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An irreversible conductor-insulator transition has been observed when heating Mg2NiH4 in the temperature interval 110 to 570 K. The disappearance of the electric conductivity is concomitant with the appearance of stacking faults (or microtwinning) in the Mg2NiH4 structure, as observed by powder x-ray diffraction. However, the stacking faults are sensitive to applied mechanical pressure or grinding, and by compressing the hydride sample in a tablet press, Mg2NiH4 regains its electric conductivity as the observable amount of stacking faults is reduced. These phenomena are attributed to peculiarities connected with the stabilization of the electron-rich tetrahedral d10 [Ni(0)H4] complex by the lattice. Formally low-valent oxidation states usually demand good electron-accepting ligands with suitable π* or d orbitals to relieve the high electron density at the central atom. This is not possible when hydrogen is the only ligand, but the easily polarizable H− ion helps to distribute electron density by outward b...

The hydride Mg2NiH4 exhibits a phase transition from a low temperature to a high temperature phase at about 508 K. The dynamics of the hydrogen in the high temperature phase have been studied by cold neutron scattering experiments. According to these, the hydrogen atoms showed a high mobility, but the motion was mainly localized, in some way, on the six available octahedral positions around each nickel atom. The translational diffusion coefficient appeared simultaneously to be very small, within the instrumental resolution, implying values

High accuracy quantum chemical calculations (CASSCF and contracted CI) have been performed on the transition metal hydrogen complexes in the Mg2FeH6 and Mg2NiH4 crystals. The effects from the surrounding crystal are simulated by a Madelung field. If the conventional charge of +2 is assumed for the magnesium ions the transition metal hydrogen complexes receive a charge of −4. Good agreement with experiment is then obtained for the bond distance and totally symmetric vibrational frequency for the iron complex. The calculated Fe–H distance in the octahedral FeH4−6 complex is 1.56 A, which is identical to the value obtained from neutron diffraction data on the Mg2FeH6 crystal. For the nickel compound also the unconventional case of Mg+ has to be considered, yielding a 2− transition metal hydrogen complex. The structure for the NiH4 complex is not as well determined as the iron complex. Experimental data has been interpreted in terms of a planar structure but a tetrahedral structure is not ruled out. Our calcu...