Mechanically reversible conductor-insulator transition in Mg2NiH4

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 d(10) [Ni(0)H-4] complex by the lattice. Formally low-valent oxidation states usually demand good electron-accepting ligands with suitable pi(*) 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 bonding to the lattice. This also stabilizes the [NiH4] complex. The present work shows that this stabilization mechanism is sensitive to disturbances in the lattice and that small disturbances can have profound effects on the electric conductivity of Mg2NiH4. (C) 2002 American Institute of Physics.