First-principle Study On Optical And Surfaces’ Properties Of Uranium Dioxide

This thesis mainly presents a study on optical and surfaces’ properties of uraniumdioxide based on first-principle. And before the study of optical property and surfaces, thecomputational parameters and crystal structure was optimized, which mainly were cutoffenergy and the K-mesh. Additionally, the uranium in uranium dioxide has a magneticmomentum, which mainly came from electrons of local non-filled5f shell. The electronsthat have different spins in one atom belong to different quantum states, so they can be inthe same point, then they impulse strongly to each other. In other words, the uraniumdioxide is a strongly correlated system. The Hubbard model should be used, and here, weused Dudarev method, in which the U and J was4.5and0.5eV respectively.We optimized the energy cutoff and K-mesh in the first place. Pseudo potentials andrelated exchange-correlation method used GGA-PBE method. When the energy-cutoffwas larger than450eV, the internal energy would be converged, and the fluctuation wasless than0.01eV. When the K-mesh concentration larger than3×3×3, the energy alsobe converged, and the fluctuation was less than0.001eV. According to experiences, whenthe MP scheme was used and the K-mesh was less than8×8×8, even divided meshwould be calculated amount favored. So we chose4×4×4.The next was the optimization of crystal structure. Two cases were considered, bothof which were all optimized in1k AFM, one fully relaxed, the other was relaxed withfixed shape. The result of former was a=b=5.558, c=5.496, and the lattera=b=c=5.531. And the former was energetically favored about4meV. They were almostthe same. Considered the construction of surfaces lately, we used the latter result. Theenergy gap was2.1eV, the same as experimental results and other theoretical results. Wecould see from the partial density of states that, the valence band were mainly constructedby U5f and O2p, while the conduct band the U6d and U5f.Thirdly was the calculation of optical properties, which were mainly of dielectric function, refractivity, reflectivity, absorptivity and the energy loss. The dielectric functioncould be calculated by VASP directly, and the reflectivity could be calculated by Fresnelformula. The refractivity, absorptivity and the energy loss could be calculated fromdielectric function. And the results were all correspond to other researchers’.At the end, properties of three different surfaces were calculated, mainly of theirstructures, Bader charge population and their magnetic properties. The result show that,the most energetically favored was (111) crystallographic plane, and the most unstablewas (100) plane. It was the largest site change of atoms in the utmost two layers thatresult in the instability of (100) plane. The Bader populations of the U and O atoms in theutmost layer become smaller. The magnetic momentums of the U atoms in the utmost twolayers become smaller while O atoms become larger. At last, the related concentrations ofdifferent planes range temperature from0to1600K were calculated. The results showthat, not only the AFM case, but the FM case, the related concentration of (111) plane wasthe largest. That was to say, the (111) plane was always the most stable in the threedifferent surfaces while the (100) plane was the most unstable.