The atomic structures and energetics of clean and Y-doped general grain boundary (GB) Σ31/(0001) models in α-Al2O3 are studied by a series of high precision ab initio calculations. A large supercell with 700 atoms and periodic boundary conditions is adopted for undoped and Y-doped GB with different substitution sites and con-centrations. It is shown that Y atoms preferably segregate to the central column of the 7-member Al ring. This is explained as more favorable bond formation for Y in this position and lower GB energy. The calculated GB formation energy for the clean and Y-doped cases is respectively 3.99 and 3.67 J/m2. On the average, the GB region in Σ31 has a slightly lower charge density than the bulk crystalline region. In addtition, the GB induces a long ranged asymmetric electrostatic potential distri-bution on each side of the grain boundary.
CHEN Jun, XU Yun, CHEN DongQuan & ZHANG JingLin Laboratory of Computational Physics, Institute of Applied Physics and Computational Mathematics, Beijing 100088, China
The equation of states （EOS） of high energy explosive HMX （octahydro-l,3,5,7-tetranitro-1,3,5,7-tetrazocine） has been studied by using the first principle method. Our results include the lattice constants, elastic constants, and the dependence of total en- ergy and pressure on volume for β- and 5-HMX. The calculated elastic constants and the pressure-volume relationship of ^-HMX are also compared with the experimental values. The theoretical tensile experiments are implemented on the 13-HMX. The atomic-scale analysis displays that the fracture originates from the intermolecule of HMX and is possibly due to the weak interaction of intermolecules.
The microscopic structures and the bonding properties of Y-doped and undoped （011^-8）/[044^-1]/180° （∑37） grain boundaries in alumina are investigated by using ab initio method. The formation energy of grain boundary and the segregation energy of Y to grain boundary are acquired. Electronic structures, potential distributions, bond orders and effective charges of Y-doped and undoped ∑37 GB systems are calculated. Our results reveal that the higher strength Y-O bond than Al-O bond is ascribed to the hybridization of Y（4p, 3d） with O（2s）. Meanwhile, dopant Y also causes a change in potential distribution in the grain boundary region, thereby further affecting the transport property of ceramic alumina.
