Superconductivity in LaT_MBN and La₃T_M2B₂N₃ (T_M = Transition Metal) Synthesized under High Pressure

Various layered boronitrides (LaN)_n(T_M2B₂) (T_M = transition metal; n = 2, 3) have been prepared using a high-pressure synthesis technique in which an inverse α-PbO-type T_M2B₂ layer is separated by two or three rock salt-type LaN layers and these layers are connected through linear (BN) units. The electronic states of the distinguishing (BN) unit and intermediate rock salt-type LaN layer are discussed on the basis of density functional theory calculations. Bulk superconductivity has been found in LaNiBN (T_c ≈ 4.1 K), CaNiBN (T_c ≈ 2.2 K), and LaPtBN (T_c ≈ 6.7 K), where the Fermi level E_F is located in the bands composed of the T_M(d)–B­(2p) antibonding state and the main T_M(d) band resides well below E_F. The non-superconductive T_M-based compounds exhibit Pauli paramagnetic behavior, in which the highly itinerant nature of the electrons caused by strong T_M(d)–B­(2p) covalent bonding suppresses the long-range magnetic ordering.