LATTICE-DISTORTIONS AROUND FROZEN AND MOBILE HYDROGEN IN NIOBIUM - A MOLECULAR-DYNAMICS

We have studied the hydrogen-induced lattice distortions in niobium us ing the molecular-dynamics simulation technique. A two-exponential int eratomic potential for the H-Nb interaction is proposed, based on rece nt first-principles calculations for the H/Nb system. The quasielastic diffuse scattering cross section is determined and compared with neut ron scattering data. The importance of the hydrogen motion is elucidat ed by comparing results from dynamic simulations with static defect mo dels and we find that the usual way of introducing the thermal motion of the atoms through a Debye-Waller factor gives a rather imprecise de scription of the q dependence of the scattering cross section. We also find some evidence that the actual time scale for the hydrogen motion influences the shape of the scattering function at high temperatures (450 K) while at lower temperatures (300 K) this effect is of minor im portance. When comparing with the experimental results we find agreeme nt for the q dependence but not for the absolute intensities.