Propriedades Estruturais, Eletrônicas, Óticas e Vibracionais das ligas Ca1-xMgxO e Ba1-ySryO via DFT

Abstract

This dissertation aims to study the structural, electronic, optical and vibrational properties of the Ca1-xMgxO and Ba1-ySryO alloys within the Density Functional Theory using a 2 x 2 x 2 (and beyond) supercell approach, which allows an expanded set of molar fractions. The optimization of the pure oxides and alloys structures were done using the CASTEP code combined with the Generalized Gradient Approximation (GGA) parameterized by Perdew, Burke and Ernzerrof (PBE) with the dispersion correction method. Norm-conserving pseudopotentials and a plane-wave basis set with energy cut-off 800 eV were used. From the optimization of all structures it was possible to calculate all properties of our interest: X-ray diffraction pattern, band structure, effective mass, optical absorption, dielectric function and infrared and Raman vibrational spectra. The results show that when we changed calcium (barium) by magnesium (strontium), for the same molar fraction x (y), different lattice symmetries were found. The enthalpy of formation, in general, increases in modulus as the Mg (Sr) content increases, and there is no appreciable difference in enthalpy values among alloys of different lattice symmetries with identical molar fraction, so different polymorphic species may co-exist. The calculated X-ray diffraction pattern show that with increase of x or y the peaks move to larger values of diffraction angles. The band gap energy depends strongly on the lattice structures, thus band gap does not show a linear variation with composition. The Ca1-xMgxO alloys exhibits the indirect-to-direct band gap transition occurring in the range 0,072 ≤ x ≤ 0,1094. The effective masses for electron and hole were calculated and the charge transport occurs preferentially through electrons. For optical properties, all alloys exhibit their absorption spectrum in the ultraviolet region with an intense band around 50 nm and a broad absorption range with intensities as function of lattice structure. We also observed that the dielectric response of all alloys changes drastically as the Mg (Sr) content increases. Infrared and Raman spectra of the alloys shows singularities which it is possible to use to differentiate one from another symmetry.