Metal-Metal Distances, Electron Counts, and Superconducting TCs in AM2B2C

We present first principles band structure calculations on representative boron carbides belonging to the class of superconducting compounds with the general formula AM2B2C with A=Lu, La, or Th and M=Ni or Pd. The compounds are analyzed within the framework of the so-called van Hove scenario, where superconductivity is linked to certain kinds of instabilities in the band structure. We attempt to determine why the addition of the extra electron on replacing the rare earth with Th does not make a significant difference to the superconducting properties, and why the compound LaNi2B2C is not superconducting.