Be-Be π-Bonding and Predicted Superconductivity in MBe2 (M=Zr, Hf).

Beryllium, an s-block element, forms an aromatic network of delocalized Be-Be π bonds in alloys ZrBe2 and HfBe2 . This gives rise to stacked [Be2 ]4- layers with tetravalent cations in between. The [Be2 ]4- sublattice is isoelectronic and isostructural to graphite, as well as the [B]-2 sublattice in MgB2 , and it bears identical manifestations of π bonding in its electronic band structure. These come in the form of degeneracies at K and H in the Brillouin zone, separated in energy as the result of interlayer orbital interactions. Zr and Hf use their valence d orbitals to form bonds with the layers, leading to nearly identical band structures. Like MgB2 , ZrBe2 and HfBe2 are computed to be phonon-mediated superconductors at ambient pressures, with respective critical temperatures of 11.4 K and 8.8 K. The coupling strength between phonons and free electrons is very similar, so that the difference in critical temperatures is controlled by the mass of constituent interlayer ions.