First Principles Study Of The Electronic Structure And Optical Properties Of Doped Laalo₃

In the present paper, the lattice structure, band structure and density of state of LaAlO₃ and LaAlO₃: Mg are calculated by first-principle method based on density functional theory. Firstly, we select the different cutoff energy and k-point grid in the calculations, and obtain the most stable geometry structure of single crystal LaAlO₃. The calculated lattice parameters are a=b=5.441 (A|°), c=13.266 (A|°), which matches with experimental values (a=b=5.365 (A|°),c=13.111 (A|°)). In order to obtain the quantitative nature of chemical bonds, the analysis on the overlap populations of the electron cloud are carried out, which show that the overlap populations of O-Al is 0.38, and that of O-La bond is 0.18. O-Al bond is covalent and its bond energy is stronger than O-La. From the band structure and density of states, It can be clearly seen that LaAlO₃ have a direct band gap (Eg = 3.6eV), whose width is smaller than that of the actual band gap 5.6eV. This result is related to the calculation of the electrons'exchange correlation potentials by using the generalized gradient approximation method. The valence band attributes mainly to the electrons of O 2p orbit, which also has the mixed contributions of the electrons of Al 3s3p and La 5p orbits. But at the Fermi surface, the main contribution comes from the electrons at O 2s orbit and the contribution on conduction band mainly come from the electrons respectively at La 5d and the Al 3s3p orbits. The investigation of the atoms'PDOS shows that Al 3s3p and O 2s, O 2p produce strong hybridizations respectively at the ranges of -18.3eV - 16.1 eV and -6.8eV - 1.4eV. LaAlO₃ is doped with Mg in B sites. Through analysis of the state density, it is found that the density of states is move to the direction of high energy with 0.5eV. If LaAlO₃ is doped with Mg in A sites, the same phenomenon is found. The optical properties of pure LaAlO₃, Mg: LaAlO₃ and Sr: LaAlO₃ are respectively studied, and the dielectric function, absorption coefficient spectrum, reflectance spectrum and the energy loss spectrum of these materials are also investigated with the theoretical analysis. It is found that the imaginary part of dielectric function exist a new peak near 0.23eV for the doped crystal. By comparing with the densities of states, it indicates that the new peak is resulted in the transition between the impurity electronic energy band and the La 5d orbit. By comparing the the absorption spectra, reflection spectra and energy loss spectroscopy of the pure LaAlO₃ and doped crystals, it is found that transmittance in the visible region for the doped LaAlO₃ is greatly increased.