The monoclinic and cubic phases of metaboric acid (precise redeterminations)

In monoclinic β-HBO2, endless [B3O3(OH)(H2O)(O2/2)] zigzag chains are linked via an extensive system of hydrogen bonds with stronger major [H⋯O between 1.67 (1) and 1.77 (1) Å] and weaker minor components [H⋯O between 2.48 (1) and 2.63 (1) Å]. The unique three-dimensional tetrahedral [BO2/2O2/2(H)] network structure of cubic γ-­HBO2 is stabilized by very short asymmetric hydrogen bonds [H⋯O2 1.48 (1) Å] with a donor–acceptor distance of 2.485 (1) Å and possesses small empty cages with a free diameter of ca 3.2 Å.