Browsing by Author "Yamauchi, H."

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    Anisotropic thermoelectric properties associated with dimensional crossover in quasi-one-dimensional SrNbO3.4+d(d~0.03)
    (American Physical Society (APS), 2011) Kobayashi, W.; Hayashi, Y.; Matsushita, M.; Yamamoto, Y.; Terasaki, I.; Nakao, A.; Nakao, H.; Murakami, Y.; Moritomo, Y.; Yamauchi, H.; Karppinen, Maarit; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    We have grown large single crystals of SrNbO3.4+d (d~0.03) with n=5 in the homologous series SrnNbnO3n+2 by using a traveling solvent floating-zone method and measured resistivity, thermopower, and thermal conductivity along all crystallographic axes. The thermoelectric properties are found to be highly anisotropic, which reflects a quasi-one-dimensional electronic structure. In particular, the thermopower along the b axis is −170 μV/K at 300 K, which is 1 order of magnitude higher than the −15 μV/K along the a axis and −25 μV/K along the c axis. A possible origin of the high anisotropy in the thermopower is discussed in terms of dimensional crossover associated with a structural modification.
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    Ca-substitution and O-doping effects in superconducting Cu(Ba0.8Sr0.2)2(Yb1-xCax)Cu2O6+z obtained from neutron diffraction refinements
    (American Physical Society (APS), 1999) Karppinen, Maarit; Yamauchi, H.; Fujinami, K.; Nakane, T.; Peitola, K.; Rundlöf, H.; Tellgren, R.; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    Distinct calcium and oxygen doping effects were studied in the Cu(Ba0.8Sr0.2)2(Yb1−xCax)Cu2O6+z (Cu−1212:P) system by means of neutron diffraction and superconducting quantum interference device experiments in the wide substitution ranges of 0<~x<~0.35 and 0
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    Ca-substitution and O-doping effects in superconducting Cu(Ba0.8Sr0.2)2(Yb1−xCax)Cu2O6+z obtained from neutron diffraction refinements
    (1999) Karppinen, M.; Yamauchi, H.; Fujinami, K.; Nakane, T.; Peitola, K.; Rundlöf, H.; Tellgren, R.; Department of Chemistry
    Distinct calcium and oxygen doping effects were studied in the Cu(Ba0.8Sr0.2)2(Yb1−xCax)Cu2O6+z (Cu−1212:P) system by means of neutron diffraction and superconducting quantum interference device experiments in the wide substitution ranges of 0<~x<~0.35 and 0
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    Coexistence of intrinsic and extrinsic magnetoresistance in the double-perovskite Sr2Fe(Mo1-xWx)O6-w system
    (2001-04-30) LINDÉN, J.; YAMAMOTO, T.; NAKAMURA, J.; Karppinen, M.; Yamauchi, H.; Tokyo Institute of Technology; Department of Chemistry and Materials Science
    In a recent study, it was shown that by partially substituting Mo with W in the double-perovskite Sr2FeMoO6-x system, the magnetoresistivity can be enhanced. [K.-I. Kobayashi, T. Okuda, Y. Tomioka, T. Kimura, and Y. Tokura, J. Magn. Magn. Mater. 218, 17 (2000).] In order to explain the increase in the magnetoresistivity a series of W-substituted Sr2Fe(Mo1-xWx)O6-x samples with 0 ≤ x ≤ 1 was synthesized. Upon increasing the W content, the samples began to exhibit coexistence of paramagnetism and ferrimagnetism at 300 K. Signatures of antiferromagnetic ordering appeared around TN ≈ 50 K for x ≥ 0.6. In samples with 0.7 ≤ x ≤ 0.8 a broad peak was observed in the magnetoresistance data at temperatures corresponding to the Néel temperature. The peak was found to have its origin in the colossal magnetoresistance effect. The W-substituted samples were partially ferrimagnetic and therefore also exhibited the tunneling-type magnetoresistance, which is characteristic of pure Sr2FeMo6-w. The coexistence of the two typesof magnetoresistance effect is responsible for the enhancement of the overall magnetoresistance value. A slight enhancement in the magnetoresistance values around 300 K for the strongly W-substituted samples was found to be related to a second colossal magnetoresistance peak related to the para-to ferrimagnetic transition at Tc.
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    Coexistence of intrinsic and extrinsic magnetoresistance in the double-perovskite Sr[sub 2]Fe(Mo[sub 1-x]W[sub x])O[sub 6-w] system
    (AIP Publishing, 2001) Lindén, J.; Yamamoto, T.; Nakamura, J.; Karppinen, Maarit; Yamauchi, H.; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    In a recent study, it was shown that by partially substituting Mo with W in the double-perovskite Sr2FeMoO6−w system, the magnetoresistivity can be enhanced. [K.-I. Kobayashi, T. Okuda, Y. Tomioka, T. Kimura, and Y. Tokura, J. Magn. Magn. Mater. 218, 17 (2000).] In order to explain the increase in the magnetoresistivity a series of W-substituted Sr2Fe(Mo1−xWx)O6−w samples with 0⩽x⩽1 was synthesized. Upon increasing the W content, the samples began to exhibit coexistence of paramagnetism and ferrimagnetism at 300 K. Signatures of antiferromagnetic ordering appeared around TN≈50 K for x⩾0.6. In samples with 0.7⩽x⩽0.8 a broad peak was observed in the magnetoresistance data at temperatures corresponding to the Néel temperature. The peak was found to have its origin in the colossal magnetoresistanceeffect. The W-substituted samples were partially ferrimagnetic and therefore also exhibited the tunneling-type magnetoresistance, which is characteristic of pure Sr2FeMoO6−w. The coexistence of the two types of magnetoresistanceeffect is responsible for the enhancement of the overall magnetoresistance value. A slight enhancement in the magnetoresistance values around 300 K for the strongly W-substituted samples was found to be related to a second colossal magnetoresistance peak related to the para- to ferrimagnetic transition at Tc.
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    Control of hole distribution through isovalent R-cation substitution in Cu2Ba2RCu2O8 superconductors
    (2007) Karppinen, Maarit; Abe, Y.; Grigoraviciute, I.; Chen, J.M.; Liu, R.S.; Yamauchi, H.; Department of Chemistry
    Superconductive Cu2Ba2RCu2O8 samples with R ranging from Gd to Tm have been synthesized through a sol-gel route. Both iodometric titration and Cu L-edge x-ray absorption near-edge structure (XANES) spectroscopy data indicate that the average Cu valence remains constant, whereas Tc increases with decreasing size of the R constituent. An explanation for this trend is revealed from O K-edge XANES spectra, which show that the smaller-for-larger R-cation substitution results in a shift of holes from the Cu2O2 charge reservoir to the superconductive CuO2 planes. Since Cu2Ba2RCu2O8 samples are underdoped, such a shift of holes raises the value of Tc.
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    Control of hole distribution through isovalent R-cation substitution in Cu[sub 2]Ba[sub 2]RCu[sub 2]O[sub 8] superconductors
    (AIP Publishing, 2007) Karppinen, Maarit; Abe, Y.; Grigoraviciute, I.; Chen, J. M.; Liu, R. S.; Yamauchi, H.; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    Superconductive Cu2Ba2RCu2O8 samples with R ranging from Gd to Tm have been synthesized through a sol-gel route. Both iodometric titration and CuL-edge x-rayabsorption near-edge structure (XANES)spectroscopy data indicate that the average Cu valence remains constant, whereas Tc increases with decreasing size of the R constituent. An explanation for this trend is revealed from O K-edge XANESspectra, which show that the smaller-for-larger R-cation substitution results in a shift of holes from the Cu2O2 charge reservoir to the superconductive CuO2 planes. Since Cu2Ba2RCu2O8 samples are underdoped, such a shift of holes raises the value of Tc.
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    Electrochemical synthesis and properties of CoO2, the x = 0 phase of the AxCoO2 systems (A = Li, Na)
    (2008) Motohashi, T.; Ono, T.; Katsumata, Y.; Kanno, R.; Karppinen, M.; Yamauchi, H.; Department of Chemistry
    Single-phase bulk samples of the “exotic” CoO2, the x=0 phase of the AxCoO2 systems (A=Li,Na), were successfully synthesized through electrochemical deintercalation 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 10−3emu/molOe, 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.
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    Electrochemical synthesis and properties of CoO[sub 2], the x=0 phase of the A[sub x]CoO[sub 2] systems (A=Li,Na)
    (AIP Publishing, 2008) Motohashi, T.; Ono, T.; Katsumata, Y.; Kanno, R.; Karppinen, Maarit; Yamauchi, H.; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    Single-phase bulk samples of the “exotic” CoO2, the x=0 phase of the AxCoO2 systems (A=Li,Na), were successfully synthesized through electrochemical deintercalation 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 10−3emu/molOe, 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.
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    Electronic phase diagram of the layered cobalt oxide system LixCoO2 (0.0≤x≤1.0)
    (2009) Motohashi, T.; Ono, T.; Sugimoto, Y.; Masubuchi, Y.; Kikkawa, S.; Kanno, R.; Karppinen, Maarit; Yamauchi, H.; Department of Chemistry
    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 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (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 the Pauli-paramagnetic and 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 noncorrelated 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 antiferromagnetic 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).
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    Electronic phase diagram of the layered cobalt oxide system LixCoO2(0.0
    (American Physical Society (APS), 2009) Motohashi, T.; Ono, T.; Sugimoto, Y.; Masubuchi, Y.; Kikkawa, S.; Kanno, R.; Karppinen, Maarit; Yamauchi, H.; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    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 Co59 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (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 the Pauli-paramagnetic and 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 noncorrelated 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 antiferromagnetic 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).
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    Establishment of strongly overdoped states in the HgBa2Ca2Cu3O8+[delta] superconductor
    (American Physical Society (APS), 1997) Fujinami, K.; Ito, T.; Suematsu, H.; Matsuura, K.; Karppinen, Maarit; Yamauchi, H.; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    Strongly overdoped HgBa2Ca2Cu3O8+δ (Hg-1223) samples with superconducting transitions as low as 97 K were successfully obtained by using HgO, Ca2CuO3, CuO, and mixtures of highly oxidized BaCuO2+ε (ε≈0.13) and/or BaO2 powders as starting materials for the high-pressure synthesis at 5 GPa and 950 °C. The overdoped state was confirmed by observing negative values for the Seebeck coefficient throughout the temperature range from Tc to 320 K in a thermoelectric power measurement. Also, both of the cell parameters a and c were found to decrease with decreasing Tc, reflecting, respectively, an increase in hole concentration in the CuO2 planes and the incorporation of oxygen into the HgOδ layer. The oxygen excess δ as determined by the Cu(+I)/Cu(+II) coulometric titration method, was ∼0.19 in the overdoped sample with Tc=107K. Subsequent reducing annealing in an Ar atmosphere at 280 °C increased the Tc to 131 K. Consistently, only positive Seebeck coefficient values were observed up to 320 K for the Ar-annealed sample. A clearly underdoped material with Tc=118K was obtained by annealing the same sample in Ar at a higher temperature (∼400°C). Finally, high-pressure synthesis starting from less-oxidized BaCuO2+ε (ε≈0.06) yielded a Hg-1223 material with δ≈0.10 and Tc=132K.
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    Establishment of strongly overdoped states in the HgBa2Ca2Cu3O8+δ superconductor
    (1997-12-01) Fujinami, K.; Ito, T.; Suematsu, H.; Matsuura, K.; Karppinen, M.; Yamauchi, H.; Tokyo Institute of Technology; Hokkaido Electric Power Co.; Department of Chemistry
    Strongly overdoped HgBa2Ca2Cu3O8+δ (Hg-1223) samples with superconducting transitions as low as 97 K were successfully obtained by using HgO, Ca2CuO3, CuO, and mixtures of highly oxidized BaCuO2+∈ (∈ ≈0.13) and/or BaO2 powders as starting materials for the high-pressure synthesis at 5 CPa and 950°C. The overdoped state was confirmed by observing negative values for the Seebeck coefficient throughout the temperature range from Tc to 320 K in a thermoelectric power measurement. Also, both of the cell parameters a and c were found to decrease with decreasing Tc, reflecting, respectively, an increase in hole concentration in the CuO2 planes and the incorporation of oxygen into the HgOδ layer. The oxygen excess δ as determined by the Cu(+I)/Cu(+II) coulometric titration method, was ∼0.19 in the overdoped sample with Tc=107 K. Subsequent reducing annealing in an Ar atmosphere at 280°C increased the Tc to 131 K. Consistently, only positive Seebeck coefficient values were observed up to 320 K for the Ar-annealed sample. A clearly underdoped material with Tc=118 K was obtained by annealing the same sample in Ar at a higher temperature (∼400°C). Finally, high-pressure synthesis starting from less-oxidized BaCuO2+∈ (∈≈0.06) yielded a Hg-1223 material with δ≈0.10 and Tc= 132 K.
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    Evidence for valence fluctuation of Fe in Sr[sub 2]FeMoO[sub 6-w] double perovskite
    (AIP Publishing, 2000) Lindén, J.; Yamamoto, T.; Karppinen, Maarit; Yamauchi, H.; Pietari, T.; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    In this letter evidence for the formation of a valence-fluctuation state of iron, formally denoted as Fe2.5+, is presented. The system under study is the Sr2FeMoO6−w double perovskite, known for exhibiting a very large magnetoresistance. Samples of Sr2FeMoO6−w were synthesized by means of an encapsulation technique utilizing an Fe getter technique and characterized by 57Fe Mössbauer spectroscopy. From 5 K to room temperature the Mössbauer spectrum is dominated by a component with hyperfine parameter values between those expected for high-spin Fe3+ and high-spin Fe2+.
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    Evidence for valence fluctuation of Fe in Sr_(2)FeMoO_(6-w) double perovskite
    (2000) Linden, J.; Yamamoto, T.; Karppinen, M.; Yamauchi, H.; Pietari, T.; Department of Chemistry
    In this letter evidence for the formation of a valence-fluctuation state of iron, formally denoted as Fe2.5+, is presented. The system under study is the Sr2FeMoO6−w double perovskite, known for exhibiting a very large magnetoresistance. Samples of Sr2FeMoO6−w were synthesized by means of an encapsulation technique utilizing an Fe getter technique and characterized by 57Fe Mössbauer spectroscopy. From 5 K to room temperature the Mössbauer spectrum is dominated by a component with hyperfine parameter values between those expected for high-spin Fe3+ and high-spin Fe2+.
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    Fabrication and thermoelectric characteristics of [(Bi,Pb)2Ba2O4±w]0.5CoO2 bulks with highly aligned grain structure
    (2008) Motohashi, T.; Nonaka, Y.; Sakai, K.; Karppinen, M.; Yamauchi, H.; Department of Chemistry
    Here, we report fabrication and thermoelectric characteristics of polycrystalline bulks with aligned grains of the misfit-layered cobalt oxides, [(Bi1−xPbx)2Ba2O4±w]0.5CoO2. High-quality precursor powder was prepared with a spray-drying technique. The precursor was then sintered under a uniaxial pressure to yield dense bulk samples consisting of highly aligned grains. The electrical conductivity of such samples was four times higher than that of conventionally sintered bulks. Accordingly, the thermoelectric characteristics of the grain-aligned specimens were enhanced.
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    Fabrication and thermoelectric characteristics of [(Bi,Pb)[sub 2]Ba[sub 2]O[sub 4±w]][sub 0.5]CoO[sub 2] bulks with highly aligned grain structure
    (AIP Publishing, 2008) Motohashi, T.; Nonaka, Y.; Sakai, K.; Karppinen, Maarit; Yamauchi, H.; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    Here, we report fabrication and thermoelectric characteristics of polycrystalline bulks with aligned grains of the misfit-layered cobalt oxides, [(Bi1−xPbx)2Ba2O4±w]0.5CoO2. High-quality precursor powder was prepared with a spray-drying technique. The precursor was then sintered under a uniaxial pressure to yield dense bulk samples consisting of highly aligned grains. The electrical conductivity of such samples was four times higher than that of conventionally sintered bulks. Accordingly, the thermoelectric characteristics of the grain-aligned specimens were enhanced.
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    High-Tc superconducting cuprates, (Ce,Y)sO2s-2Sr2(Cu2.75Mo0.25)O6+d: Tc-increase with apical Cu-O decrease at constant Cu-O planar distance
    (2014) Marezio, M.; Chmaissem, O.; Bougerol, C.; Karppinen, M.; Yamauchi, H.; Geballe, T.H.; Department of Chemistry
    Evidence for high-Tc cuprate superconductivity is found in a region of the phase diagram where non-superconducting Fermi liquid metals are expected. Cu valences estimated independently from both XANES measurements and bond valence sum calculations are greater than 2.25 and are in close agreement with each other for structures of the homologous series given in the title with s = 1, 2, 3, 4 and 5. Two questions arise from the present perspective: 1) Is all the action in the CuO2 layers? 2) Is there superconductivity beyond the usual dome? The record short apical oxygen distance found in the homologous series especially in the s = 1 member, at odds with the current theory, suggests the possibility of a new pairing mechanism. The apical Cu-O distance in the s = 1 member decreases upon oxygenation from 2.29 to 2.15 dot A while the Cu valence increases to 2.45 dot A.
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    Layer-specific hole concentrations in Bi2Sr2(Y1-xCax) Cu2O8+δ as probed by XANES spectroscopy and coulometric redox analysis
    (2003-04) Karppinen, M.; Kotiranta, Miia; Nakane, T.; Yamauchi, H.; Chang, Sungchun; Liu, R.-S.; Chen, J. M.; Department of Chemistry; Tokyo Institute of Technology; National Taiwan University; National Synchrotron Radiation Research Center
    Induction of holes not only in the superconductive CuO2 plane but also in the Bi2O2-δ charge reservoir of the Bi2Sr2(Y1-xCax) Cu2O8+δ superconductor upon CaII-for-YIII substitution is evidenced by means of two independent techniques, i.e., high-resolution x-ray-absorption near-edge structure (XANES) spectroscopy measurements and coulometric redox titrations. The absolute values derived for the CuO2-plane hole concentration from the Cu L2,3-edge XANES spectra are in good agreement with those obtained from the coulometric redox analysis. The CuO2-plane hole concentration is found to increase from 0.03 to 0.14 concomitantly with the increase in the BiO1+δ/2-layer hole concentration from 0.00 to 0.13 as the Ca-substitution level, x, increases from 0 to 1. The threshold CuO2-plane hole concentration for the appearance of superconductivity is determined at 0.06, while the highest Tc is obtained at the hole concentration of 0.12. In the O K-edge XANES spectrum, the increases in the CuO2-plane and BiO1+δ/2-layer holeconcentrations with increasing x are seen as enhancement in the relative intensities of the pre-edge peaks at ∼528.3 and ∼530.5 eV, respectively.
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    Layer-specific hole concentrations in Bi2Sr2(Y1-xCax)Cu208+[delta] as probed by XANES spectroscopy and coulometric redox analysis
    (American Physical Society (APS), 2003) Karppinen, Maarit; Kotiranta, M.; Nakane, T.; Yamauchi, H.; Chang, S. C.; Liu, R. S.; Chen, J. M.; Kemian laitos; Department of Chemistry; Kemian tekniikan korkeakoulu; School of Chemical Technology
    Induction of holes not only in the superconductive CuO2 plane but also in the Bi2O2+δ charge reservoir of the Bi2Sr2(Y1-xCax)Cu2O8+δ superconductor upon CaII-for-YIII substitution is evidenced by means of two independent techniques, i.e., high-resolution x-ray-absorption near-edge structure (XANES) spectroscopy measurements and coulometric redox titrations. The absolute values derived for the CuO2-plane hole concentration from the Cu L2,3-edge XANES spectra are in good agreement with those obtained from the coulometric redox analysis. The CuO2-plane hole concentration is found to increase from 0.03 to 0.14 concomitantly with the increase in the BiO1+δ/2-layer hole concentration from 0.00 to 0.13 as the Ca-substitution level, x, increases from 0 to 1. The threshold CuO2-plane hole concentration for the appearance of superconductivity is determined at 0.06, while the highest Tc is obtained at the hole concentration of 0.12. In the O K-edge XANES spectrum, the increases in the CuO2-plane and BiO1+δ/2-layer hole concentrations with increasing x are seen as enhancement in the relative intensities of the pre-edge peaks at ∼528.3 and ∼530.5 eV, respectively.