Elastic constants and chemical bonding of LaNi₅and LaNi₅H₇by first principles calculations

Abstract

Elastic constants of LaNi5H7 and LaNi5 are calculated by a first principles pseudopotential method using plane-wave basis sets. Some extra calculations using model clusters were made in order to discuss the magnitude of chemical bondings. Inner displacements associated with all deformation modes are taken into account. Elastic constants are smaller and more isotropic in the hydride than those in the host. The electronic mechanism to determine the change in the elastic properties is investigated from the viewpoint of chemical bonding. Strong Ni–H bonds are formed in LaNi5H7 at the expense of Ni–Ni bonds. They play key roles in determining the elastic properties. The isotropic distribution of the Ni–H bonding charge in LaNi5H7 should be responsible for the isotropic elastic constants. Hydrogen atoms are found to relax considerably during the deformation to maintain the Ni–H bond length. When the inner displacements are ignored, the elastic constants of LaNi5H7 are as large as those of LaNi5. However, the remarkable displacement of hydrogen atoms during the elastic deformation plays an essential role in softening by hydrogenation.