The effect of nitrogen vacancies in La3Ni2B2N3-δ

Polycrystalline samples of the layered boronitride superconductor La₃Ni₂B₂N_3-δ with various nominal nitrogen stoichiometries and identified an approximate width of formation ranging from δ ~ 0.1 to δ ~ 0.4 were prepared. The relation between nitrogen stoichiometry and superconducting properties were studied by resistivity and specific heat measurements. With increasing nitrogen off-stoichiometry δ an increase of the residual resistivity ρ₀ and a simultaneous decrease of the superconducting transition temperature T_c from 14.2 K to 12.2 K is observed. The trend of T_c(p₀) extrapolates to a clean limit T_c ~ 16K for terminal La₃Ni₂B₂N₃. The pressure dependence of T_c yields an initial slope dT_c/dp = -80mK/kbar for La₃Ni₂B₂N_2.92(5).