Fabrication, superconducting T-c and charge transfer of VNx, NbNx and TaNx foils

Group V transition metal nitride foils were fabricated in an ultrahigh vacuum chamber and superconducting transition temperature (T-c), lattice parameter (a(0)), and electron energy loss (EELS) measurements were performed on these samples. It is found that as a function of increasing nitrogen-to-transition-metal atomic ratio, T-c first increases with increasing nitrogen content then decreases before reaching a 1:1 transition metal to nitrogen stoichiometry ratio. T-c also increases with increasing lattice parameter a(0) and then decreases before reaching the maximum values for a(0) which are about 4.14 Angstrom for NVx and 4.392 +/- 0.002 Angstrom for NbNx. The EELS data indicate that the outermost metal d electrons transfer to nitrogen atoms during formation of transition metal nitrides. However, the possibility of electrons forming covalent bonds cannot be ruled out.