Thermodynamic ground state of MgB₆ predicted from first principles structure search methods

Crystalline structures of magnesium hexaboride, MgB₆, were investigated using unbiased structure searching methods combined with first principles density functional calculations. An orthorhombic Cmcm structure was predicted as the thermodynamic ground state of MgB₆. The energy of the Cmcm structure is significantly lower than the theoretical MgB₆ models previously considered based on a primitive cubic arrangement of boron octahedra. The Cmcm structure is stable against the decomposition to elemental magnesium and boron solids at atmospheric pressure and high pressures up to 18.3 GPa. A unique feature of the predicted Cmcm structure is that the boron atoms are clustered into two forms: localized B₆ octahedra and extended B_∞ ribbons. Within the boron ribbons, the electrons are delocalized and this leads to a metallic ground state with vanished electric dipoles. The present prediction is in contrast to the previous proposal that the crystalline MgB₆ maintains a semiconducting state with permanent dipole moments. MgB₆ is estimated to have much weaker electron-phonon coupling compared with that of MgB₂, and therefore it is not expected to be able to sustain superconductivity at high temperatures.