Superconducting properties of ${\mathrm{La}}_3$${\mathrm{Ni}}_2$${\mathrm{B}}_2$${\mathrm{N}}_{3\mathrm{-}\mathrm{\delta }}$

Nearly-single-phase ${\mathrm{La}}_3$ ${\mathrm{Ni}}_2$ ${\mathrm{B}}_2$ ${\mathrm{N}}_{3\mathrm{-}\mathrm{\delta }}$ prepared by arc melting and subsequent annealing at 1100 °C is characterized by x-ray powder techniques, electron microprobe analysis, and high-resolution electron microscopy. A standardized set is provided for the crystallographic parameters, and structural chemistry is described from the point of view of nonmetal-nonmetal interactions as well as of the metal coordination around the nonmetal atoms. The superconducting properties of ${\mathrm{La}}_3$ ${\mathrm{Ni}}_2$ ${\mathrm{B}}_2$ ${\mathrm{N}}_{3\mathrm{-}\mathrm{\delta }}$ are studied by means of resistivity, magnetization, and heat capacity measurements. The obtained critical fields ${\mathrm{\mu }}_0$ ${\mathit{H}}_{\mathit{c}}$(0)=0.16 T, ${\mathrm{\mu }}_0$ ${\mathit{H}}_{\mathit{c}1}$(0)=13(2) mT, and ${\mathrm{\mu }}_0$ ${\mathit{H}}_{\mathit{c}2}$(0)=7.5(5) T classify ${\mathrm{La}}_3$ ${\mathrm{Ni}}_2$ ${\mathrm{B}}_2$ ${\mathrm{N}}_{3\mathrm{-}\mathrm{\delta }}$ as a hard type-II superconductor with a Ginzburg-Landau parameter κ=33(2), a coherence length ${\xi }_0$=7.0(5) nm, and a penetration depth λ(0) of about 210 nm. The pressure dependence of ${\mathit{T}}_{\mathit{c}}$ with ${\mathit{dT}}_{\mathit{c}}$/dp=-130 mK/kbar is found to be one order of magnitude larger than for R${\mathrm{Ni}}_2$ ${\mathrm{B}}_2$C (R=Y, Lu). Since the deviations of the thermodynamic ratios as ΔC/γ${\mathit{T}}_{\mathit{c}}$, γ${\mathit{T}}_{\mathit{c}}^2$/${\mathit{H}}_{\mathit{c}}^2$(0), ${\mathit{H}}_{\mathit{c}}$(0)/${\mathit{H}}_{\mathit{c}}^{\prime }$(${\mathit{T}}_{\mathit{c}}$)${\mathit{T}}_{\mathit{c}}$, and Δ(0)/${\mathit{k}}_{\mathrm{B}}$ ${\mathit{T}}_{\mathit{c}}$ from their BCS values are rather small, ${\mathrm{La}}_3$ ${\mathrm{Ni}}_2$ ${\mathrm{B}}_2$ ${\mathrm{N}}_{3\mathrm{-}\mathrm{\delta }}$ is characterized as a phonon-mediated weak- to medium-coupling BCS superconductor. © 1996 The American Physical Society.