Xing Gao

EFFECT OF STRAIN FIELD ON THRESHOLD DISPLACEMENT ENERGY OF TUNGSTEN STUDIED BY MOLECULAR DYNAMICS SIMULATION

The influence of strain field on defect formation energy and threshold displacement energy (Ed) in body-centered cubic tungsten (W) is studied with molecular dynamics simulation. Two different W potentials (Fikar and Juslin) are compared and the results indicate that the connection distance and selected function linking the short-range and long-range portions of the potentials affect the threshold displacement energy and its direction-specific values. The minimum Ed direction calculated with the Fikar potential is 〈100〉 and with the Juslin potential is 〈111〉. Nevertheless, the most stable self-interstitial configuration is found to be a 〈111〉-crowdion for both the potentials. This stable configuration does not change with the applied strain. Varying the strain from compression to tension increases the vacancy formation energy while decreases the self-interstitial formation energy. The formation energy of a self-interstitial changes more significantly than a vacancy such that Ed decreases with the applied hydrostatic strain from compression to tension.

First-principles investigation of vacancies in LiTaO3.

Formation energies of neutral and charged vacancies in lithium tantalate, as well as their electronic states have been investigated through first-principles calculations. It is found out that [Formula: see text], [Formula: see text] and [Formula: see text] are the most energy favorable vacancies on O, Li and Ta sites respectively. The formation energy of vacancy on the O site is lower in oxygen poor environments than that in oxygen rich environments, and the formation energy of vacancy on the cation site is lower in oxygen rich environments than that in oxygen poor environments. Among all types of neutral vacancies considered in this study, the energy of Li partial Schottky reaction, 2[Formula: see text]u2009u2009+u2009u2009[Formula: see text], is the lowest at almost all considered chemical environments. Taking into account the distribution of vacancy induced energy levels, we suggest that either [Formula: see text] or [Formula: see text] is responsible for the light absorption band around 460u2009nm observed in annealed LiTaO3 crystal.

Structural Distortion and Defects in Ti

Ti3AlC2 samples are irradiated in advance by 3.5MeV Fe-ion to the fluence of 1.0×10 16 ion/cm 2 , and then are implanted by 500 keV He-ion with the fluence of 1.0×10 17 ion/cm 2 at room temperature. The irradiated samples are investigated by grazing incidence x-ray diffraction (GIXRD) and transmission electron microscopy (TEM). GIXRD results show serious structural distortion, but without amorphization in the irradiated samples. Fe-ion irradiation and He-ion implantation create much more serious structural distortion than single Fe-ion irradiation. TEM results reveal that there are a large number of defect clusters in the damage region, and dense spherical He bubbles appear in the He depositional region. It seems that the pre-damage does not influence the growth of He bubbles, but He-ion implantation influences the pre-created defect configurations.