  Properties of point defects resulting from the incorporation of inert-gas
atoms in bcc tungsten are investigated systematically using first-principles
density functional theory (DFT) calculations. The most stable configuration for
the interstitial neon, argon, krypton and xenon atoms is the tetrahedral site,
similarly to what was found earlier for helium in W. The calculated formation
energies for single inert-gas atoms at interstitial sites as well as at
substitutional sites are much larger for Ne, Ar, Kr and Xe than for He. While
the variation of the energy of insertion of inert-gas defects into interstitial
configurations can be explained by a strong effect of their large atomic size,
the trend exhibited by their substitutional energies is more likely related to
the covalent interaction between the noble gas impurity atoms and the tungsten
atoms. There is a remarkable variation exhibited by the energy of interaction
between inert-gas impurities and vacancies, where a pronounced size effect is
observed when going from He to Ne, Ar, Kr, Xe. The origin of this trend is
explained by electronic structure calculations showing that p-orbitals play an
important part in the formation of chemical bonds between a vacancy and an atom
of any of the four inert-gas elements in comparison with helium, where the
latter contains only 1s2 electrons in the outer shell. The binding energies of
a helium atom trapped by five different defects (He-v, Ne-v, Ar-v, Kr-v, Xe-v,
where v denotes a vacancy in bcc-W) are all in excellent agreement with
experimental data derived from thermal desorption spectroscopy. Attachment of
He clusters to inert gas impurity atom traps in tungsten is analysed as a
function of the number of successive trapping helium atoms. Variation of the
Young modulus due to inert-gas impurities is analysed on the basis of data
derived from DFT calculations.
