The Properties Of Y₃Al₅O₁₂ From First Principle

The dissertation is devoted to the study of physical properties of Y3Al5O12 from first principles . The concerned properties which include geometry structure,electronic structure, optical properties,thermodynamic properties and thermal transport properties are researched in this dissertation. The study can deepen people's understanding of the properties of YAG above.The simulated x-ray diffraction spectrum of YAG agrees well with the experimental one.And then, the geometry structure is optimized ,and lattice constant a=1.2223nm is achieved. We use General Gradient Approximation(GGA)to study the electronic structure of YAG. Both density of states and partial density of states are obtained. It is considered that the valance band is mainly offered by the 2p electronic states of O , while conduction band by the 4d electronic states of Y. At the same time, we study the optical properties .The dielectric spectrum, refraction index spectrum, absorption spectrum, reflection spectrum and loss spectrum are achieved. The calculation results show that the peak located -10eV is mainly derived from the translation between the 4d electronic states of Y and the 2p electronic states of O, while the peak located -25eV from the translation between the 4d electronic states of Y and the 2s electronic states of O,and the peak located -30eV from the translation between the 4d electronic states of Y and the 3d electronic states of Y. The calculation results also show that YAG is transparent in visible band.Based on density functional perturbation theory (DFPT) and combined with the norm-conserving pseudopotential method,the lattice dynamic simulation is presented. The total phonon density of states, partial phonon density of states and phonon dispersion spectrum are obtained. By using the special point sampling method within the first Brillouin zone, the special heat capacity and the population averaged group speed of phonon are calculated. The anharmonic phonon mean free path is calculated theoretically within anharmonic interaction and by using the Fermi's golden rule scheme combined with the special point sampling method within the first Brillouin zone. With two types of the phonon scattering mechanisms considered comprehensively, the thermal conductivity of YAG ceramic is achieved. Our work indicates that the grain boundary scattering will play a major role in the thermal resistance at low temperature in YAG ceramic, while the three phonon interaction contribution to the thermal resistance will prevail above a certain temperature. Meanwhile, the viewpoint, put forward by Y.Sato et al., is theoretically proved that the difference of the thermal conductivity of between YAG ceramic and single crystal can be ignored above room temperature. The temperature variation of the calculated thermal conductivity and special heat capacity agrees well with the experimental results.