H Motion in Pd and Nb: A Molecular-Dynamics Study
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H MOTION IN Pd AND Nb: A MOLECULAR-DYNAMICS STUDY Yinggang Li* and Gbran Wahnstrbm** *Institute of Theoretical Physics, Chalmers University of Technology and University of G6teborg, S-412 96 GCteborg, Sweden "*Department of Applied Physics, Chalmers University of Technology and University of Gbteborg, S-412 96 G6teborg, Sweden ABSTRACT Based on realistic many-body potentials molecular-dynamics simulations are carried out for PdH0 .0 3 and NbH0 .02 . The H motion is investigated at two different temperatures, T = 300 and 600K, paying attention to the vibrational and diffusive motion. We find that the motion of H in Nb, a bcc metal, is more complicated than in Pd, a fcc metal, and the differences are discussed. When detailed comparison is made with quasielastic neutron scattering data for H in Pd at 600K, we argue that in order to characterize the diffusion correctly, one has to include nonadiabatic effects. INTRODUCTION Metal-hydrogen systems are of applied as well as of basic physical interest and have attracted great attention from both the experimental and the theoretical communities [1]. Over the last 20 years extensive neutron scattering studies have been performed which give detailed information on the H motion on an atomistic scale. Numerical molecular-dynamics (MD) simulations provide complementary information and recently this method has been used for theoretical studies of metal-hydrogen systems. In that case a sufficiently accurate model for the interatomic interactions is required as input. New potential models have been developed during the last decade for metals and their alloys. The embedded-atom method (EAM)[2] and the similar scheme by Finnis and Sinclair [31 are two such approaches, which are suited for fcc and bcc transition metals, respectively. The EAM has been used by Pratt and Eckert [4] and more recently by us [5, 6] to study H motion in Pd. The Finnis-Sinclair model has been applied by Gillan and his coworkers[7, 8] in the study of quantum vibrational states and quantum diffusion of H in Nb. This paper presents our MD results for the motion of hydrogen in Pd and some preliminary results for H in Nb. PdH., and NbHZ are typical representatives of fcc and bcc metalhydrogen systems, respectively, and they are extensively studied experimentally. First we present the models to be used. This is followed by a presentation of the results. We show the stationary positions for H and give the numerical values for the diffusion constant. We then determine the H motion at room temperature, where the vibrational properties are investigated, and at higher temperatures, where the detailed features of H diffusion are studied. The results are compared with quasielastic-neutron-scattering experiment and the effect of electron-hole pair excitations is discussed. THE SYSTEM AND THE MANY-ATOM INTERACTION MODELS For PdHZ we use 256 Pd atoms and 8 H atoms, except in the study of the vibrational motion where only 1 H atom is included. Periodic boundary conditions are always emMat. Res. Soc. Symp. Proc. Vol. 291. 0199
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