Chemical bonding topology of superconductors. III. Layered quaternary lanthanide nickel borocarbides and boronitrides

The concept of porous delocalization has been extended to the conducting skeletons of quaternary lanthanide nickel borocarbide and boronitride superconductors and related nonsuperconducting materials. Such materials exhibit layered structures of the general type (LnX)n(Ni2B2) (Ln = lanthanide, particularly the nonmagnetic LaIIIor LuIII; X = C or N; n = 1, 2, or 3) containing planar square Ni nets bonded to BCB2- ligands in the superconducting LnNi2B2c, BC3- ligands in the nonsuperconducting LnNiBC and Ln2NiBc2, and BN2- ligands in the superconducting La3Ni2B2N3. The linear BCB2- bridges linking adjacent Ni layers in LnNi2B2c lead to a porous conducting skeleton in LnNi2B2c. The BC3- ligands in the nonsuperconducting LuNiBC and the closely related Ln2NiBc2 = Ln2c2.2LnNiBC do not link adjacent Ni layers in LuNiBC so that LuNiBC has only a two-dimensional conducting skeleton, which is not necessarily porous in these two dimensions. The superconducting La3Ni2B2N3 = LaN. 2LaNiBN has a similar local Ni environment as LuNiBC but with BN2- rather than the isoelectronic BC3- ligands above and below each Ni layer. However, in La3Ni2B2N3 unlike LuNiBC the LaNiBN units are linked electronically by the La atoms in the intermediate LaN layer leading to a three-dimensional conducting skeleton in La3Ni2B2N3 rather than the two-dimensional conducting skeleton of LuNiBC. The linear BNLaNB units linking the Ni layers in La3Ni2B2N3 lead to a porous conducting skeleton similar to that in the LnNi2B2c superconductors.