Structure of the 13-K superconductor La₃Ni₂B ₂N₃ and the related phase LaNiBN

THE recent discovery of superconductivity in a number of alloys containing boron and carbon has revived interest in the field of intermetallic superconductors^1-3. Although two structural forms have now been identified^4-6, most of the new intermetallic supercon-ductors have a crystal structure typified by that of the quaternary alloy LuNi ₂B₂C, in which layers of rock-salt-structured LuC alternate with nickel boride tetrahedra^4'5. The range of supercon-ducting intermetallic compounds was extended recently by the dis-covery of superconductivity in a system of quaternary boro-nitride alloys, La-Ni-B-N (ref. 7). Here we report the structure of the superconducting phase in this system, La₃Ni₂B₂N₃, together with that of the related non-superconducting phase, LaNiBN. We find that these two phases are members of a homologous series (LaN)_n(Ni₂B ₂), with n Combining double low line 3 and n Combining double low line 2, respectively. Furthermore, our samples contain planar defects that suggest the existence of phases with larger n. Varying the number, n, of LaN layers within this system might thus provide a way to investigate the effect of dimensionality on the superconducting properties of intermetallics.