Absence of superconductivity in the high-pressure polymorph of MgB 2

We report a high-pressure orthorhombic ${\text{KHg}}_{2}$-type polymorph (space group Imma, 4 f.u./cell) of ${\text{MgB}}_{2}$ stable above 190 GPa predicted through ab-initio evolutionary simulations. The formation of this new phase results from the strong out-of-plane distortions of the two-dimensional honeycomb boron sublattice of the low-pressure ${\text{AlB}}_{2}$-type structure creating a peculiar tetrahedrally bonded three-dimensional boron network. This high-pressure phase is a weak metal and not superconducting, re-highlighting the key role of the planar boron sublattice in forming the superconducting state and clear structure-property relations that can enable design of new superconductors.