Structure of La2Ni7 hydride from first-principles calculations

Change in the structure of La2Ni7 during hydrogenation was investigated using first-principles calculations. The most stable site for hydrogen atoms is the tetrahedral site (4f) surrounded by one La and three Ni atoms in the La2Ni4 cell. The 12k site surrounded by three La and two Ni atoms in the La2Ni4 cell is also a stable site for hydrogen atoms. Occupation of these two types of sites induces the lattice expansion of 26.7% along the c-axis in the La2Ni4 cell. It is still possible to insert extra hydrogen atoms at the 4e site surrounded by four Ni atoms in the La2Ni4 cell, which leads to the lattice expansion of more than 70% along the c-axis in the La2Ni4 cell. The change in the structures by hexagonal to orthorhombic transformation during hydrogenation can be reproduced by considering partial occupation of hydrogen atoms.

► The stable sites for H atoms are the 4f and 12k sites in the La2N4 cell.
► The expansion of c-axis is caused by breaking Ni–Ni bonds and forming Ni–H bonds.
► Orthorhombic transformation can be reproduced by partial occupation of H atoms.