Industrial Applications of Positron Annihilation
J. Phys. IV France 05 (1995) C1-135-C1-142
Positron Annihilation Characteristics in Perfect and Imperfect Transition Metal Carbides and NitridesM.J. Puska1, M. Sob2, G. Brauer3 and T. Korhonen1
1 Laboratory of Physics, Helsinki University of Technology, SF-02150 Espoo, Finland
2 Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zizkova 22, CZ-616 62 Brno, Czech Republic
3 Positron Annihilation Spectroscopy Group of the Technical University Dresden, Research Center Rossendorf Inc., P.O. Box 510119, D-01314 Dresden, Germany
First-principles electronic structure and positron state calculations for transition metal carbides and nitrides are performed. Perfect NaCl-structures as well as structures with metal or carbon/nitrogen vacancies are considered. The positron affinities and lifetimes are determined. The trends are discussed and the results are compared with recent positron lifetime measurements for group IV and V refractory metal carbides and in hexagonal WC. The present analysis indicates, contradictory to an earlier interpretation, that positrons are trapped and annihilated both at carbon and metal vacancies. The concentration of metal vacancies seen by positrons is probably very low, below the sensitivity limit of the other experimental methods. Experimental data for WC may be well understood if a Bloch-like state in the lattice with carbon vacancies is employed as an initial positron state in the trapping model.
© EDP Sciences 1995