The A-15 phase of chemically vapor-deposited Nb3Ge has been investigated by annealing techniques in the temperature range 700–1450°C and by irradiation with high-energy neutrons (E>1 MeV). By x-ray diffraction analysis the samples were found to contain about 55 wt % of the A-15 phase, 35 wt % of tetragonal Nb5Ge3, and minor amounts of NbO2, NbO, and hexagonal Nb5Ge3. The superconducting transition temperatureT c and lattice parametera 0 of the A-15 phase have been measured as a function of annealing temperature and neutron irradiation.T c for the unirradiated sample remained above 19 K for long-term anneals in the temperature range 700–1000°C but decreased rapidly above 1000°C, reaching a limiting value of about 6 K at 1250°C, compared to the as-deposited value of about 20 K. The decrease inT c was accompanied by an increase ina 0 and an increase in the amount of tetragonal Nb5Ge3 to about 50 wt %. The observed decrease inT c was irreversible in that subsequent annealing at 1100°C did not result in any increase inT c or decrease ina 0 . Large depressions inT c were also observed in a sample irradiated with high-energy neutrons. The sample having aT c of about 21 K was found to exhibit no superconductivity to 1.5 K after exposure to a fluence of ∼5×1019 n/cm2. An increase ina 0 was also found as a function of neutron irradiation. However, in this case the effects were reversible in thatT c anda 0 of the irradiated sample could be restored to close to their original values by annealing in the 800–900°C range. The results are discussed in terms of stability of the A-15 phase and the effect of site-exchange disorder and composition on the superconducting properties and lattice parameters. It is concluded that the stoichiometric composition is metastable and upon annealing transforms to a stable, Nb-rich A-15 phase containing about 83 at % Nb with aT c of ∼6 K accompanied by precipitation of tetragonal Nb5Ge3.
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On leave from Universidade Estadual de Campinas, Brazil, supported by Conselho Nacional de Pesquisas.
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Sweedler, A.R., Cox, D.E., Moehlecke, S. et al. Superconductivity and phase stability of Nb3Ge. J Low Temp Phys 24, 645–661 (1976). https://doi.org/10.1007/BF00657172
- Diffraction Analysis
- Phase Stability