Masayuki Udagawa

Unusual hydrogen absorption properties in graphite mechanically milled under various hydrogen pressures up to 6 MPa

We examined hydrogen absorption/desorption behaviors in graphite mechanically milled under hydrogen pressures ranging from 0.3 to 6.0 MPa. Experimental results indicated that the chemisorbed hydrogen concentration during 80 h milling decreased from 6.1 to 4.1 wt.% with increasing milling hydrogen pressure, while the physisorption-like hydrogen concentration increased with increasing milling hydrogen pressure and reached more than 0.5 wt.% at 6 MPa. Moreover, higher pressure hydrogen suppressed the fracture rate of the milled graphite more effectively, suggesting that the hydrogen atoms trapped at the edges of the graphene sheets and between the graphene layers near the surface are responsible for preserving the lamellar nanocrystalline structure, which may be the host for physisorption-like hydrogen absorption.

Raman Scattering Study on Rb2ZnBr4

Raman spectra of Rb 2 ZnBr 4 were measured in the temperature region including the normal-incommensurate ( T i =73°C) and incommensurate-commensurate ( T c =-86°C) phase transition points. At the optical geometries of c ( a a ) b and a ( c c ) b , a soft mode has been observed whose frequency shift decreases as T i is approached from below. At the geometry of a ( c a ) b , the change in the Rayleigh wing has been observed on passing. through T c . Detailed measurements with an I 2 -filter were also made.

ESR and Magnetic Susceptibility in Y2BaCuO5

ESR and magnetic susceptibility measurements were made in insulating Y2BaCuO5. A uniaxial type of the ESR spectrum was observed at room temperature and was found to be due to Cu2+ ions by an intensity measurement. The g values parallel and perpendicular to the symmetry axis are 2.24±0.02 and 2.06±0.02, respectively. Susceptibility measurements showed that the Curie constant is (0.22±0.1) emuK/mol and the paramagnetic Curie temperature is (-8±1) K. The small value of the Curie constant suggests that the sample contains some oxygen deficiency.

Raman Scattering and Infrared Absorption Investigation of Hydrogen Configuration State in Mechanically Milled Graphite under H2 Gas Atmosphere

The hydrogen binding state in the mechanically milled graphite has been investigated by Raman scattering and infrared absorption. There is no C–H vibration signal in Raman scattering spectra, while, a C–H vibration signal has been clearly observed by infrared absorption. The dominant hydrogen configuration is C–H 2 and the contribution of C–H 3 slightly increases for a longer milling time. This result indicates the first experimental determination of the hydrogen state in the mechanically milled graphite under hydrogen gas atmosphere.

Pressure-induced superconductor-insulator transition in the spinel compound CuRh2S4

We performed resistivity measurements in CuRh2S4 under quasihydrostatic pressure of up to 8.0 GPa, and found a pressure-induced superconductor-insulator transition. Initially, with increasing pressure, the superconducting transition temperature T(c) increases from 4.7 K at ambient pressure to 6.4 K at 4.0 GPa, but decreases at higher pressures. With further compression, superconductivity in CuRh2S4 disappears abruptly at a critical pressure P(SI) between 5.0 and 5.6 GPa, when it becomes an insulator.

Brillouin Scattering Study on Lithium Ammonium Tartrate Monohydrate in Paraelectric Phase

The Brillouin scattering studies on lithium ammonium tartrate monohydrate (LAT) have been made in paraelectric phase. The elastic stiffness constants C p 55 and C E 55 vanish at T 0 =94.5 K and T c =97.5 K, respectively. Some Raman scattering spectroscopies of this crystal have been also made. From the phenomenological theory and the discussion of the elastic dispersion, LAT has been concluded to undergo the proper ferroelastic transition, or, the soft mode in LAT is an acoustic phonon of which energy is determined by C p 55 . Measurements of refractive indices at room temperature are reported.

Thermodynamic properties of lithium amide under hydrogen pressure determined by Raman spectroscopy

Partial pressures and equilibrium constants for the reaction from lithium amide and hydrogen to lithium hydride and ammonia at different temperatures were evaluated by Raman spectroscopy to determine standard enthalpy ΔHo and entropy ΔSo of the reaction. Raman intensity measurements were used for estimating ammonia partial pressures in gaseous products after heat treatment of lithium amide under hydrogen atmosphere. The van’t Hoff plot of fractions of the partial pressures of ammonia and hydrogen indicated ΔHo=49.9±9.1 kJ mol−1 and ΔSo=59±16 J mol−1 K−1.

Infrared and Raman Spectroscopy of High Tc Superconducting System (La1-xBax)2CuO4

Infrared absorption spectra of (La1-xBax)2CuO4-y; (x=0, 0.02, 0.03, 0.04, 0.06, 0.075, 0.10, 0.15 and 0.20) have been measured at room temperature between the energy region from 400 to 4000 cm-1. The phonon spectra have been found to change sensitively with the change in concentration. The spectral shape has been found to show good correspondence to the crystal structure. The absorption edge of some electrical transition has been clearly observed at all compositions. The energy gap corresponding to this absorption edge has been found to be almost independent of the concentration.

Correlation of Superconductivity with the Infrared Anomaly in (La1-xMx)2(Cu1-yNy)O4 (M=Ca, Sr and Ba; N=Ni and Zn)

For La 2 CuO 4 based high- T c superconducting oxide systems, three infrared absorption peaks by phonons, which were labeled as P 1 , P 2 and P 3 from the higher energy side, were observed in the energy region between 250 and 800 cm -1 . The intensity of the peak P 1 changed sensitively depending on the concentration of substituted ions. We call the fact the infrared anomaly. Superconductivity appears for samples for which the intensity ratio I (P 1 )/ I (P 2 ) is nearly zero. This correlation between the infrared anomaly and supeconductivity has been confirmed to be common not only for (La 1- x M x ) 2 CuO 4 (M=Ca, Sr and Ba) but also for (La 1- x Sr x ) 2 (Cu 1- y N y )O 4 (N=Ni and Zn). From mode assignments, the anomalous peak P 1 is inferred to correspond to stretching motions of Cu-O bonds along the bond direction perpendicular to the c -axis.

Infrared and Raman Study of (La1-xSrx)2NiO4 and La2(Ni1-xCux)O4

Motivated by the observation of the close correlation between disappearance of the infrared absorption peak at about 680 cm -1 and the appearance of high- T c superconductivity in (La 1- x M x ) 2 CuO 4 (M=Ca, Sr, and Ba), we have measured infrared absorption spectra of (La 1- x Sr x ) 2 NiO 4 ( x =0.0, 0.01, 0.02, 0.03 0.04, 0.07, 0.10 and 0.15), La 2 (Ni 1- x Cu x )O 4 ( x =0.0, 0.20, 0.40 0.60, 0.70, 0.80, 0.85, 0.90, 0.95 and 1.00), and Raman spectrum of La 2 NiO 4 . No disappearance of the infrared absorption peak at about 680 cm -1 has been observed in (La 1- x Sr x ) 2 NiO 4 . By assigning the peaks to characteristic lattice vibrational modes, discussions have been devoted to the possible origin of the infrared anomaly.