Mizuka Sano

Polymerization of benzene occluded in graphite-alkali metal intercalation compounds

Abstract Benzene accommodated within the interlayer space of graphite-alkali metal (potassium and rubidium) intercalation compounds was found to be polymerized not only to biphenyl, but also to terphenyl and quaterphenyl, while only biphenyl was formed by the action of potassium or rubidium metal alone under the same conditions. The formation of higher oligomers of benzene in the interlayer space of graphite intercalation compounds is to be ascribed to the amphoteric nature of the compounds, which are capable of both donating and accepting electrons.

Partial charge transfer in HMTTeF-TCNQF4

Abstract The degree of charge transfer in a solid complex of tetrafluorotetracyano- p -quinodimethane (TCNQF 4 ) with hexamethylenetetratellurafulvalene (HMTTeF) was estimated at ≈ 0.75 by Raman spectroscopy. X-ray photoelectron spectra of N 1s electrons also demonstrate the mixed-valency state of the complex, which can explain its high electrical conductivity.

Benzene in graphite-potassium intercalation compounds studied by resonance raman spectroscopy

Abstract Benzene occluded in the interlayer gaps of graphite-potassium intercalation compounds KC 24 and KC 8 , was found to be dimerized to biphenyl (Bp) at room temperature, and the biphenyl thus formed is accommodated in the gaps as anions (Bp 2− and Bp − ).

Positron annihilation in a hydrogen-physisorbed graphite-caesium intercalation compound

Abstract The annihilation spectrum for CsC 24 obtained from the Doppler broadening of 511 keV γ-radiation consists of a narrow band superimposed on a broad band. Upon hydrogen physisorption at 77 K, the spectral height decreased. These facts are explained in terms of an interlayer state, the electrons of which are blocked to interact with positrons through the hydrogen physisorption.

Positron annihilation in potassium-intercalated graphite

Abstract The peak of the Doppler broadening spectrum of KC 24 is depressed by ≈10% upon hydrogen physisorption into its interstices. The depression shows that positrons are distributed in the potassium layers. The lifetime of positrons in KC 8 is measured to be 290 ps which is longer than the lifetime in graphite. It is, therefore, deduced that the positrons are not confined in a special area, but distributed over negatively charged graphitic carbon layers and also positively charged potassium layers in KC 24 and KC 8 . The narrow component of the spectrum is assigned to the annihilation of positrons with electrons in potassium layers. Imperfections included in the intercalant layers of KC 8 are detected as defects in which the positrons have a long lifetime of ≈ = 530 ps.

Hydrogen in a graphite-rubidium intercalation compound RbC8 studied by positron annihilation

Abstract Doppler-broadened spectra of positron-annihilation radiation have been measured on RbC 8 absorbing hydrogen at 300 or 373 K. The spectrum of RbC 8 was composed of a narrow component and a broad one. The intensity of the narrow component was suppressed through hydrogen absorption at 300 K, while it decreased at first and then increased through the absorption at 373 K. From the change in line-shape of the spectra, the hydrogen accommodated in RbC[in8] is considered to be atomic at 300 K and to be in the form of the hydride ion at 373 K. The spectrum was compared with that calculated for the lattice accommodating hydrogen atoms.

Dissociation of hydrogen molecules by a first-stage graphite-caesium intercalation compound

Abstract A caesium-intercalated graphite compound, CsC 8 , which has been identified with the first-stage through the observation of an in situ Raman spectrum, is found to dissociate hydrogen molecules on its surface with the aid of a hydrogen-deuterium equilibrium reaction. The specific rate constants for the reaction over alkali-metal-graphite intercalation compounds are found to be in the order CsC 8 >RbC 24 >KC 24 , RbC 8 , CsC 24 >KC 8 at around room temperature.

Electric conduction in HMTTeF radical salts

Abstract Single crystals of HMTTeF·X n (X=Cl, Br, I, PF 6 , AsF 6 , and ClO 4 ) were prepared electrochemically. The in-plane conductivities of the plate-like crystals of Cl, Br, and I salts were found to show metallic temperature dependence in the range from 300 to ca. 100 K, and to maintain fairly high values even at 5 K. In these metallic salts, HMTTeF was found to be in the state of mixed-valency by Raman spectroscopy.

Two-dimensional electron momentum distribution in graphite revealed by means of angular correlation of positron annihilation

Abstract Two-dimensional angular correlation measurements with positron annihilation radiation (2D-ACAR) on a single crystal of Kish graphite reveal that the motion of the annihilating electrons is nearly free along the graphite layers and restricted within some distance shorter than the lattice constant in the direction perpendicular to the layers. The hollows appearing on the 2D-ACAR surfaces are explained in terms of preferential annihilation of positrons with π-electrons in graphite.

A-axis thermal expansion of donor graphite intercalation compounds

Abstract Temperature dependence of linear thermal expansion coefficients parallel to the c -axis, α c , is investigated for donor graphite intercalation compounds, C 8 K, C 4 KHg and C 4 n KH x ( n = 1, 2 for stage 1 and 2). The experimental results of α c suggest that the anharmonicity of the (001) longitudinal acoustic phonon mode increases in the order of C 4 KH x 4 KHg 8 K. The trend of the anharmonicity and the lattice stiffness are discussed in relation to the electronic properties of these compounds.