Superconducting properties of La 3 Ni 2 B 2 N 3 − δ

Nearly-single-phase La 3Ni 2B 2N 32d 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 La 3Ni 2B 2N 32d are studied by means of resistivity, magnetization, and heat capacity measurements. The obtained critical fields m 0Hc(0)50.16 T, m 0Hc1(0)513(2) mT, and m 0Hc2(0)57.5(5) T classify La 3Ni 2B 2N 32d as a hard type-II superconductor with a Ginzburg-Landau parameter k533(2), a coherence length j 057.0(5) nm, and a penetration depth l(0) of about 210 nm. The pressure dependence of Tc with dTc / dp 52130 mK/kbar is found to be one order of magnitude larger than for RNi 2B 2 C( R 5Y, Lu!. Since the deviations of the thermodynamic ratios as DC/gTc , gTc /Hc (0), Hc(0)/H c(Tc)Tc , and D(0)/kBTc from their BCS values are rather small, La 3Ni 2B 2N 32d is characterized as a phonon-mediated weak- to medium-coupling BCS superconductor. @S0163-1829~96!01638-4#