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First-Principles Study of Superabundant Vacancy Formation in Metal Hydrides

Summary: First-Principles Study of Superabundant Vacancy Formation
in Metal Hydrides
Changjun Zhang and Ali Alavi*
Contribution from the Department of Chemistry, UniVersity of Cambridge, Lensfield Road,
Cambridge CB2 1EW, United Kingdom
Received January 24, 2005; E-mail: asa10@cam.ac.uk
Abstract: Recent experiments have established the generality of superabundant vacancies (SAV) formation
in metal hydrides. Aiming to elucidate this intriguing phenomenon and to clarify previous interpretations,
we employ density-functional theory to investigate atomic mechanisms of SAV formation in fcc hydrides of
Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au. We have found that upon H insertion, vacancy formation energies
reduce substantially. This is consistent with experimental suggestions. We demonstrate that the entropy
effect, which has been proposed to explain SAV formation, is not the main cause. Instead, it is the drastic
change of electronic structure induced by the H in the SAV hydrides, which is to a large extent responsible.
Interesting trends in systems investigated are also found: ideal hydrides of 5d metals and noble metals
are unstable compared to the corresponding pure metals, but the SAV hydrides are more stable than the
corresponding ideal hydrides, whereas opposite results exist in the cases of Ni, Rh, and Pd. These trends
of stabilities of the SAV hydrides are discussed in detail and a general understanding for SAV formation is
provided. Finally, we propose an alternative reaction pathway to generate a SAV hydride from a metal
1. Introduction


Source: Alavi, Ali - Department of Chemistry, University of Cambridge


Collections: Chemistry