摘要:

The disappearance of microtwins in the high- Tc, superconductor Y1Ba2Cu3O7–xunder the influence of electron irradiation is reported, and is shown to occur when the structure transforms from the orthorhombic to the tetragonal phase. Convergent-beam electron diffraction studies establish the crystallography of this transformation. The observations are explained in terms of an increase inxunder electron irradiation. The space group of the orthorhombic structure is determined as Pm2m.

ISSN:0950-0839    

DOI:10.1080/09500838808203765

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The change in annihilation line-shape for positrons implanted into a molybdenum surface with a variable-energy positron beam has been analysed in terms of a back-diffusion model. Closer agreement with the experimental data was found if account was taken of epithermal positron and positronium emission using a model of elastic scattering of positrons near the surface.

ISSN:0950-0839    

DOI:10.1080/09500838808203766

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A new criterion for the classification of quasicrystalline structures is proposed. A quasicrystal with composition close to a Frank-Kasper phase, which consists exclusively of tetrahedra, should belong to the i((Al, Zn)49Mg32) class whereas one with composition close to that of the crystal, which consists of both tetrahedra and octahedra, should belong to the i(A1MnSi) class. The Ti2(Ni, V) building block consists of four icosahedra and a truncated tetrahedron. Replacing the eight carbon atoms of a diamond unit cell with this building block produces the Ti2(Ni, V) crystal. Placing a distorted Ti2(Ni, V) building block on a threefold axis of an icosahedrally symmetric point group and operating on it by this point group gives a double Mackay icosahedron. It can be concluded that the Ti2(Ni, V) and α-AlMnSi quasicrystals are isomorphic structures since Ti2(Ni, V) and α-A1MnSi crystals consist of both tetrahedra and octahedra.

ISSN:0950-0839    

DOI:10.1080/09500838808203767

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

An enhancement of the green-edge emission in polycrystalline CdS films after low-energy electron bombardment has been observed. The opposite behaviour occurred in CdS single crystals. Both thermal and athermal processes (including ejection and diffusion of species) have been accounted for in explaining the cathodoluminescence modulation under the action of electron bombardment.

ISSN:0950-0839    

DOI:10.1080/09500838808203768

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

X-ray diffraction patterns of GeS2and GeO2, glasses under pressure up to 100 kbar have been measured in order to examine their medium-range structure. Qualitative differences have been found between the pressure dependences of the first sharp diffraction peaks, suggesting distinct glassy structures. The first peak in chalcogenide glasses can be ascribed to inter-layer correlations, which become degraded at high pressures.

ISSN:0950-0839    

DOI:10.1080/09500838808203769

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The experimental Lα1,2and Lβ1X-ray emission spectra of Zn, Ga and Ge are presented. The high-energy satellite structure of the lines is considered. It appears that the intensity of these satellites relative to the main lines is much larger for Zn than for Ga and Ge. This enhancement effect is interpreted as arising mainly from the 2p−11/2→ 2p−13/23d−13/2,5/2Coster-Kronig transition occurring in metallic Zn, which is absent in the case of Ga and Ge. Furthermore, for all these elements it is suggested that a significant contribution to the high-energy satellites of the Lα1,2; X-ray emission band arises from the 2s−11/2→ 2p−13/23d−13/2,5/2Coster-Kronig process, whereas in the case of the satellites of the Lβ1X-ray emission spectrum the main cause would be the 2s−11/2→ 2p−11/23d−13/2,5/2transition.

ISSN:0950-0839    

DOI:10.1080/09500838808203770

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The cut-and-project method is employed to construct octagonal, decagonal and dodecagonal quasilattices. We also simulate the diffraction patterns with eight-, tenand twelvefold symmetries, which show striking similarities with the electron diffraction images observed from the octagonal, decagonal and dodecagonal quasicrystals.

ISSN:0950-0839    

DOI:10.1080/09500838808203771

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The hydrostatic pressure dependences of the velocities of longitudinal and shear ultrasonic waves propagated in amorphous selenium have been measured as a function of temperature through the glass-transition region. Both δC11,δP and δC44/δP increase rapidly as temperature is increased, a finding attributed to a combination of increasing vibrational anharmonicity as Tgis approached from below and to free-volume effects. At room temperature, substantially below Tg. the thermal Grüncisen parameter 7this only 1·05 whereas the mean long-wavelength acoustic parameter 7elis 3·0. This arises because the intermolecular volume is much more compressible than the intramolecular volume: the Gruneisen parameters for purely internal modes of vibration are small whereas those for the external modes are larger.

ISSN:0950-0839    

DOI:10.1080/09500838808203772

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

ISSN:0950-0839    

DOI:10.1080/09500838808203773

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor