SELF-TRAPPED HYDROGEN STATES IN METALS DETERMINED FROM QUANTUM-MECHANICAL CALCULATIONS USING POTENTIALS BASED ON AB-INITIO DATA .2. HYDROGEN ISOTOPES IN FE

An auxiliary short-ranged Fe-H pair potential is constructed on the ba sis of ab initio pseudopotential calculations for hydrogen in alpha-Fe . This pair potential is used in self-consistent quantum mechanical ca lculations of hydrogen states and lattice displacements which have bee n performed in order to investigate the relative stability of self-tra pped states formed at tetrahedral (T) or octahedredal (O) sites as a f unction 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 where as it is lower at the T site for the heavy isotopes p, d and t. The si te with higher energy is found to be unstable. The physical origin of the stabilization of O sites for the lighter particles is the lower vi brational energy at the O site whereas the isotope effect in the latti ce displacements is of minor importance in this respect.