The structural, dielectric and piezoelectric properties of (1-x)(Bi 1/2 Na 1/2 ) TiO 3 -xBaTiO 3 ceramics were investigated for the compositional range, x=0.02, 0.04, 0.06, 0.08, 0.10. The samples were synthesized by a conventional solid-state reaction technique. All compositions show a single perovskite structure, and X-ray powder diffraction patterns can be indexed using a rhombohedral structure. Lattice constants and lattice distortion increase while the amount of BaTiO 3 increases. The X-ray diffraction results show the morphotropic phase boundary (MPB) of (1-x)(Bi 1/2 Na 1/2 ) TiO 3 -xBaTiO 3 exists in near x=0.06-0.08. Temperature dependence of dielectric constant ε T 33 /ε 0 measurement reveals that all compositions experience one structural phase and two ferroelectric phases transition below 400 o C: rhombohedral (or rhombohedral plus tetragonal) ferroelectric phase tetragonal antiferroelectric phase tetragonal paraelectric phase. Relaxor behaviors exist in the course of ferroelectric to antiferroelectric phase transition. Dielectric and piezoelectric properties are enhanced in the MPB range for (1-x)(Bi 1/2 Na 1/2 )TiO 3 -xBaTiO 3 .

The electronic structures and optical properties, the atomic population, bond lengths, band structures and density of states (DOS) of undoped and doped rare earth elements (Y, La) in γ-Si3N4 have been calculated by means of plane wave pseudo-potential method (PWP) with generalized gradient approximation (GGA). The gap after doping will decrease and enables the formation of new semiconductor，which affords to help finding new semiconductor. We researched optical properties after doping of rare earth elements (Y, La), and found that the static dielectric constant of doped γ-Si3N4 is much higher than that of the undoped material，which may serve as new dielectric and refractive material，and may have special applications in certain optical devices.

Electrical structures and optical properties of β-Si3N4 have been calculated by means of plane wave pseudo-potential method (PWP) with GGA. The results such as lattice constants and band structures are in good agreement with experimental data. Furthermore, the pressure-dependent coefficient and band gap of β-Si3N4 have also been calculated, which will be helpful in the application of Si3N4 under high pressure.

In this paper, we construct a ferromagnet/semiconductor/ferromagnet parabolic well magnetic tunneling junction with double barriers as research object by inserting another semiconductor as a barrier between ferromagnetic and semiconductor potential wells. On the basis of the quantum coherent transport theory and transfer matrix method, we investigate the spin polarized electron transport and the tunnel magnetic resistance (TMR) in parabolic well magnetic tunneling junction with double barriers. We derive the analytical expressions of transmission probability, tunnel magnetic resistance and spin polarization from the new magnetic tunneling junction mode. The significant quantum size, Rashba spin orbit interaction, the angle effect and the thickness of the double barriers layer are discussed simultaneously. The results indicate that the tunnel magnetic resistance shows periodic variation as the width of the parabolic-well at different angles. The TMR is monotonically decreasing when the angle varying from 0 to up, which reflects the structure of the spin valve effect. Meanwhile, results also show that the spin polarization and the tunnel magnetic resistance oscillate with the same period for different barriers thickness. The phase difference appears after inserting the barriers. With increasing the barriers width, phase difference becomes large. The amplitude and peak to alley ratio of the spin polarization and the tunnel magnetic resistance are increase with the barrier width increases. Furthermore, the spin polarization make quasiperiodic oscillation that the oscillation amplitudes become large, the period and peak to alley ratio are decrease as the Rashba spin-orbit coupling strength increases. It appears the spin flip phenomenon as increasing the thickness of the barriers. The TMR shows the typical properties of resonant tunneling with the increasing of the spin orbit coupling strength. In order to better reveal the role of the symmetry double tunnel barriers in the parabolic well structure, we calculate TMR against the thickness of the double barriers. It is found that the existence of the double tunnel barriers increase the TMR and the spin polarization significantly, which shows that the large TMR value can be obtained with the suitable layer thickness of the double barriers layer and the Rashba spin-orbital coupling coefficients. These characteristics are helpful to promote the development and application of new magnetic tunnel junctions.

The effects of vacant, O defects and As doping on the structures and properties of Ge nanocrystals (Ge-ncs) are investigated by using first-principles calculation based on the density functional theory (DFT). The calculation results indicate that the O defects induced by thermal annealing cannot compensate for the defects caused by neutron irradiation in Ge nanocrystals, while the introduction of As produced by neutron transmutation doping (NTD) will do the jop. We also show that the strong attraction between O and Ge atoms inhibits the formation of vacant defects in Ge nanocrystals, and further improve the luminescent property of Ge-SiO2 system. This suggests that it is necessary to perform thermal annealing for Ge-ncs structures before NTD. Our calculations well support our previous experimental results.