采用第一原理方法，计算了Pb掺杂锐钛矿型TiO2的晶胞几何结构和电子结构。由于Pb的原子半径大于Ti，掺Pb后晶胞体积明显增大；水平方向和顶点方向的Pb-O键键长比相应的Ti-O键键长增长。由于Pb与O原子的sp轨道杂化作用，与Pb近邻的O原子p电子态出现尖锐的峰，并使处于第一价带的p电子态的带宽由4.6eV增至7.3eV；同时O的p轨道产生空态，掺杂后系统的导带底由Pb的p轨道构成。Pb掺杂使得锐钛矿型TiO2的能隙大幅度减小，因此，可以通过掺杂Pb实现对于锐钛矿型TiO2的能带调制，从而改变TiO2的光催化性能。计算结果与实验趋势相符

We have investigated the effects of O on the structure and mechanical properties of NiAl intermetallics using a first-principles pseudopotential total-energy method based on the density functional theory with the generalized gradient approximation. We found that the impurity O atom can either replace Ni atom or go into the tetrahedron interstitial site depending on the surrounding environment. In both cases, O tends to form an Al2O3-like tetrahedron structure with its nearest Al or Ni atoms, leading to the formation of the stronger O-Al bond containing covalent component. We demonstrate that the presence of O will cause an increase of the brittleness and a decrease of the ductility of NiAl based on the calculated elastic constants and the empirical criterions for both substitutional and interstitial cases. Our calculations provide a way to suppress the negative effect of O by adding the alloying element with lower electronegativity than that of Al.

Using a first-principles method, we have investigated structure, stability and diffusion of hydrogen (H) in tungsten (W). We found that single H atom prefers to occupy the tetrahedral interstitial site with the formation energy of～-2.45 eV. Two H in the tetrahedral interstitial sites form a pairing cluster along the <110> directions with the H–H distance of ～2.22, while the corresponding binding energy is only 0.02 eV, indicating a very weak attractive interaction. This suggests that H itself is not capable of trapping other H atoms to form a H2 molecule. The kinetics of H in intrinsic W is discussed, and the diffusion barrier of H that jumps between the tetrahedral interstitials is calculated to be 0.20 eV.

采用基于密度泛函理论和局域密度近似的第一性原理赝势方法，计算了纯Al晶界和杂质Sr偏析Al晶界的原子结构和电子结构, 结果表明Sr偏析引起了晶界膨胀和晶界处电子密度的大幅度降低，从而导致晶界结合力的减弱, 这应为Sr杂质偏析引起的Al晶界脆化的主要根源所在。

A thermodynamic free energy function in the form of an eighth-order polynomial has been developed for bulk potassium niobate KNbO3, based on the Landau-Ginsburg-Devonshire phenomenological theory. The obtained free energy function successfully models the ferroelectricsingle domain intrinsic properties of KNbO3, including the three sequential phase transition temperatures as well as polarizations, dielectric constants, and lattice constants along the whole ferroelectric temperature range. It is applicable to explaining and predicting the extrinsic contributions such as hydrostatic pressure on the bulks properties and substrate constraints on the thin films properties.

Neutral oxygen vacancies in amorphous (a-) SiO2 and GeO2 as well as bulk a-SiO2 and a-GeO2 configurations have been investigated by using the first-principles pseudopotential method in order to elucidate the mechanism of the photorefractive effect of Ge-doped SiO2. Amorphous configurations of SiO2 and GeO2 have been constructed by quenching using classical molecular-dynamics method and subsequent relaxation using the first-principles method. Obtained configurations and electronic properties are in good agreement with experiments of a-SiO2 and a-GeO2, and the gross features of the densities of states(DOS's)of these glasses are similar to each other. The highest valence band is the O-2p nonbonding band and the lowest conduction band consists of Si or Ge orbitals. However, the structural differences between the a-SiO2 and a-GeO2 configurations such as the larger O-O distance in a-GeO2 and the relatively shorter Ge-Ge distance in a-GeO2 induce the peculiar differences in the DOS's. And the band gap of a-GeO2 is much smaller than that of a-SiO2. The oxygen deficient centers (ODC's) are formed by removing identical oxygen atoms from the Si-O-Si and Ge-O-Ge networks. One occupied defect state is generated in the band gap. The Ge-Ge bond at the Ge-ODC is shorter than the Si-Si bond at the Si-ODC in the present configurations. The larger network flexibility and theless electrostatic repulsion in a-GeO2 than in a-SiO2 should cause the shorter Ge-Ge bond length at the Ge-ODC, which results in the lower occupied defect level in the gap in a-GeO2 and the much lower ODC formation energy in a-GeO2. This smaller formation energy indicates much more ODC's in a-GeO2 than in a-SiO2. Therefore more E8 centers may be generated in Ge-doped a-SiO2 than in pure a-SiO2 if a-GeO2 clusters exist in Ge-doped a-SiO2. Furthermore, we have discussed the electron trapping defects.

We investigate the interaction between interstitial oxygen (Oi) and As dopant in heavily As-doped Si using first-principles total-energy calculations. The interaction between Oi and As (substitutional) is found to be short ranged. The most stable configuration is with As and Oi as second nearest neighbors, forming-Si-O-Si-As-type complexes, with a binding energy of 0.14-0.21 eV. These complexes can trap Oi in their vicinity and thus reduce the Oi mobility. But the magnitude of trapping energy by such complexes is about fives times smaller than the value estimated from the experimentally observed retardation of Oi diffusion in heavily As-doped Si. We suggest that structural complexes involving other defects may resolve this discrepancy.

We have investigated the interstitial oxygen (Oi) diffusion in heavily arsenic (As)-doped Si using first-principles calculations. We show that it is not the As per se but the Si vacancy (V) that trap Oi to reduce its diffusion. Arsenic actually plays the role of an arbitrator to activate thermal generation of As-V pairs, which in turn trap Oi with a large binding energy of～1.0 eV, in quantitative agreement with experiments. Our finding solves a long-standing puzzle on the atomistic mechanismunderlying the retardation of Oi precipitation in heavily As-doped Si.

Using a first-principles computational tensile test, we show that the ideal tensile strength of an Al grain boundary GB is reduced with both Na and Ca GB segregation. We demonstrate that the fracture occurs in the GB interface, dominated by the break of the interfacial bonds. Experimentally, we further show that the presence of Na or Ca impurity, which causes intergranular fracture, reduces the ultimate tensile strength when embrittlement occurs. These results suggest that the Na/Ca-induced intergranular embrittlement of an Al alloy originates mainly from the GB weakening due to the Na/Ca segregation.

Using first-principles total-energy calculations, we have investigated the adsorption and diffusion of Si and Ge adatoms on Ge/Si(001)-(2 8) and Ge/Si(105)-(1 2) surfaces. The dimer vacancy lines on Ge/Si(001)-(2 8) and the alternate SA and rebonded SB steps on Ge/Si(105)-(1 2) are found to strongly influence the adatom kinetics. On Ge/Si(001)-(2 8) surface, the fast diffusion path is found to be along the dimer vacancy line (DVL), reversing the diffusion anisotropy on Si(001). Also, there exists a repulsion between the adatom and the DVL, which is expected to increase the adatom density and hence island nucleation rate in between the DVLs. On Ge/Si(105)-(1 2) surface, the overall diffusion barrier of Si(Ge) along e(501T direction is relative fast with a barrier of) 0.83(0.61) eV, despite of the large surface undulation. This indicates that the adatoms can rapidly diffuse up and down the (105)-faceted Ge hut island. The diffusion is also almost isotropic along [010] and ½(501) directions.