SUPERCONDUCTING PROPERTIES OF LA3NI2B2N3-DELTA

Nearly-single-phase La3Ni2B2N3-delta prepared by are melting and subse quent annealing at 1100 degrees C is characterized by x-ray powder tec hniques, electron microprobe analysis, and high-resolution electron mi croscopy. A standardized set is provided for the crystallographic para meters, and structural chemistry is described from the point of view o f nonmetal-nonmetal interactions as well as of the metal coordination around the nonmetal atoms. The superconducting properties of La3Ni2B2N 3-delta are studied by means of resistivity, magnetization, and heat c apacity measurements. The obtained critical fields mu(0)H(c)(0) = 0.16 T, mu(0)H(c1)(0) = 13(2) mT, and mu(0)H(c2)(0) = 7.5(5) T classify La 3Ni2B2N3-delta as a hard type-II superconductor with a Ginzburg-Landau parameter kappa = 33(2), a coherence length xi(0) = 7.0(5) nm, and a penetration depth lambda(0) of about 210 nm. The pressure dependence o f T-c with dT(c)/dp = -130 mK/kbar is found to be one order of magnitu de larger than for RhNi2B2C (R = Y, Lu). Since the deviations of the t hermodynamic ratios as Delta C/gamma T-c, gamma T-c(2)/H-c(2)(0), H-c( 0)/H-c'(T-c)T-c, and Delta(0)/k(B)T(c) from their BCS values are rathe r small, La3Ni2B2N3-delta is characterized as a phonon-mediated weak- to medium-coupling BCS superconductor.