No abstract

Raman scattering and infrared studies are reported for the layer compound GaSe under hydro static pressures up to 4 GPa. The rigid-layer mode shifts towards higher frequencies with an initial pressure coefficient of 0.234 GPa"1. The overall behavior is explained in terms of the volume anharmonicity characteristic of the van der Waals bonding. The internal bond-bending mode softens and the Born effective charge decreases linearly. By adopting the single-layer compressibili ty /c/~0.015 GPa"1 we obtain a Gruneisen parameter of -1.9 for the bond-bending mode and -0.55 ±0.05 for the effective charge, comparable to those of a tetrahedral semiconductor. Coex istence of such molecular and covalent characters leads to a nonlinear shift of the energy, Eg, of the Penn-Phillips oscillator. This behavior is shown to be described well by AEg=Da{Aa/a0) + Dc(Ac/c0) with the deformation potentials Da = -7.6± 1.0 eV for the a axis and Dc = 1.06±0.16 eV for the c axis. These deformation potentials reflect the dimensionality of bonding network as well as the nature of the electronic structure.

Convergent-beam electron diffraction (eBED) patterns do not always show the true symmetries of a specimen crystal, but can show higher s~~etries than those of the specimen (s~~etry enhancement). If the phase of the structure factor can not be detected, a symmetry enhancement may take place. Lacking of an inversion center in a crystal can not be detected by X-ray diffraction, but can be unarnbiguously knmffi by CBED.~Vhen the structure factors at two reciprocal. lattice points which are related by a twofold axis have-the same amplitude but have different phases, we say that a pseudo twofold a~is 2* is present. If a mirror plane is present, a pseudo twofold axis is accomp~~ied perpendicularly to the mirror plane. In this case, two structure factors related by the pseudo tHO fold axis are complex conjugate each other. A pseudo mirror plane m* is defined in a similar manner.A pseudo twofold axis and a pseudo mirror plane can cause a symmetry-enhancement in the bright field pattern of CBED. However, the enhanc~~ent can not be observed, since the true mirror plane and the true twofold axis which give the same sy~metries caused by the 2* and m* coexist in usual cases. In a general case, the syLmnetry enhancement c~T), not be expected in the \-lhole pattern and dark field pattern.In a zincblende type crystal, Hhose space group is F43m, the approximate sy~metry enhancement takes place in the 220 dark field pattern by 2* at [Ill] electron incidence, that is, HOLZ lines in the reflection shm-ls the symmetry m2. The reflections belonging to the O-th Laue zone produce 2mm symmetry in the 220 dark-field pattern, since the phases of the reflections are ze~o. The imaginary part of the 220 reflection caused by two successive HOLZ reflections (Umweganregung) is proportional to fA2fBl -fAlfB2, where fxi is the st~cture factor of X atom for the i-th reflection. wnen the scattering angle dependence of the structure factor for A atom is similar to that of B atom, the imaginary part and the resultant phase of the reflection becomes small, resulting a symmetry enhancement of m2' The contribution of the UmweganregQ~g via more th~~ three reflections may be small. A 34 bea~ dynamical computation really shows m2 symmetry in the 220 reflection.The point group of 10H-SiC is 3m. However, it is known that the point group determined by X-ray diffraction is 6/rnrnm, which is Ra~sdell type symmetry enhancement [L.S. Rfu"sdell and J.A. Kohn, Acta Cryst. i (1951) Ill]. When the crystal is exa~ined by CBED, the diffraction group and point group obtained at 0001 incidence were 3mlR and 6m2, and those obtained at 1100 incidence were mlR and 6rnrn. The symmetry enhanc~~ent occurs in the bright field pattern and the approXL~ate symmetry enhancement occurs in the dark field patterns by m* in the (0001) and the (1120) planes and by 2* in the c axis and in the [1100] and the [1120] directions.