الفهرس 
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Abstract
In the present work, a theoretical study of some of the electronic, optical, vibrational, thermal and mechanical properties of the quaternary semiconductor alloy InP_x 〖As〗_y 〖Sb〗_(1-x-y) had been achieved using the empirical pseudopotential method (EPM) within the framework of the virtual crystal approximation (VCA). In order to study the quaternary alloy, it is needed to study the binary compounds that constitute it, which is done in this research. The binary compounds that constitute the quaternary alloy are InAs, InP, InSb, which are all III-V semiconductor compounds. An EPM calculations had been done for the three binary compounds at standard temperature (300 K) and pressure (1 atm).The EPM calculations had been done using an algorithm written in MATLAB. The EPM calculation takes an input consists of the pseudopotential form factors, of the studied materials and the lattice constant and yields the electronic band structure. Usually the pseudopotential form factors are adjusted many times before the algorithm yield a perfect match with the experimental values of the fundamental energy gaps at points of high symmetry in the first Brillouin zone, for example the Γ,L,X points.In this research, the effect of temperature and pressure on the physical properties had been done, and to avoid adjusting the pseudopotential form factors for every single temperature or pressure, a form factors temperature and pressure dependencies was proposed and tested, the proposed models showed a good agreement with the experimental values.The physical properties under study for the binary compounds was, the energy gaps E_Γ,E_L,E_X, high frequency dielectric constant ε_∞ and static dielectric constant ε_s , the refractive index ’n’ , the longitudinal phonon frequency ω_LO and the transverse phonon frequency ω_TO, Debye temperature θ_D, the molar specific heat C_p, the elastic constants c_11,c_12,c_44, mechanical moduli: Young modulus Y_o, Bulk modulus B_u, shear modulus C_s, the effective charge Z^*, the transverse dynamic effective charge e_T^* , Poisson ratio P_o, Cauchy ratio C_a, Born ratio B_o, isotropy factor I_s , linear compressibility C_o, bond stretching parameter α, bond bending force constant β and Internal strain parameter . Finally the sound velocity had been studied for different propagation directions and polarization planes.The study of the quaternary alloy was done using the EPM calculations, and the interpolation scheme for the pseudopotential form factors and lattice constant assuming Virtual Crystal Approximation (VCA). A study of the compositional effect of InPxAsySb1-x-y had been done at standard temperature and pressure.A study of the thermal and compositional effects together on the physical properties of the quaternary alloy had been done, and a 3D maps of this dependence was introduced.A study of the pressure and compositional effects together on the physical properties of the quaternary alloy had been done, and a 3D maps of this dependence was introduced. The inversion of the quaternary semiconductor alloy from a direct to an indirect band gap semiconductor was observed and listed.