Discovery of a New Layered Crystal, Hexagonal Beryllium Oxide*

The reported work was prompted by the intriguing question whether sp²-coordinated, layered polymorphs of octet compounds other than BN can exist. While such polymorphs with thicknesses within a thermodynamic limit are predicted, the possibility of their thicker layered crystals has long been dismissed. Here, using high-resolution transmission electron microscopy and electron energy loss spectroscopy, we show that BeO crystallizes in the sp²-coordinated, layered structure in liquid cells formed by sheets of graphene. We further reveal that the layered crystals can be thicker than the thermodynamically determined ultra-thin limit, beyond which the layered phase is energetically unfavored. The discovery counters a long-held dismissal and calls for a reevaluation of the possible existence of sp²-coordinated, layered polymorphs of octet compounds beyond the ultra-thin limit.