First Principles Study Of Ceramic Materials Rbaco <sub> 4 </ Sub> O <sub> 7 </ Sub> And Pb Films Quantum Size Effect

In this paper, the first-principles, i.e the density functional theory (DFT) methods was used to calculate the nature of the material. In the first part a new type of composite material RBaCo₄O₇ perovskite (R=Ca, Y, Yb) was studied to optimize their lattice constant, and investigate their band structure and density of states in the ground state. Problems encountered in the present study was discussed and ideas to solve these problems was proposed.The results show that the low-level energy band is provided by O-2s Y-4p electrons, and most level of the band is from the contribution of Ba and O atoms of BaO₁₂, Y and O atoms of YO₈, and Co,O atoms of CoO₄. A mixed-valenceelectronic images of arbitrary ratio of valence of Co⁺² and Co⁺³ was given. It is pointed out that the electrons near the Fermi level mainly occupied by Co-3d, the localization and the strong correlation of Co-3d electron is the main reason that leads to a incorrect band structure near the Fermi energy in our present calculations.In the second part of this paper, we study the quantum size effect(QSE) in.growth of free-standing Pb films, Pb films on the substrate of GaAs (111) with different polarities, and Pb films with doped Bi. From the perspective of the film and the substrate, the change of surface energy,work function and lattice relaxation of different film thickness were calculated and their effects on the film growth were compared.Calculations for free-standing Pb films with different thickness shows that the change of the surface energy, work function, and the lattice relaxation of two layers near the surface with the thickness has oscillation mode in which a long 9ML periodicity is superimposed on a small 2ML periodicity. This conclusion is consists with reported experimental results and calculations of others work and reveals the role of the quantum size effect in modulating the growth of thin film.Finally the quantum size effect of the Bi atoms doped Bi-Pb alloy films was studied. The results show that the double oscillation phenomena in the surface caused by the quantum size effect is still existed, however, such oscillations decay quickly above 20ML due to the interaction between Bi-Bi. We also calculated the effect of Ge (111) and Si (111) substrates on Pb films. It was found that these two kinds of substrates have almost the same impact on Pb alloy films, which is consists well with the experimental results.