Hydrogen insertion effects on the magnetic properties and chemical bonding within C14 Laves phases
Résumé
The Laves phases family, AB2 (A = 3B, 4B, rare earth or actinide; B = transition metal) is among the largest of binary intermetallic systems. They readily absorb hydrogen and as such they are of potential use for energy storage. This work presents a thorough electronic and magnetic structure study within these systems among which we select ScFe2 and its hydride ScFe2H2 which crystallize in the C14 (2H) hexagonal structure. The relevance of this study for solid state chemistry pertains to the complexity brought by the presence of two distinct crystal sites for Fe with different magnetic properties (ordered moments, hyperfine fields) and bonding with hydrogen whose insertion sites were to be defined. Such issues, not considered experimentally, are addressed here within the well-established quantum mechanical density functional theoretical framework (DFT) using both pseudo-potential calculations for geometry optimization and all-electrons investigations for full study of the electronic, chemical bonding and magnetic structure properties. From energy–volume quadratic curves providing the equation of state, the hydride is found more compressible at a higher equilibrium volume than the pristine intermetallic. This stresses the negative pressure brought by hydrogen. Cohesive energy studies show the stability of hydrogen within ScFe2. From electron localization function (ELF) plots the expected picture of a negatively charged hydrogen within an alloy lattice is obtained. The chemical bond of H within the A2B2 tetrahedron formed of Sc and one type of Fe is discussed. Magnetic moments and Fermi contact term HFC of the effective hyperfine field Heff are found within range of the average experimental values in both the alloy and its hydride. For ScFe2H2 a peculiar feature of the magnetic moment magnitude inversion and of HFC for the two iron sites is found to be connected with a change in magnetic characters of the two Fe sites, becoming strongly and weakly ferromagnetic, respectively, for the isolated iron and the H connected Fe.