Koun Shirai

Infrared study of amorphous B1−xCx films

Amorphous boron carbide (a‐B1−xCx) is believed to have an icosahedron‐based random network. In this paper, vibrational properties of a‐B1−xCx films are studied by IR and Raman spectra, placing particular emphasis on the interpretation of the most prominent 1100‐cm−1 band associated with the B–C bond. The 1100‐cm−1 band appears in both IR and Raman spectra, and the frequency variation and the intensity as a function of the C content are examined, together with evaluation of the absolute absorption coefficients. Within the framework of the impurity‐induced vibration theory, the 1100‐cm−1 band is characterized. The estimation of the force constants by the observed frequencies leads to a conclusion that the vibration can be classified as an extrinsic and preferably a local mode. This vibration is well described as the stretching mode of a localized two‐center bond between the B and C atoms. In this sense, the C atom in amorphous B1−xCx is not regarded as a network constituent. The frequency shift with the C c...

Characterization of hydrogenated amorphous boron films prepared by electron cyclotron resonance plasma chemical vapor deposition method

This paper presents characterization of amorphous boron (a‐B) films prepared by the electron cyclotron resonance plasma chemical vapor deposition method using diborane (B2H6) and hydrogen mixtures. Compositional analyses show that hydrogen is the only impurity which influences film properties appreciably. The infrared absorption study shows that the hydrogen is incorporated into the film in the terminal bonding form. The hydrogen concentration has been estimated using the semiempirical equation given by Blum, Feldman, and Satkiewicz [Phys. Status Solidi A 41, 481 (1977)], which can be verified by the more fundamental principle. The infrared absorption study also shows that the amorphous films possess some short‐range order related with the icosahedral structure. These observations allow us to consider that this film belongs to one of alloyed materials and is true hydrogenated a‐B. In order to study the contributions of hydrogen to the film properties, variations of the color and the electrical resistivity...

Raman scattering and isotopic phonon effects in dodecaborides

The Raman spectra of numerous dodecaborides have been measured on high-quality single crystals at ambient conditions with high spectral resolution and signal-to-noise ratio. Besides the strong Raman-active modes, numerous Raman-inactive modes occur in the spectra, indicating distortions of the structures. Ab initio calculation of the phonon spectra on ZrB(12) excellently agrees with the experimental results. Force constants are theoretically calculated and force parameters are estimated from the Raman frequencies. The influence of the surface range on the Raman spectra is evident. The different isotopic effects (virtual crystal approximation, the polarization effect and the effect of isotopic disorder) on the phonon frequencies are determined, separated and discussed.

Lattice vibrations and the bonding nature of boron carbide

The vibrations of boron carbide have been studied by the valence force model. There have been so far several ambiguities in the interpretation of experimental spectra, which made theoretical study difficult. This paper solves the main difficulties, that is, characterization of the highest band and most of the Raman bands, providing reasonable interpretations for low-frequency modes. The band is ascribed to the anti-symmetric stretching of the linear chain. The necessity of large angle-bending forces of carbon atom arose for better fitting, which fact can be regarded as evidence for formation of strong covalent bonds around this atom. The present analysis also throws a new light on the Longuet-Higgins interpretation of the role of carbon incorporation. The coexistence of the strong covalent bonds of carbon atoms and the relatively weak icosahedral bonds results in such a balance of bonding that the intra-icosahedral and rhombohedral bonds are further weakened, and the covalent bonds of the carbon chain are instead reinforced.

Mechanism behind the high thermoelectric power factor of SrTiO3 by calculating the transport coefficients

The thermoelectric power factor of SrTiO3 is unusually high with respect to its mobility and band gap. Good thermoelectrics usually have high mobility and a narrow band gap, but such properties are not found in SrTiO3. We have determined the mechanism behind the high power factor by calculating the transport coefficients. The key to understanding the power factor is that different effective masses contribute to different transport phenomena. The discrepancy between the effective mass for the conductivity and the thermoelectric power showed that the conductivity and thermoelectric power are conveyed by electrons with different effective masses in the Brillouin zone. Light electrons were responsible for the high conductivity, whereas heavy electrons were responsible for the high thermoelectric power. The high carrier concentrations of more than 1020 cm−3 did not reduce the thermoelectric power of SrTiO3 above the classical limit. This indicates that the electrons carrying the thermoelectric power were not d...

Electronic Structure and Electrical Resistivity of α-Boron under High Pressure

Recently, it has been discovered that the semiconductor α-boron becomes metallic at high pressures and finally undergoes a superconducting transition at 160 GPa, without causing any phase transition. Before the superconducting transition, a step is often observed in the pressure dependence of electrical resistivity in this class of boron crystals, which possess icosahedron-based structures. This step structure used to be thought to occur due to a phase transitions. In the present paper, we show that the step of α-boron at 50 GPa is not due to a phase transition. It is caused by a gradual change in the bonding character from semiconductor to metal. The increase in the metallic character is caused by the shortening of the three-center bond, which is a characteristic feature of icosahedron-based boron crystals. This shortening of the three-center bond enhances the bonding character of the conduction bottom band and finally closes the band gap. However, even far before the gap closing, the shortening has impo...

The Narrow Raman Linewidth of a Librational Mode of α-Rhombohedral Boron and Its Anharmonic Effects

The 527- cm -1 Raman band of α-boron, which has been recently assigned as a librational mode, exhibits a very narrow linewidth. The mechanism of this narrow linewidth is studied from the viewpoint of anharmonic effects of phonons. The density of states which is responsible to the decay processes is rather large. Anharmonic force constants of individual bonds are evaluated from the pressure dependence on the phonon frequencies, and are found to be not small. Accordingly, by these two factors only, the linewidth could not be as narrow as observed. It is found that the essential cause of the narrow linewidth is its exceptionally high frequency. This makes the thermal factor for the up-conversion processes small even at room temperature.

Optical properties of B12P2

Abstract On polycrystalline B 12 P 2 obtained by chemical vapor deposition on silicon substrates, the absorption edge and its low-energy tail were optically determined and analyzed with respect to interband transitions (indirect allowed transitions at 1.62, 1.80, 2.17, 2.46 and 2.75 eV) and ten gap-state-related transitions. Six of them exhibit energies which agree with electron traps generated by electron-phonon interaction in β-rhombohedral boron. The number of one-phonon resonance frequencies agrees with that group-theoretically determined. The two-phonon processes are quantitatively measured as well. Strong luminescence radiation is superimposed on the FT-Raman spectrum, which differs qualitatively from those traditionally measured. This difference is explained by fluctuating distortions of the icosahedra related to the strong optical excitation of electrons from intrinsic electron traps.

Polar vibrations and the effective charges of the icosahedral boron solid

Abstract Boron crystals exhibit polar vibrations, despite the fact that the crystal is comprised of the same chemical species. In this paper, occurrence of the first-order polar vibrations for the icosahedral B 12 molecule and α-boron crystals is demonstrated within the harmonic approximation, by using the shell model. The dynamic effective charges are expressed by both the normal coordinates and the cartesian coordinates, while the tensor nature of the effective charges is emphasized. By use of different expressions for the effective charges, the dielectric properties of anisotropic and multi-mode polar vibrations are formulated for general crystals whose symmetry is higher than the monoclinic system. Numerical estimations have been examined for the B 12 molecule and α-boron. It is shown that the effective charge of the B 12 molecule is very small (≈ 10 −2 ), while that of α-boron is enhanced by the deformation of the isosahedra. The non-central coreshell force has the dominant effect of this enhancement. Although there still remains some uncertainties in the interpretation of experimental spectra, the present calculation provides some restrictions on the allowable values of the force constants and leads to relationships between the frequencies of Raman and i.r. bands and the intensity of i.r. bands.

Structural Study of α-Rhombohedral Boron at High Pressures

Recently, it has been shown that, like β-rhombohedral boron (β-boron), α-rhombohedral boron (α-boron) exhibits superconductivity at high pressure. The transition pressure is similar to that of β-boron, that is, about 160 GPa. This paper presents a theoretical and experimental study of the structural change in α-boron at high pressures. High-pressure (up to 200 GPa) X-ray experiments were performed at a synchrotron-radiation facility. At ambient temperature, the crystal of α-boron is stable over the whole range of pressures examined. There is no phase transitions, such as to α-Ga type, which were previously predicted. All the structural parameters vary continuously with pressure. The cause of metallization is a significant contraction of the inter icosahedral three-center bond, which brings a higher coordination to the icosahedron and a transfer of charge from the intra icosahedoral bond to the three-center bond. Although everything is continuous, a careful analysis of the pressure dependence of the struct...