Magnetic grain boundaries in Ni and Fe and defect structure in zirconia-based materials

Abstract:
In the first part of the lecture, the results of grain boundary (GB) studies in magnetic materials Ni and Fe will be presented. In particular, we studied the Σ5(210)[001] GB in Ni where we examined its structure taking into account imperfections like vacancies and impurities. For this purpose, the Vienna ab initio simulation package (VASP) was employed. It turns out that vacancies significantly influence segregation of studied impurities at this GB. In addition, we found that vacancies in the studied GB undergo the so called ‘delocalization’ when the open volume connected with them effectively disappears. This is in agreement with well known fact that GBs behave as a sink for vacancies. First results for the magnetic anisotropy of the Σ5(210) GB will be also presented. Next, we investigated two GBs in Fe, namely Σ5(210)[001] and Σ3(111)[-111]. Their structure was first determined using molecular dynamics (MD). As for the Σ5(210) GB, we found totally five different configurations. Subsequent relaxations using VASP confirmed the stability of four from five GB configurations found using MD. In the case of the Σ3(111) GB, MD studies resulted in three different configurations and VASP relaxations are currently in progress to confirm these structures. Finally, the segregation of Cr on selected configurations of the Σ5(210) and Σ3(111) GBs was examined using the Metropolis Monte Carlo method. First results show that Cr atoms do segregate at the GBs studied and the segregation effect decreases with increasing temperature and decreasing Cr concentration. In the next part of the lecture, principles of positron annihilation employed often for defect studies will be explained including methods that can be used to calculate positron characteristics like positron lifetime and momentum distribution of electron-positron pairs. In materials based on zirconia (ZrO­2) the defect structure is rather complicated and its current understanding will be shortly discussed. Furthermore, we shall present results of structure relaxations and positron calculations for selected defects in ZrO2 like oxygen (VO) and zirconium vacancies and VO-2Y complexes that exist in yttria stabilized zirconia (YSZ) in large concentrations. Results for hydrogen-related defects will be also shown. Possibilities of positron annihilation in defect studies of YSZ will be then summarized.