Superconductivity in La<i>T</i>_<i>M</i>BN and La₃<i>T</i>_<i>M</i>2B₂N₃ (<i>T</i>_<i>M</i> = Transition Metal)\nSynthesized under High\nPressure

Abstract

Various layered boronitrides (LaN)_<i>n</i>(<i>T</i>_<i>M</i>2B₂) (<i>T</i>_<i>M</i> = transition metal; <i>n</i> = 2, 3) have been prepared using a high-pressure synthesis\ntechnique in which an inverse α-PbO-type <i>T</i>_<i>M</i>2B₂ layer is separated by two\nor three rock salt-type LaN layers and these layers are connected\nthrough linear (BN) units. The electronic states of the distinguishing\n(BN) unit and intermediate rock salt-type LaN layer are discussed\non the basis of density functional theory calculations. Bulk superconductivity\nhas been found in LaNiBN (<i>T</i>_c ≈\n4.1 K), CaNiBN (<i>T</i>_c ≈ 2.2 K), and\nLaPtBN (<i>T</i>_c ≈ 6.7 K), where the\nFermi level <i>E</i>_F is located in the bands\ncomposed of the <i>T</i>_<i>M</i>(d)–B­(2p)\nantibonding state and the main <i>T</i>_<i>M</i>(d) band resides well below <i>E</i>_F.\nThe non-superconductive <i>T</i>_<i>M</i>-based compounds exhibit Pauli paramagnetic behavior, in which the\nhighly itinerant nature of the electrons caused by strong <i>T</i>_<i>M</i>(d)–B­(2p) covalent\nbonding suppresses the long-range magnetic ordering.