High–temperature positron annihilation and vacancy formation in refractory metals

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Abstract
By means of the new technique of sealed–source specimens the Doppler broadening of the positron–annihilation γM line has been investigated in the refractory metals V, Kb, Ta, Mo and W between 4·2 K and the melting points. Tho ' high-temperature S ' characteristic for positrons trapped at vacancies in thermal equilibrium was observed for Mo and W. In V, Nb and Ta the high–temperature vacancy effect is less developed and no saturation of the temperature variation of the γM lineshape can be seen. It is concluded that in the latter metals detrapping of positrons from vacancies has to be taken into account. The following mono–vacancy formation enthalpies H1vF (in eV) were determined: 2·1 ± 0·2 (V), 2·6 ± 0·3 (Nb), 2·8± 0·6 (Ta, 3·0 ± 0·2 (Mo) and 4·0 ± 0·3 (W). The vacancy migration enthalpies H1vM deduced from the present results, in combination with self–diffusion data, indicate that in the b.c.c. refractory metals vacancies become mobile only above room temperature. The existing annealing data are reconsidered from this view point.