Lanthanide(III) Nitride Bismuthides M 2 NBi ( M = La–Nd) and Their Potential as Topological Insulators

Four lanthanide(III) nitride bismuthides of the new class M 2 NBi ( M = La–Nd) have been synthesized by the reaction of the respective lanthanide metal ( M ) with caesium azide (CsN 3 ), elemental bismuth (Bi), the corresponding lanthanide trichloride ( M Cl 3 ), and caesium chloride (CsCl) as flux in evacuated silica tubes for 7 days at 900 °C. The M 2 NBi compounds crystallize tetragonally in the space group P 4/ nmm with Z = 2 with the lattice parameters a = 480.61(4) pm, c = 948.34(9) pm ( M = La), a = 475.48(4) pm, c = 938.76(9) pm ( M = Ce), a = 471.80(4) pm, c = 929.51(9) pm ( M = Pr), and a = 468.13(4), c = 921.08(9) pm ( M = Nd). As the main structural feature of the crystal structures, ∞ 2 {[N M 2 ] 3+ } layers of condensed nitride‐centered square pyramids of M 3+ cations dominate. These pyramids, in which the base is built of ( M 1) 3+ and the top of terminal ( M 2) 3+ cations ( t ), are connected by all four basal edges ( e ) to form a two‐dimensional network according to ∞ 2 {[N( M 1) e 4/4 ( M 2) t 1/1 ] 3+ } that is separated by square double layers of Bi 3– anions. Both crystallographically independent M 3+ cations bear two types of anions (N 3– and Bi 3– ) as ligands. ( M 1) 3+ is coordinated in the shape of a square antiprism by four N 3– and four Bi 3– anions, and ( M 2) 3+ is surrounded by a distorted octahedron consisting of one N 3– and five Bi 3– anions. The Bi 3– anions reside in capped square antiprisms of nine M 3+ cations. The partial density of states (PDOS) and the band structure of La 2 NBi show that this compound has to be considered as a zero‐band‐gap semiconductor with a nontrivial topology and a high potential as a topological insulator.