The anomalous temperature behavior of the Raman scattering bands at superionic phase transition of Cu 6 PS 5 Br is revealed. Using density functional perturbation theory within local density approximation, ab initio long-wavelength phonons have been obtained for high-symmetry cubic structure. In the calculated phonon spectrum two unstable modes indicate the experimentally observed sequence of ferroelastic and superionic phase transitions.
In the work it was shown that the In 4 Se 3 crystal is similar by its dynamical properties to the InSe modulated crystal. The transformation of the D group of In 4 Se 3 structure has been investigated. In spite of structure corrugation of In 4 Se 3 layer, it is shown that the region of the weak bond in this crystal by the dynamical characteristics is similar to the weak bond region in InSe crystal.
The effect of the shear strains on the energy spectrum of the strongly anisotropic SbSI crystal has been investigated by group-theoretical method in combination with the Pikus method of invariants. The first-principles local density approximation has been implemented to determine the band structure of the crystal. Ab initio calculations of the band structure have shown an exact localization of twofold degenerate maximum of the valence band in the T point. It turned out that the shear strains result in the band topology changes in the vicinity of the T point and the k -linear term appears in the corresponding dispersion law. : 71.15.Ap, 71.15.Hx, 71.15.Mb, 61.50.Ah The inhomogeneous bonding in the strongly anisotropic crystals with a complicated unit cell results in some peculiarities of their energy band structure caused by external effect. Such crystals are the crystals with distinguished structural units in the form of layers or chains. Some of them belong to the group of ferroelectricssemiconductors. The SbSI crystal is one of the representatives of the mentioned group.
Key words: ab initio, deformations, SbSI
PACSAs it is well known, the main characteristic of semiconductors is their energy spectrum. The changes in the energy spectrum under external effects are crucial in the manifestation of such effects on the electronic physical properties. One of the above mentioned factors is the effect of external strains.The kp-theory and its modern modifications make it possible to investigate dispersion laws of electrons and holes for strained and unstrained crystals. Nevertheless, *
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