TERNARY NITRIDES BINCA3 AND PBNCA3 - UNUSUAL IONIC BONDING IN THE ANTIPEROVSKITE STRUCTURE

The compounds BiNCa3 and PbNCa3, a narrow-gap semiconductor and a metal, respectively, with the antiperovskite structure, are investigated with the local-density augmented-plane-wave method. The calculations confirm that both N and the heavy ions Bi and Pb are close to the nominal 3- charge states of an ionic picture; however, there is considerable covalent mixing between the two sets of p states. The filled p bands of the BiNCa3 compound are separated from the conductions bands, with a calcualated gap of approximately 0.1 eV consistent with its semiconducting behavior. PbNCa3, with one less electron per cell and therefore an unfilled p band, is predicted correctly to be metallic, consistent with a rigid-band picture.