The bonding in hexagonal Ba2/3Pt3B2 and CeCo3B2 type ternary metal borides

Abstract

Tight-binding calculations with an extended Huckel Hamiltonian were performed on Ba2/3Pt3B2 and LuOs3B2. Hypothetical linear metal boride chains present in these materials are analyzed with a three-dimensional model that contains a trigonal bipyramidal T3B2 (T = transition metal) building unit for the compounds. The geometrical structure for the T3B2 trigonal bipyramids depends on the number of electrons. For systems that have greater than 36 electrons in its trigonal bipyramidal building unit, a structural distortion is expected. Electron back donation from the electron-rich M3 fragment to the empty e′ set on B2 creates boron–boron interaction along the z-axis. Boron–boron pairing then participates as an electron sink and causes a trigonal distortion of the platinum Kagome net. On the other hand, a system with <35 electrons should have an undistorted, CeCo3B2 type structure. The electronic factors that create the breathing motion are discussed and analyzed with the aid of molecular and solid-state models. The metal–metal bonding associated with the structural properties also has been examined.