Toshiro Ban

Preparation and ionicity of C60 charge transfer complexes

Abstract The electron-accepting ability of a C 60 molecule in solution was evaluated by the measurements of the charge transfer (CT) absorption energy and the redox potential. It was concluded that C 60 is a weak electron acceptor comparable to 2,5-dibromo- p -xyloquinone, s -tetracyanobenzene or 2,3-dichloro-1,4-naphthoquinone. The adibatic electron affinity of a C 60 molecule was estimated as (2.10–2.21)±0.1 eV. Several black solid CT complexes of C 60 were prepared with mainly TTF derivatives and the ionicity of the ground state was examined by their CT band energies and vibrational spectra. These CT complexes were found to be neutral. A crystal structure of the OMTTF complex from benzene was determined.

Photo-Induced Absorption Band in One-Dimensional Halogen-Bridged Mixed-Valence Platinum Complex: [Pt(en)2][PtI2(en)2](SO4)2·6H2O and its Au-Doped Complex: [AuxPt1-xI(en)2]SO4·3H2O

The intervalence charge-transfer absorption spectra in the mixed-valence complexes [Pt(en) 2 ][PtI 2 (en) 2 ](SO 4 ) 2 ·6H 2 O and [Au x Pt 1- x I(en) 2 ]SO 4 ·3H 2 O ( x =0.03) have been investigated at 4.2 K. In these complexes, below the charge-transfer absorption edge, a weak absorption band ( A -band) has been observed for the light polarized parallel to the chain axis. When these complexes were irradiated with the light in the region of the charge-transfer transition from Pt II to Pt IV , the absorption coefficient of the A -band increased remarkably, which implies that the A -band is a photo-induced absorption band. The photo-induced effect for the A -band in [Au x Pt 1- x I(en) 2 ]SO 4 ·3H 2 O ( x =0.03) is more intense than that in [Pt(en) 2 ][PtI 2 (en) 2 ](SO 4 ) 2 ·6H 2 O. The A -band suggests the existence of new excitations such as a soliton-like excitation in the electronic state of the platinum chains.

Semiconductor-to-metal and metal-to-metal transitions in the three-dimensional mixed-valence compound Cs2Au2I6 under high pressures

Electrical resistivity of three-dimensional Au mixed-valence compound Cs2Au2I6 was investigated under hydrostatic pressures up to 7.0 GPa and temperatures between room temperature and 480 K, where a semiconductor-to-metal transition (P=4.5 GPa, T=room temperature) and a metal-to-metal transition (P=6.5 GPa, T≈330 K) were observed. The metallic phase appeared at P=6.5 GPa and T⪆ 330 K could be obtained at room temperature and ambient pressure as a metastable phase, whose structure was found to be a cubic perovskite structure from the X-ray diffraction pattern. In connection with these results, the Au valence state of Cs2Au2I6 under high pressures was discussed.

Magnetic and Transport Properties of Ce-Ta-S Ternary System

Synthesis, transport and magnetic properties of a new ternary system Ce-Ta-S have been investigated. (CeS) 0.6 TaS 2 and (CeS) 1.2 TaS 2 order antiferromagnetically with Neel temperatures of 2.3 K and 2.7 K, respectively. In the former compound, a diverging anomaly in the susceptibility at T N , a small spontaneous magnetization below T N and a magnetic phase transition at 2.5 kOe are observed. From these results, (CeS) 0.6 TaS 2 is supposed to be a quasi-2D antiferromagnet with a weak ferromagnetism composed of ferromagnetic layers. The fractional superconductivity is observed with the magnetic ordering. Ce 4 f electrons are well localized and the correlation between Ce 4 f and conduction electrons is weak in these compounds.

A Search for New Ionic C60 Charge Transfer Complexes

Abstract As one of the strategies to obtain ionic C60 charge transfer complexes, a doping of alkali metals to neutral CT complexes has been attempted under a mild condition. Superconductivity has been detected by magnetic measurements in OMTTF·C60·benzene doped with potassium or rubidium, where OMTTF is octamethylene-tetrathiafulvalene.

Three photo-induced absorption bands of one-dimensional halogen-bridged mixed-valence complex, [Pt(en)2][PtBr2(en)2](SO4)2.6H2O

Abstract Weak absorption bands below the low-energy absorption edge of the intervalence charge-transfer (IVCT) transition in one-dimensional halogen-bridged mixed-valence complex [Pt(en) 2 ][PtBr 2 (en) 2 ](SO 4 ) 2 .6H 2 O have been investigated at 4.2K for the light polarized parallel to the Pt chain axis. In this complex, we found that three weak absorption bands (A-, B-, C-bands) were observed below the IVCT band. It is the first example of a non-doped halogen-bridged mixed-valence complex having three weak absorption bands. The absorption intensity of the C-band is much weaker than those of the A- and B-bands, but it is remarkably enhanced by the Au-doping. The A-, B- and C-bands have the photo-induced absorption effect, and its effect depends sensitively on the crystal quality and on the condition of the crystal growth, and is enhanced by the Au-doping. The photo-induced absorption decays during several minutes since stopping the irradiating light in the region of the IVCT band. From these behaviors of the A-, B-, and C-bands, we discuss the relation between the origin of these bands and the mismatch of the valence alternation.

Preparation and physical properties of some C60 charge transfer complexes

Abstract A variety of electron donors including aromatic amines, phenothiazines, phenazines, tetrathiafulvalene (TTF) derivatives, and metallocenes were examined to form charge transfer (CT) complexes with C 60 . Black solid CT complexes were obtained from mainly TTF derivatives and these CT complexes were found to be neutral. Dark brown solid products were prepared by treating decamethylferrocene, cobaltocene, or nickelocene with C 60 . Although these dark brown products with metallocenes have not been completely identified, ferromagnetic behavior was observed in some cases.

Hydrostatic pressure effect on the charge-density wave transition in the low-dimensional metal K3Cu8S6

Abstract The electrical resistivity of K3Cu8S6 was measured between 290K and 1.5K under hydrostatic pressure up to 1.1GPa. The two phase transitions, which are related to the formation of charge density waves (CDWs), were strongly influenced by pressure; the temperature of the first-order transition (50K under ambient pressure) rose and that of the second-order transition (152K under ambient pressure) fell with increasing pressure. The non-metallic behavior of the resistivity due to the CDWs was almost suppressed by the pressure of 1.1GPa.

Behaviour of the electrical conductivity of the three-dimensional mixed-valence compounds Cs2Au2X6 (X=C1, Br, I) under high pressures

Abstract Electrical resistivity of the three-dimensional Au mixed-valence compounds Cs2Au2X6 (X=Cl, Br, I) was investigated under hydrostatic pressures up to 7.0 GPa and temperatures between room temperature and 480K. The resistivities and the activation energies of these compounds decrease rapidly with increasing pressure. The activation energies of Cs2Au2Cl6, Cs2Au2Br6 and Cs2Au2I6 are estimated by extrapolation as zero at 4.4 GPa, 7.2 GPa and 7.5 GPa, respectively. In the case of Cs2Au2I6, a sonductor-to-metal transition (P= Gpa, T= room temp.) and a metal-to-metal transition (P=6.5 GPa, T≈330 K) were observed. The metallic phase appeared at P =6.5 GPa and T ⪆ 330 K could be obtained at room temperature and ambient pressure as a metastable phase, whose structure was found to be a cubic perovskite structure from the X-ray diffraction pattern.

Magnon Sideband of the 4T2g(4D) State in the Quasi Two-Dimensional Antiferromagnet (C2H5NH3)2MnCl4

A magnon sideband which has an anomalous shape with the cut-offs at the low as well as the high energy sides was observed in the lowest energy region of the absorption spectra 6 A 1g → 4 T 2g ( 4 D ) in (C 2 H 5 NH 3 ) 2 MnCl 4 . This band shape is well fitted by the calculation of the shape of magnon sideband, considering both the inter- and intra-sublattice exciton transfers. The magnitudes of the inter- and intra-sublattice exciton transfers are estimated to be 10.2 cm -1 and 6.8 cm -1 , respectively, from the analysis of the band shape. The behaviour of this magnon sideband in the magnetic field was measured at 4.2 K. From its behaviour, the spin flop field is estimated to be 30±5 kOe, which is confirmed by the measurement of magnetization. The existence of exciton dispersions is supported by the effects of magnetic field and uniaxial stress on this magnon sideband.