Positron lifetime in non-stoichiometric carbides with a B1(NaCl) structure

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Abstract
The positron lifetimes in the carbides of the tetravalent group IV transition metals are found to be similar to those in the corresponding pure metals whereas in the carbides of the pentavalent group V metals the positron lifetimes are by about 40 ps longer than in the pure metals. A representation of the positron annihilation rate versus the density of metal valence electrons demonstrates that the positrons are predominantly annihilated with metal valence electrons in these carbides. This result is strongly supported by the decrease of the positron lifetime in NbCy with carbon vacancy content 1-y and is interpreted as positron annihilation on vacant sites of the carbon sublattice.