| Autor: |
Blomqvist, Helen, Nore´us, Dag |
| Předmět: |
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| Zdroj: |
Journal of Applied Physics; 4/15/2002, Vol. 91 Issue 8, p5141, 8p, 1 Color Photograph, 1 Black and White Photograph, 2 Diagrams, 7 Graphs |
| Abstrakt: |
An irreversible conductor-insulator transition has been observed when heating Mg[sub 2]NiH[sub 4] 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 Mg[sub 2]NiH[sub 4] 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, Mg[sub 2]NiH[sub 4] 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[sup 10] [Ni(0)H[sub 4]] complex by the lattice. Formally low-valent oxidation states usually demand good electron-accepting ligands with suitable π[sup *] 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[sup -] ion helps to distribute electron density by outward bonding to the lattice. This also stabilizes the [NiH[sub 4]] 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 Mg[sub 2]NiH[sub 4]. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
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