

The magnetic and magnetotransport properties of the oxygen deficient perovskites, SrCo1-xMxO3-d with M = Nb and Ru, were investigated. Both Nb- and Ru-substituted cobaltites are weak ferromagnets, with transition temperatures Tm of 130-150 K and 130-180 K, respectively, and both exhibit a spin glass behavior at temperatures below Tf = 80-90 K. It is demonstrated that there exists a strong competition between ferromagnetism and spin glass state, where Co4+ induces ferromagnetism, whereas Nb or Ru substitution at the cobalt sites induces magnetic disorder, and this particular magnetic behavior is the origin of large negative magnetoresistance of these oxides, reaching up to 30% at 5 K in 7 T. The differences between Nb- and Ru-substituted cobaltites are discussed on the basis of the different electronic configuration of niobium and ruthenium cations.

cond-mat.str-el↗

arXiv2004

Oxygen and Cation Ordered Perovskite, Ba2Y2Mn4O11

A three-step route has been developed for the synthesis of a new oxygen-ordered double perovskite, BaYMn2O5.5 or Ba2Y2Mn4O11. (i) The A-site cation ordered perovskite, BaYMn2O5+d, is first synthesized at d ~ 0 by an oxygen-getter-controlled low-O2-pressure encapsulation technique utilizing FeO as the getter for excess oxygen. (ii) The as-synthesized, oxygen-deficient BaYMn2O5.0 phase is then readily oxygenated to the d ~ 1 level by means of 1-atm-O2 annealing at low temperatures. (iii) By annealing this fully-oxygenated BaYMn2O6.0 in flowing N2 gas at moderate temperatures the new intermediate-oxygen-content oxide, BaYMn2O5.5 or Ba2Y2Mn4O11, is finally obtained. From thermogravimetric observation it is seen that the final oxygen depletion from d ~ 1.0 to 0.5 occurs in a single sharp step about 600 C, implying that the oxygen stoichiometry of BaYMn2O5+d is not continuously tunable within 0.5 < d < 1.0. For BaYMn2O5.5 synchrotron x-ray diffraction analysis reveals an orthorhombic crystal lattice and a long-range ordering of the excess oxygen atoms in the YO0.5 layer. The magnetic behavior of BaYMn2O5.5 (with a ferromagnetic transition at ~ 133 K) is found different from those previously reported for the known phases, BaYMn2O5.0 and BaYMn2O6.0.

cond-mat.str-el↗

arXiv2003

Kinetics and thermodynamics of the degree of order of the B cations in double-perovskite Sr2FeMoO6

Here we show that the B-site cation (Fe/Mo) ordering in the double-perovskite magnetoresistor, Sr2FeMoO6, is controlled by either kinetics or thermal equilibrium depending on the temperature range. In order to enhance the ordering, long synthesis periods at moderate temperatures are thus required. Employment of a special oxygen-getter-controlled low-O2-pressure encapsulation technique for the sample synthesis enabled us to use long heating periods without evident evaporation of Mo, and thereby reach the thermal equilibrium state and a high degree of B-site cation order. For samples fired at 1150C for long periods (> 36 hours), record-high Ms values of 3.7 ~ 3.9 muB were obtained.

cond-mat.str-el↗

arXiv2003

Iron and molybdenum valences in double-perovskite (Sr,Nd)2FeMoO6: electron-doping effect

Double perovskite, (Sr1-xNdx)2FeMoO6, was doped with electrons through partial substitution of divalent Sr by trivalent Nd (0 < x < 0.2). The Fe valence and the degree of B-site order were probed by 57Fe Mossbauer spectroscopy. Replacing Sr by Nd increased the fraction of Fe and Mo atoms occupying wrong sites, i.e. antisite disorder. It had very little effect on the Fe valence: a small but visible increase in the isomer shift was seen for the mixed-valent FeII/III atoms occupying the right site indicating a slight movement towards divalency of these atoms, which was more than counterbalanced by the increase in the fraction of antisite Fe atoms with III valence state. It is therefore argued that the bulk of the electron doping is received by antisite Mo atoms, which - being surrounded by six MoV/VI atoms - prefer the lower IV/V valence state. Thus under Nd substitution, the charge-neutrality requirement inflicts a lattice disorder such that low-valent MoIV/V can exist.

cond-mat.str-el↗

arXiv2003

Enhanced thermoelectric properties of NaxCoO2 whisker crystals

Single-crystalline whiskers of thermoelectric cobalt oxide, NaxCoO2, have been grown by an unconventional method from potassium-containing compositions, and their transport properties, and chemical compositions were determined. The growth mechanism was analyzed and interpreted by means of thermogravimetric analysis. At 300 K, electrical resistivity r and thermoelectric power S of the whisker are 102 u ohm cm and 83 uV/K, respectively. The power-factor, S2/r, thus is enhanced up to ~ 68 uW/K2 cm.

cond-mat.str-el↗

arXiv2003

Sodium and oxygen nonstoichiometry and thermoelectric properties of NaxCoO2+d

The sodium-cobalt oxide NaxCoO2+d is a promising candidate for thermoelectric applications since it may possess simultaneously large thermoelectric power and low resistivity. It is difficult to understand the simultaneous appearance of the two properties within the framework of conventional one-electron models. It has been suggested that the spin state of cobalt ions plays a crucially important role in enhancing the thermoelectric power. In order to study the relationship between the cobalt spin state and thermoelectric properties, NaxCoO2+d samples with precisely controlled sodium and oxygen contents are indispensable. Recently, we established an uncon-ventional sample-synthesis method that enables us to precisely control the Na content in NaxCoO2+d samples. In this article, we discuss the effects of sodium and oxygen nonstoichiometry on the thermoelectric properties of NaxCoO2+d.

cond-mat.str-el↗

arXiv2002

Unconventional magnetic transition and transport behavior in Na0.75CoO2

Here we report an unconventional magnetic and transport phenomenon in a layered cobalt oxide, NaxCoO2. Only for x = 0.75, a magnetic transition of the second order was clearly detected at Tm ~ 22 K where an apparent specific-heat jump, an onset of extremely small spontaneous magnetization, and a kink in resistivity came in. Moreover large positive magnetoresistance effect was observed below Tm. These features of the transition strongly indicate the appearance of an unusual electronic state that may be attributed to the strongly-correlated electrons in Na0.75CoO2.

cond-mat.str-el↗