Self-trapped hydrogen states in metals determined from quantum mechanical calculations using potentials based on ab initio data: II. Hydrogen isotopes in Fe

For pt.I. see ibid. vol.6, 7679-7704, (1994). An auxiliary short-ranged Fe-H pair potential is constructed on the basis of ab initio pseudopotential calculations for hydrogen in alpha -Fe. This pair potential is used in self-consistent quantum mechanical calculations of hydrogen states and lattice displacements which have been performed in order to investigate the relative stability of self-trapped states formed at tetrahedral (T) or octahedral (O) sites as a function of the isotopic mass. It is found that for the light hydrogen isotopes mu ⁺ and pi ⁺ the total energy is lower at the O site whereas it is lower at the T site for the heavy isotopes p, d and t. The site with higher energy is found to be unstable. The physical origin of the stabilization of O sites for the lighter particles is the lower vibrational energy at the O site whereas the isotope effect in the lattice displacements is of minor importance in this respect.