Deuterium clusters in a strained palladium lattice
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Ricerche
Fabrizio Cleri Dipartimento Reattori Veloci, Divisione Metodi di Progetto, E.N.E.A.-Centro Ricerche Energia Casaccia, P.O. Box 2400, 1-00100 Roma A.D, Italy (Received 9 April 1990; accepted 9 July 1990)
We report a first observation of the behavior of clusters of deuterium atoms confined in a lattice vacancy in a Pd crystal, either at equilibrium or under the influence of externally applied lattice strains. Molecular dynamics simulations have been performed on both Pd perfect crystals and Pd crystals with a single vacancy, showing the energetic behavior and the dynamical values of the interatomic distance among deuterium atoms belonging to clusters of different sizes confined within the vacancy. At low temperatures, the smallest interatomic D-D distances are of the order of the D2 molecular bond length and are obtained during the transitory regimes induced by hydrostatic strains applied to the lattice.
I. INTRODUCTION
The study of the microscopic behavior of hydrogenlike atoms in transition metals has received renewed impetus after the recent debate on the possible neutron emission from deuterated metal samples in nonequilibrium conditions.1"5 Several theoretical investigations have been performed so far to study the microscopic scale behavior of deuterium in solid palladium either in the low concentration limit6'7 or in the form of PdD or PdD2 clusters.8-9 The occurrence of two physical conditions has been claimed to be required for a mechanism capable of producing solid-state nuclear reactions: (1) the possibility of concentrating a quite large amount of energy (of the order, at least, of some hundreds of electronvolts10) in a relatively small volume of the lattice (of the order of the unit cell volume or less); (2) the possibility of confining two or more deuterium atoms within such a small volume. Possible scenarios taking into account these conditions have recently been proposed.1112 We would in turn focus attention on the fact that the energy-concentration mechanism could not be sufficient, alone, to account for the release of the necessary amount of energy to the couple of deuterium atoms. As has recently been stressed, deuterium interstitials cannot reduce their relative distance down to values where the probability of a D-D fusion would become appreciable (rDD < 0.3 A) within a perfect lattice (i.e., in configurations where multiple occupancy of the interstitial sites should be allowed6'8). Vacancies, dislocations, and grain boundaries could, in principle, provide a more efficient trapping for deuterium atoms,13 as has recently been reasserted by Nordlander et a/.8 We have investigated the possibil2094
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 10, Oct 1990
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ity of a more efficient spatial confinement by means of a joint mechanism where the trapping of a few deuterons occurs through the action of a lattice vacancy, and the energy required to reduce the D-D distance is furnished by elastic strains imposed on the lattice. Strong distortions, in fact, are
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