The lattice response to embedding of helium impurities in BCC metals

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Abstract
A calculation scheme which combines a pair-potential approach with the effective-medium theory has been used to study the influence of impurity-induced relaxations on formation and trapping energies and self-trapping phenomena for helium impurities in several metals (K, Fe, Ni, Mo, W). The main effort has been devoted to BCC metals covering a wide range of elastic properties and interstitial electron densities. Only one FCC metal has been included. In all the BCC metals the calculations indicate that when relaxations are taken into consideration the He formation energies (heats of solution) are nearly equal at the tetrahedral and octahedral positions. The energy difference between the two sites is always less than 0.03 eV. In the FCC metal Ni, the octahedral site is found to be the favourable position for interstitial helium. Relaxations are found to decrease the He formation energies by 10-20%, in agreement with a simple model. The calculations reproduce the trends in the experimental trapping energies to monovacancies. Finally, the potential energies of helium in the self-trapped states corresponding to the octahedral, tetrahedral and substitutional positions are mapped and rough estimates of the zero-point energies of helium are given.

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