Chemical Bonding Topology of Superconductors: III. Layered Quaternary Lanthanide Nickel Borocarbides and Boronitrides

✍ Scribed by R.B. King

• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 238 KB
• Volume: 124
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.1996.0245

✦ Synopsis

DEDICATED TO PROF. DR. ERWIN WEISS ON THE OCCASION OF HIS 70TH BIRTHDAY designated as porous delocalization. Such ideas were sub-The concept of porous delocalization has been extended to sequently extended to other types of compounds (7) includthe conducting skeletons of quaternary lanthanide nickel boroing other ternary metal borides and silicides, the A-15 alloy carbide and boronitride superconductors and related nonsupersuperconductors M 3 E (M ϭ V, Nb, Ta; E ϭ Si, Ge, Sn), conducting materials. Such materials exhibit layered structures and transition metal alloys. In addition, these ideas were of the general type (LnX) n (Ni 2 B 2 ) (Ln ‫؍‬ lanthanide, particularly shown to apply to the high T c copper oxide superconducthe nonmagnetic La III or Lu III ; X ‫؍‬ C or N; n ‫؍‬ 1, 2, or 3) tors to provide a simple rationalization why the T c 's of the containing planar square Ni nets bonded to BCB 2؊ ligands in copper oxide superconductors are much higher than any the superconducting LnNi 2 B 2 C, BC 3؊ ligands in the nonsuperof the known metal cluster superconductors (8, 9). conducting LnNiBC and Ln 2 NiBC 2 , and BN 2؊ ligands in the Since completion of this theoretical work in 1990, a new superconducting La 3 Ni 2 B 2 N 3 . The linear BCB 2؊ bridges linking adjacent Ni layers in LnNi 2 B 2 C lead to a porous conducting class of layered quaternary metal boride superconductors skeleton in LnNi 2 B 2 C. The BC 3؊ ligands in the nonsuperconhas been discovered, which exhibits significantly higher ducting LuNiBC and the closely related Ln 2 NiBC 2 ‫؍‬ T c 's than the ternary metal borides studied earlier (Table Ln 2 C 2 ؒ 2LnNiBC do not link adjacent Ni layers in LuNiBC so 1) (1, 7). The first members of this new class of superconthat LuNiBC has only a two-dimensional conducting skeleton, ducting metal borides to be discovered were metal borocarwhich is not necessarily porous in these two dimensions. The bide superconductors with the general formula LnM 2 B 2 C superconducting La 3 Ni 2 B 2 N 3 ‫؍‬ LaN ؒ 2LaNiBN has a similar (Ln ϭ lanthanide, particularly the nonmagnetic La III or local Ni environment as LuNiBC but with BN 2؊ rather than Lu III , M ϭ Ni, Pd, Pt) and were found to exhibit T c 's as the isoelectronic BC 3؊ ligands above and below each Ni layer. high as 16.6 K in the case of LuNi 2 B 2 C (10). Subsequent However, in La 3 Ni 2 B 2 N 3 unlike LuNiBC the LaNiBN units are very recent work (11) led to the discovery of related borolinked electronically by the La atoms in the intermediate LaN layer leading to a three-dimensional conducting skeleton in nitride superconductors with the general formula La 3 Ni 2 B 2 N 3 rather than the two-dimensional conducting skele-Ln 3 Ni 2 B 2 N 3 , among which the lanthanum derivative ton of LuNiBC. The linear BNLaNB units linking the Ni layers La 3 Ni 2 B 2 N 3 was found to exhibit a T c around 12 K. This in La 3 Ni 2 B 2 N 3 lead to a porous conducting skeleton similar to paper discusses the extension of the previously used graphthat in the LnNi 2 B 2 C superconductors.