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.
The ab initio full potential linearized augmented plane wave (FP-LAPW) method within density functional theory was applied to study the effect of composition on the structural and electronic properties of Zn 1-x Be x S, Zn 1-x Be x Se and Zn 1-x Be x Te ternary alloys. The effect of composition on lattice parameter, bulk modulus, band gap and effective mass was investigated. Deviations of the lattice constant from Vegard's law and the bulk modulus from linear concentration dependence were observed for all three alloys. It was deduced that increasing the Be composition in the alloys increases the hardness of the materials. In addition, the calculated band structures showed that the band gap undergoes a direct-to-indirect transition at a given concentration. Using the approach of Zunger and co-workers, the microscopic origins of band gap bowing have been explained. The electron (hole) effective masses were also calculated. The band gap and effective mass were found to vary non-linearly with Be composition.
Zn 1−x Mg x S, Zn 1−x Mg x Se and Zn 1−x Mg x Te ternary wide-gap semiconductor alloys were investigated using the full potential-linearized augmented plane wave (FP-LAPW) method. We have studied the effect of composition on structural properties such as lattice constants, bulk modulus and bond ionicity. The bandgap and the microscopic origins of compositional disorder have also been explained in detail. In addition, from the obtained band structures, the electron (hole) conduction and valence effective masses are deduced. These parameters were found to depend non-linearly on alloy composition x, except the lattice parameter for Zn 1−x Mg x S, which follows Vegard's law. The calculated band structures for all three alloys show a direct bandgap in the whole range of x composition. We have paid special attention to the disorder parameter (gap bowing). Using the approach of Zunger and co-workers, we have concluded that the total bandgap energy bowing was mainly caused by the charge exchange effect for the alloys of interest.
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