Faculty Profiles - OKUNO Tsunehisa

Published Papers - OKUNO Tsunehisa

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Magnetic interaction in the 1:1 salts of nitronyl nitroxide cations with the dicyanophthalocyaninatocobalt(III) anion

K Mitsuya, T Inabe, T Okuno, K Awaga

MOLECULAR CRYSTALS AND LIQUID CRYSTALS SCIENCE AND TECHNOLOGY SECTION A-MOLECULAR CRYSTALS AND LIQUID CRYSTALS ( GORDON BREACH SCI PUBL LTD ) 296 293 - 303 1997 [Refereed]

Two cationic derivatives of alpha-nitronyl nitroxide (NN), p-PPYNN (p-N-n-propylpyridinium alpha-nitronyl nitroxide) and p-BPYNN (p-N-n-butylpyridinium alpha-nitronyl nitroxide), have been crystallized with the [Co(Pc)(CN)2](-) (dicyanophthalocyaninatocobalt(III)) counter anion. Obtained two kinds of 1:1 salt crystals have been found to be isomorphous; monoclinic, P2(1)/c, a = 15.585(6), b = 11.015(4), c = 26.111(3) Angstrom, beta = 89.95(2)degrees, V = 4482(3) Angstrom(3), Z = 4 for p-PPYNN[Co(Pc)(CN)(2)] and a = 15.830(3), b = 11.201(4), c = 25.709(3) Angstrom, beta = 90.29(1)degrees, V = 4558.4(17) Angstrom(3), Z = 4 for p-BPYNN[Co(Pc)(CN)(2)]. The magnetic interaction between the NN radicals in these two kinds of crystals has been found not to be the same; weak antiferromagnetic interaction operates in p-PPYNN[Co(Pc)(CN)(2)], while only paramagnetic behavior is observed for p-BPYNN[Co(Pc)(CN)(2)].

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Ferromagnetic linear chain of p-NPNN center dot Cu(hfac)(2) with enhanced interchain interaction

T Origuchi, W Fujita, A Yamaguchi, T Okuno, K Awaga, H Yano, N Wada

MOLECULAR CRYSTALS AND LIQUID CRYSTALS SCIENCE AND TECHNOLOGY SECTION A-MOLECULAR CRYSTALS AND LIQUID CRYSTALS ( GORDON BREACH PUBLISHING, TAYLOR & FRANCIS GROUP ) 296 281 - 292 1997 [Refereed]

The complex p-NPNN . Cu(hfac)(2) (p - NPNN =p-nitrophenyl nitronylnitroxide and hfac=hexafluoroacetylacetonate) crystallizes into the triclinic P (1) over bar space group [a=12.515(3) Angstrom, b=12.540(2) Angstrom, c=11.610(3) Angstrom, alpha=106.18(2)degrees, beta=119.15(2)degrees, gamma=91.14(2)degrees, V=1501.7(6) Angstrom(3), Z=2]. The structure consists of an alternating chain of p-NPNN and Cu(hfac)(2) in which p-NPNN bridges two nonequivalent copper ions occupying the axial positions. The chains are connected by a pi-pi and head-to-tail overlap of the p-NPNN molecules, resulting in a two-dimensional network. The high-temperature magnetic susceptibilities can be interpreted in terms of a ferromagnetic intrachain interaction of J/k(B)=14.3 K and an antiferromagnetic interchain interaction of zJ'/k(B)=-2.6 K. The observed interchain interaction which would originate in the overlap of p-NPNN is much stronger than those in the related Cu(II) materials reported so far. The low-temperature magnetic susceptibilities indicate an antiferromagnetic order at T-N=1.22+/-0.01 K, followed by an enhancement of the susceptibility below T-c=0.6+/-0.1 K, probably due to a spin canting.

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Molecular spin ladder in the Ni(dmit)(2) (dmit=1,3-dithiol-2-thione-4,5-dithiolate) salt with a nitronyl nitroxide cation

H Imai, T Inabe, T Otsuka, T Okuno, K Awaga

PHYSICAL REVIEW B ( AMER PHYSICAL SOC ) 54 ( 10 ) R6838 - R6840 1996.09 [Refereed]

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Magnetic behavior of the layered perovskite ferromagnet (p-cyanoanilinium(+))(2)CuCl42- under hydrostatic pressure

T Sekine, T Okuno, K Awaga

CHEMICAL PHYSICS LETTERS ( ELSEVIER SCIENCE BV ) 249 ( 3-4 ) 201 - 204 1996.02 [Refereed]

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Two-dimensional spin gap organic antiferromagnet m-MPYNN・BF_4 II

Wada N., Kobayashi T., Yano H., Yamaguchi A., Okuno T., Awaga K.

Meeting Abstracts of the Physical Society of Japan ( The Physical Society of Japan ) 1996 60 - 60 1996

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Molecular spin ladder in the N (dmit=1,3-dithiol-2-thione-4,5-dithiolate) salt with a nitronyl nitroxide cation

Hiroyuki Imai, Tamotsu Inabe, Takeo Otsuka, Tsunehisa Okuno, Kunio Awaga

Physical Review B - Condensed Matter and Materials Physics 54 ( 10 ) R6838 - R6840 1996 [Refereed]

The crystal and magnetic structure of the 1:1 salt of Ni(Formula presented) anionic radical and [Ni(Formula presented)] (Formula presented)-ethylpyridinium (Formula presented)-nitronyl nitroxide ((Formula presented)-EPYNN) cationic radical have been studied. Ferromagnetic interaction with (Formula presented) K has been found to operate in the one-dimensional (Formula presented)-EPYNN chains, while the first molecular antiferromagnetic spin ladder has been found to form in the Ni(Formula presented) chains. © 1996 The American Physical Society.

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Nuclear magnetic relaxation of H-1 in organic cation radical salt m-MPYNN(+)X(-) (X=ClO4, BF4)

Y Fujii, T Goto, K Awaga, T Okuno

CZECHOSLOVAK JOURNAL OF PHYSICS ( CZECHOSLOVAK JNL OF PHYSICS ) 46 2213 - 2214 1996 [Refereed]

The nuclear spill-lattice relaxation time T-1 of H-1 in the organic cation radical salt m-MPYNN(+)X(-) (S=ClO4, BF4) has been measured in the temperature range 1.5 similar to 200K and in the field range 1 similar to 70kOe. The experimental results in high fields were well interpreted in terms of flip-flop relaxation process involving the pairs of ferromagnetic dimers, whose energy scheme consists of lower triplet- and upper singlet-states with the gap energy of 20K. An appreciable increase of the relaxation rate in low fields below about 10kOe was explained by the direct relaxation process due to the band-like spread of each Zeeman level.

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Crystal Structures and Magnetic Properties of MCl4 2- (M=Mn(II) and Co(II)) Salts of m- and p-N-Methylpyridinium Nitronyl Nitroxides

Akira Yamaguchi, Tsunehisa Okuno, Kunio Awaga

Bulletin of the Chemical Society of Japan ( Chemical Society of Japan ) 69 ( 4 ) 875 - 882 1996 [Refereed]

Organic radical cations, 4,4,5,5-tetramethyl-2-(1-methyl-3 or 4-pyridinio)-3-oxide-4,5-dihydro-1H-1-imidazolyloxyl (or, m- and p-N-methylpyridinium nitronyl nitroxides, abbreviated as m- and p-MPYNN+, respectively), were found to crystallize with magnetic counter anions, MCl4 2- (M = Mn2+ (S = 5/2) and Co2+ (S = 3/2)), although the MnCl4 2- salts were rather air-sensitive. X-Ray full-crystal analyses were carried out on the stable CoCl4 2- salts. The structure of (m-MPYNN+)2CoCl4 2- crystallizes in the monoclinic P21/c space group, while that of (p-MPYNN+)2CoCl4 2- belongs to the triclinic P1 space group. Although (m-MPYNN+)2MnCl2- 4 and (p-MPYNN+)2MnCl4 2- were not stable enough for an X-ray full data collection, they were indicated to be isostructural to the corresponding CoCl4 2- salts, respectively, by their diffractions of 20 &lt
θ &lt
25°. Variable-temperature magnetic susceptibility measurements reveal a clear contrast between the (m-MPYNN+)2MCl4 2- and (p-MPYNN+)2MCl4 2- salts. The two (m-MPYNN+)2MCl4 2- salts exhibit a ferromagnetic behavior independently of MCl4 2-, which is attributable to a m-MPYNN+ dimer. However, the magnetic properties of two (p-MPYNN+)2MCl4 2- salts are strongly dependent on the MCl4 2- anion: (p-MPYNN+)2MnCl4 2- exhibits an antiferromagnetic interaction in opposition to a ferromagnetic one in (p-MPYNN+)CoCl4 2-. The magnetic difference between them can be qualitatively understood in terms of a charge-transfer interaction between MCl4 2- and p-MPYNN+, in which the difference in the electronic structure between the Mn2+ and Co2+ ions is reflected.

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Intercalation of functional organic molecules into copper(II) magnetic materials

K Awaga, W Fujita, T Sekine, T Okuno

MOLECULAR CRYSTALS AND LIQUID CRYSTALS SCIENCE AND TECHNOLOGY SECTION A-MOLECULAR CRYSTALS AND LIQUID CRYSTALS ( GORDON BREACH SCI PUBL LTD ) 285 323 - 330 1996 [Refereed]

(i) The magnetic properties of the series of a layered material, Cu-2(OH)(3)(n-CmH2m+1COO), are found to show an anomalous change, depending on the alkyl-chain length: the m=0 and 1 materials are metamagnetic while the m=7-9 materials become weak ferromagnets below 22 K. Further, 4-phenylazobenzoate and 2-anthracene carboxylate, which are derivatives of well-known photo-functional molecules, are intercalated into the copper hydroxides. The obtained materials are both antiferromagnetic in the whole temperature range of 3-280 K, However, the photochemical reactions do not take place by irradiation with a xenon lump. (ii) The magnetic properties of the layered perovskite ferromagnets (p-cyanoanilinium(+))(2)CuCl42- and (p-chloroanilinium(+))(2)CuBr42- are studied under hydrostatic high pressure of 1.0 GPa. Above T-c, they show an increase of the magnetization, which would due to enhancement of the ferromagnetic coupling, but, below T-c, they show different magnetic response to the pressure.

Mossbauer spectroscopic study of the reentrant phase transitions in TBA[Fe(tdas)(2)]

M Takahashi, M Takeda, K Awaga, T Okuno, Y Maruyama, A Kobayashi, H Kobayashi, S Schenk, N Robertson, AE Underhill

MOLECULAR CRYSTALS AND LIQUID CRYSTALS SCIENCE AND TECHNOLOGY SECTION A-MOLECULAR CRYSTALS AND LIQUID CRYSTALS ( GORDON BREACH SCI PUBL LTD ) 285 399 - 404 1996 [Refereed]

Temperature dependence of Fe-57 Mossbauer spectra for TBA[Fe(tdas)(2)] is measured to obtain the information on the spin state and the phase transitions. The large quadrupole splitting due to the S = 3/2 spin state increases reenteringly in the intermediate temperature region (190 - 230 K on cooling and 200 - 240 K on heating). The small increase in the quadrupole splitting and the continuous decrease in the isomer shift suggests the 3/2 spin state does not change, supporting the interpretation of the magnetic susceptibility.

[NHN](+) hydrogen bond in organic radical solid

T Okuno, T Otsuka, K Awaga

MOLECULAR CRYSTALS AND LIQUID CRYSTALS SCIENCE AND TECHNOLOGY SECTION A-MOLECULAR CRYSTALS AND LIQUID CRYSTALS ( GORDON BREACH SCI PUBL LTD ) 278 A57 - A63 1996 [Refereed]

Reaction of m- or p-pyridyl nitronyl nitroxide (m- or p-PYNN) with HBr gas gave a 2:1 complex between them. The IR spectra of (m- and p-PYNN)(2)HBr indicates an intermolecular [NHN](+) hydrogen bond between two pyridyl rings. (m-PYNN)(2)HBr shows ferromagnetic properties, while the p-isomer does antiferromagnetic properties. Furthermore, the HCl salt of phenyl-4-pyridylcarbene was generated by irradiation of the diazo precursor. The ESR spectrum shows signals of triplet and quintet spin species, latter of which indicates a ferromagnetic interaction between the triplet carbenes.

Intermolecular arrangements of p-substituted aniliniums in the interlayer of cupric chloride

T Sekine, T Okuno, K Awaga

MOLECULAR CRYSTALS AND LIQUID CRYSTALS SCIENCE AND TECHNOLOGY SECTION A-MOLECULAR CRYSTALS AND LIQUID CRYSTALS ( GORDON BREACH SCI PUBL LTD ) 278 A65 - A72 1996 [Refereed]

Crystal structures and magnetic properties of the layered pervoskite compounds, (p-X-C6H4NH3+)(2)CuCl42- with X=CN (1), Cl (2), and NO2 (3), were studied. The compound 1 crystallizes in the triclinic <P(1)over bar> space group. The structure consists of a CuCl layer and an organic bilayer in which the molecular axis of the cyanoanilinium is nearly perpendicular to the inorganic layer. At the middle of the organic bilayer, there is a layer of the CN group, in which the CN groups are arranged to cancel their electric dipoles. The compound 2 has a similar structure to that of 1. The crystal of 3 belongs to the orthorhombic Pbca space group, in which the structure also consists of an inorganic layer and an organic bilayer. The bilayer is formed by short contacts between the NO2 groups, which appear to incline the molecular axis of the nitroanilinium with respect to the normal to the CuCl layer. AC and DC magnetic susceptibility measurements were carried out on the three salts. Their behavior can be understood with two parameters, ferromagnetic transition temperature T-c and in-plane ferromagnetic coupling constant J. The obtained parameters are T-c=9.5 K and J/k(B)=24 K for 1, T-c=9.1 K and J/k(B)=23 K for 2, and T-c=7.0 K and J/k(B)=17 K for 3. The values of T-c and J of 3 are smaller than those of the other two. The distortion of the molecular arrangement in the organic layer of 3, is found to result in weakening the magnetic interaction in the CuCl layer.

CRYSTAL-STRUCTURES AND MAGNETIC-PROPERTIES OF (M-PYRIDINIUM-NITRONYL-NITROXIDE)+CENTER-DOT-X- - POSSIBLE SPIN-1 KAGOME ANTIFERROMAGNET

K AWAGA, T OKUNO, A YAMAGUCHI, M HASEGAWA, T INABE, Y MARUYAMA, N WADA

SYNTHETIC METALS ( ELSEVIER SCIENCE SA ) 71 ( 1-3 ) 1807 - 1808 1995.04 [Refereed]

Crystal structures and magnetic properties of m-N-methylpyridinium nitronyl nitroxide (abbreviated as m-MPYNN+) salts were studied. m-MPYNN+. ClO4-.(1/3)(acetone) is found to crystallize into the trigonal P3c1 space group, where the m-MPYNN+ molecules exist as a dimer and the dimer units form a 2-D triangular lattice. Temperature dependence of the magnetic susceptibility can be well interpreted in terms of a strong ferromagnetic intradimer interaction J(1) forming a triplet state and a weak antiferromagnetic interdimer interaction J(2). There is a possibility that this spin system can be characterized as a spin-1 Kagome antiferromagnet at very low temperatures. m-MPYNN+ makes salts with various anions, I-, Cl-, BF4-, ClO4- etc. and their mixtures. In an isostructural solid-solution system, m-MPYNN+.(ClO4-)(x) . l-(1-x).(1/3)(acetone) (0 less than or equal to x less than or equal to 1), J(2) quickly weakens with increasing the ratio of the ClO4- ion, in contrast to little dependence of J(1).

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EPR STUDIES OF THE 2-D MAGNETIC SYSTEM IN AN ORGANIC CATION-RADICAL SALT, M-MPYNN+CENTER-DOT-X-

M HASEGAWA, A YAMAGUCHI, T OKUNO, K AWAGA

SYNTHETIC METALS ( ELSEVIER SCIENCE SA ) 71 ( 1-3 ) 1797 - 1798 1995.04 [Refereed]

EPR measurements were carried out on a single crystal of m-N-methylpyridinium nitronyl nitroxide perchlorate which consists of a 2-D bond-alternated hexagonal lattice. Angular dependence of the linewidth Delta H-pp shows minimums at the magic angles, and can be well interpreted with the theoretical equation for the 2-D magnetic system,Delta H-pp=A(3cos2 theta . 1)(2)+B. Temperature dependence of Delta H-pp shows an anomaly between 50 K and 100 K, which could be relevant to freezing of the orientational disorder of the perchlorate ion. Angular dependence of the g-value confirms the 2-D character of the magnetic system. Below 50 K, the EPR signal is affected by the demagnetizing effects.

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Ferromagnetic and antiferromagnetic intermolecular arrangements of the N-alkylpyridinium nitronyl nitroxide

K Awaga, A Yamaguchi, T Okuno

MOLECULAR CRYSTALS AND LIQUID CRYSTALS SCIENCE AND TECHNOLOGY SECTION A-MOLECULAR CRYSTALS AND LIQUID CRYSTALS ( GORDON BREACH SCI PUBL LTD ) 271 97 - 105 1995 [Refereed]

The magneto-structural correlation in some p-N-alkylpyridinium nitronyl nitroxide salts has been studied. In the iodide salts of p-N-R-pyridinium nitronyl nitroxides with R=methyl, ethyl, n-propyl, and n-butyl, the magnetic property varies from antiferromagnetic to ferromagnetic with the extension of the N-alkyl chain. The nearest-neighbor molecular arrangements of the nitroxides observed in the four crystals, can be classified into two groups: the radicals are connected by a intermolecular contact between the NO groups (Type I) or between the NO group and the pyridinium ring (Type II). The observed magnetic behaviors can be interpreted in terms of an antiferromagnetic intermolecular interaction for the Type I contact and a ferromagnetic for Type II. Effects of replacement of the I- ion by Br- or Cl-, have been also studied in the p-N-methyl derivative.

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VARIABLE MAGNETIC-INTERACTIONS IN AN ORGANIC RADICAL SYSTEM OF (M-N-METHYLPYRIDINIUM ALPHA-NITRONYL NITROXIDE).X- - A POSSIBLE KAGOME ANTIFERROMAGNET

K AWAGA, T OKUNO, A YAMAGUCHI, M HASEGAWA, T INABE, Y MARUYAMA, N WADA

PHYSICAL REVIEW B ( AMER PHYSICAL SOC ) 49 ( 6 ) 3975 - 3981 1994.02 [Refereed]

In this paper we study an organic system of geometrical spin frustration. m-N-methylpyridinium alpha-nitronyl nitroxide (m-MPYNN+) is a spin-1/2 organic radical. The simple salt, m-MPYNN+.ClO4-.1/3 (acetone), crystallizes in a trigonal P3c1 space group, where the m-MPYNN+ molecules exist as a dimer and the dimer units form a two-dimensional (2D) triangular lattice. One-third of the ClO4- ions are in the organic layer, joining the m-MPYNN+ molecules, and the remainder is between the layers, compensating the excess of positive charge in the organic layers. The single-crystal EPR measurements clearly indicate a 2D Heisenberg character of the magnetic system in it. m-MPYNN+ makes a crystalline solid-solution system, m-MPYNN+.(ClO4-)x.I1-x-.1/3 (acetone) (0 less-than-or-equal-to x less-than-or-equal-to 1), which also belongs to the trigonal system. Both the a and c axes are slightly lengthened with increasing the ratio of the ClO4- ion, x, in the solid solution: The unit-cell volume is increased by 3.2% when x runs from 0 to 1. The temperature dependence of the magnetic susceptibilities of the solid solutions can be well interpreted in terms of a strong ferromagnetic intradimer interaction J1 forming a triplet state and a weak antiferromagnetic interdimer interaction J2 which is expected to give rise to spin frustration among the triplet spin species on each side of the triangles. It is found that J2 quickly weakens with an increase in x, while J1 shows little dependence. There is a possibility that this organic system can be characterized as a spin-1 kagome anti-ferromagnet at very low temperatures.

30p-PSA-39 Triangular antiferromagnet m-MPYNN salts II

Wada N., Yano H., Yamaguchi A., Okuno T., Awaga K.

Meeting Abstracts of the Physical Society of Japan ( The Physical Society of Japan ) 49 156 - 156 1994

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13p-PSA-7 Antiferromagnetic Kagome lattice in an organic radical crystal of m-MPYNN^+・X^-

Awaga K., Yamaguchi A., Hasegawa M., Okuno N., Wada N., Itakura M., Hikami S., Inabe T., Maruyama Y.

Meeting Abstracts of the Physical Society of Japan ( The Physical Society of Japan ) 1993 73 - 73 1993

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