Ultra‐Flexible Boron‐Oxygen 3D Solid‐State Networks

The existence of ultra‐flexible low‐energy forms of boron oxides (B₂O₃ and BO) is demonstrated, in particular structures in which B₃O₃ or B₄O₂ six‐membered rings are linked by single B‐O‐B bridges. The minima in the energy landscapes are remarkably broad; the variation in the internal energies is very small over a very large range of volumes. Such volume changes may even exceed 200%. This remarkable behavior is attributed predominantly to the pronounced angular flexibility of the B‐O‐B bridges linking the rings, which is unusual for a covalent bond. At larger volumes, the structures are nanoporous; the pores collapse upon compression with negligible change in energy, making these suitable as guest‐host materials. In marked contrast, in other materials where low density frameworks have been reported or predicted, such low‐density phases are considerably higher in energy. The flexibility of the structures also offers a resolution of the long‐standing controversy reconciling the structure and density of vitreous B₂O₃.