Superstoichiometry, accelerated diffusion, and nuclear reactions in deuterium-implanted palladium

Deuterium (D) was introduced into Pd at atomic ratios greater than one by ion implantation at cryogenic temperatures. The rise and saturation of the D concentration at these low temperatures, together with the decay of the superstoichiometric state during annealing, were observed by detecting charged particles from the nuclear reaction D(d,p)T when a deuteron beam impinged on the specimen. At implantation temperatures of 41 and 81 K, a saturation concentration ratio [D]/[Pd] of 1.6±0.2 was reached, substantially above the limit of 1.0 observed in gas-phase charging. As the temperature was subsequently ramped upward, [D]/[Pd] abruptly decreased to approximately 1.0 near 120 K, reflecting a process of accelerated transport unique to the superstoichiometric state. The responsible diffusion mechanism was theoretically examined using analytical modeling and molecular-dynamics simulations, leading to a picture of correlated D hopping among octahedral and tetrahedral interstitial sites. Cold fusion was not detected in the absence of external ion bombardment, implying an upper bound on the reaction rate in the high-concentration deuteride of approximately ${10}^{\mathrm{-}21}$ events/s D atom.