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. Low‐energy 3D networks of boron oxide based on six‐membered rings connected by B‐O‐B bridges are reported. The B‐O‐B connections are extremely flexible, enabling volume changes exceeding 200% with very little internal energy change (