| Summary: | 碩士 === 淡江大學 === 物理學系 === 87 === Using a first-principle pseudopotential molecular-dynamics approach within both the local density approximation (LDA) and generalised gradient approximation (GGA) for the exchange-correlation potential, we explore the structural, vibrational and electronic properties of a prototypical family of quasi-molecular layered solid of the type XI3 (where X=As,Sb) under compression. In the result s of structural calculation, there Group-V metal triiodides reveal a continuous isostructural transition from a molecular solid to a two-dimensional layered crystal under high pressure. We further find that gradient correction are necessary for an appropriate description of equilibrium structure, bonding and compression mechanisms and that local density approximation appears to fail badly. In Raman spectroscopy study, these materials reveal an unusual non-monotonic variation of the symmetric X-I stretch frequency (assigned as Ag mode). Qualitatively, we present a same trend of this unusual phenomenon in the phone calculation. On the other hand, the structural and vibrational response (including the complex pressure dependence of the bond-stretch frequency) is well accounted for by population analysis which shows that the pressure-induced molecular-nonmolecular transition corresponds to a electronic dedormation without associated crystallographic symmetry breaking. We further examine the band structure, density of states and partial density of states of these systems.
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