Band-structure calculations for Ni, Ni4H, Ni4H2, Ni4H3, and NiH

Abstract

We present a self-consistent calculation of the electronic structure of different nickel hydrides based on the linear-muffin-tin-orbital formalism and using the local-density approximation for the exchange and correlation. The calculations for the ground state show a continuous decrease of the saturation magnetization of Ni by increasing the hydrogen concentration as a result of changes in the exchange-splitting parameter and in the Fermi energy. The presence of hydrogen in the Ni matrix modifies the electronic states of the bulk. New impurity states appearing far below from the d band of the host are found in agreement with photoemission measurements. The density of states at the Fermi level increases as a function of the hydrogen concentration, giving in this way a greater γ coefficient for the electronic specific heat than that of pure Ni. The equilibrium lattice parameter of the various Ni hydrides also increases as a function of the hydrogen concentration, reaching a saturation value for a concentration of approximately 75 at. %. All features calculated are supported by experiments, and our first-principles calculation explains the increase of the electronic specific heat together with the loss of the ferromagnetism. We also found that the electronic density around the proton occupying the octahedral sites in the fcc Ni matrix is larger than that in the free atomic hydrogen. The electron transfer occurs from the Ni atoms to the hydrogen. The polarization of H in Ni is directed opposite to the bulk polarization and of the same order of magnitude as for positive muons in the same matrix. The hyperfine field of muon in Ni can be derived by use of the calculated values for the spin density around the proton and by taking into account the zero-point motion of the positive charge around its equilibrium position.