Fully hydrogenated graphene (graphane) and partially hydrogenated graphene materials are expected to possess various fundamentally different properties from graphene. We have prepared highly hydrogenated graphene containing 5% wt of hydrogen via Birch reduction of graphite oxide using elemental sodium in liquid NH3 as electron donor and methanol as proton donor in the reduction. We also investigate the influence of preparation method of graphite oxide, such as the Staudenmaier, Hofmann or Hummers methods on the hydrogenation rate. A control experiment involving NaNH2 instead of elemental Na was also performed. The materials were characterized in detail by electron microscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy both at room and low temperatures, X-ray fluorescence spectroscopy, inductively coupled plasma optical emission spectroscopy, combustible elemental analysis and electrical resistivity measurements. Magnetic measurements are provided of bulk quantities of highly hydrogenated graphene. In the whole temperature range up to room temperature, the hydrogenated graphene exhibits a weak ferromagnetism in addition to a contribution proportional to field that is caused not only by diamagnetism but also likely by an antiferromagnetic influence. The origin of the magnetism is also determined to arise from the hydrogenated graphene itself, and not as a result of any metallic impurities.
The magnetic, electric and thermal properties of the (Ln 1−y Y y ) 0.7 Ca 0.3 CoO 3 perovskites (Ln = Pr, Nd) were investigated down to very low temperatures. The main attention was given to a peculiar metal-insulator transition, which is observed in the praseodymium based samples with y = 0.075 and 0.15 at T M −I = 64 and 132 K, respectively. The study suggests that the transition, reported originally in Pr 0.5 Ca 0.5 CoO 3 , is not due to a mere change of cobalt ions from the intermediate-to the low-spin states, but is associated also with a significant electron transfer between Pr 3+ and Co 3+ /Co 4+ sites, so that the praseodymium ions occur below T M −I in a mixed Pr 3+ /Pr 4+ valence. The presence of Pr 4+ ions in the insulating phase of the yttrium doped samples (Pr 1−y Y y ) 0.7 Ca 0.3 CoO 3 is evidenced by Schottky peak originating in Zeeman splitting of the ground state Kramers doublet. The peak is absent in pure Pr 0.7 Ca 0.3 CoO 3 in which metallic phase, based solely on non-Kramers Pr 3+ ions, is retained down to the lowest temperature.
The structural, magnetic and transport properties of the LaMn 1−x Co x O 3 system were investigated over a wide temperature range, 10-900 K. Two structural types, depending on x, were detected-orthorhombic Pbnm (x 0.5) and rhombohedral R 3c (x > 0.6), separated by the bi-phasic sample x = 0.6. At both ends of the LaMn 1−x Co x O 3 system, the respective substituents are unambiguously characterized as Co 2+ (x 0.1) and Mn 4+ (x 0.9). On the Mn-rich side, up to x < 0.5, we infer on the basis of high temperature transport and magnetic data the gradual increase of the Co 2+ valence towards the prevailing Co 3+ , while for the Co-rich samples (0.6 x 0.8), the Co 3+ states tend to disproportionate at high temperatures to Co 2+ + Co 4+ , which probably controls both the transport and magnetic properties. At low temperatures, the long range ferromagnetic order was confirmed by means of neutron diffraction for the x = 0.4 sample. A non-uniform magnetic state was detected at higher cobalt substitution up to x = 0.8 and was associated with FM clusters that are formed below ∼220 K and coagulate below ∼150 K.
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