Investigation of Lattice Defects in LaNi5 by Positron Annihilation Spectroscopy and First-Principles Calculations

摘要

Positron annihilation spectroscopy and first-principles calculations have been applied to identify lattice defects introduced in LaNi5, which is widely used as hydrogen storage materials, during hydrogen absorption-desorption process. Positron lifetime measurement and calculated positron lifetimes revealed that Ni vacancies are formed during hydrogen absorption in LaNi5. First-principles calculations have been performed in order to estimate the stability of the Ni vacancy in the hydrogen absorption process. The binding energy at the Ni sites is reduced by absorbed hydrogen atoms in LaNi5 hydrides. In the case of the Ni 2b site surrounded by four hydrogen atoms, the binding energy is lowered by about 0.7 eV. Another contribution to the stability of the Ni vacancy is a decrease in the energy of additional lattice sites occupied by the atoms removed as vacancies. In the hydrogen absorption process, additional hydrogen sites are occupied by hydrogen atoms supplied in the process, which leads to the decrease in the vacancy formation energy. As a result, the formation energy of Ni vacancy is reduced to almost zero in the hydrogen absorption process of LaNi5.