Superconductivity in lanthanum nickel boro-nitride

RESEARCH into intermetallic superconductors has recently been reinvigorated by the discovery of superconductivity at 23 K in a Y-Pd-B-C quaternary alloy¹ (tying with the long-standing inter-metallic record held by Nb₃ Ge (ref. 2)), and at lower temperatures in other quaternary boro-carbide alloys based on Ni, Pd and Pt^3-6. The crystal structure of these new intermetallic superconductors, typified by LuNi₂B ₂C (ref. 7), consists of one square layer of rock-salt-type LnC (where Ln stands for a rare-earth element) alternating with one layer of transition-metal boride tetrahedra. Here we report superconductivity at 12-13 K in a new quaternary intermetallic system, lanthanum nickel boro-nitride. We have determined the formula and crystal structure of superconducting La ₃Ni₂B₂N₃, and also of a related non-superconducting phase LaNiBN. The crystal structure of La₃Ni ₂B₂N₃ is related to that of LuM ₂B₂C, but consists of three rock-salt-type LaN layers alter-nating with tetrahedral Ni₂B₂ layers, an arrangement considerably more two-dimensional than is found for the superconducting boro-carbides. These discoveries strongly suggest that a large number of unusual superconducting intermetallic phases are yet to be found, based on materials more complex than have previously been considered as candidates for superconductivity.