A theoretical study of structural and electronic properties of boron compounds
BN, BP, BAs and BSb is presented, using the full potential linearized augmented
plane wave method. In this approach, the generalized gradient
approximation was used for the exchange-correlation potential. Ground
state properties such as lattice parameter, bulk modulus and its pressure
derivative are calculated as well as structural transition pressure. The band
structure is obtained for both zincblende and rocksalt structures. We also
give the valence charge density at equilibrium lattice constant and at
transition pressure. We show from the latter quantity the inverse role between
cation and anion for BP, BAs and BSb. Results are discussed and compared with
experimental and other theoretical data with reasonable agreement.
We use the 1-bond→2-phonon percolation doublet of zincblende alloys as a 'mesoscope' for an unusual insight into their phonon behavior under pressure. We focus on (Zn,Be)Se and show by Raman scattering that the original Be-Se doublet at ambient pressure, of the stretching-bending type, turns into a pure-bending singlet at the approach of the high-pressure ZnSe-like rocksalt phase, an unnatural one for the Be-Se bonds. The 'freezing' of the Be-Se stretching mode is discussed within the scope of the percolation model (mesoscopic scale), with ab initio calculations in support (microscopic scale).
The structural and electronic properties of calcium chalcogenides CaX
(X = S,Se,Te)
under high pressure have been investigated using the full potential linearized augmented
plane wave method within density functional theory. We used both the local density
approximation and the generalized gradient approximation (GGA) that is based on
exchange–correlation energy optimization for calculating the total energy. Moreover, the
Engel–Vosko GGA formalism is applied so as to optimize the corresponding potential for
band structure calculations. The equilibrium lattice constant for CaX compounds agrees
well with the experimental results. The pressures at which these compounds undergo
a structural phase transition from NaCl-type to CsCl-type were calculated. A
numerical first-principles calculation of the elastic constants was used to calculate
C11,
C12
and C44. The energy band gaps at ambient conditions in the NaCl-type structure and
the volume dependence of band gaps in the CsCl-type structure up to the band
overlap metallization were investigated. Besides this, the nature of the chemical
bond in these compounds was analysed in terms of electronic charge density.
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