Discovery of β-HIO_3: A Metastable Polymorph of HIO_3

摘要

The β-HIO_3 polymorph, previously difficult to detect and whose existence was questioned, has been structurally characterized. The crystal structure of β-HIO_3 was solved in the same space group as α-HIO_3 (P212121); however, it was found that the unit cell axes were all different by about 1 A. Similar to that of α and γ phases, the unit cell contains only a single HIO_3 molecule in the asymmetric unit with I-O bond lengths ranging from 1.786(5) to 1.903(7) A. The I(V) atom is further coordinated by three oxygen atoms of neighboring acid molecules forming a distorted octahedral with a range of I-O distances (2.498(6) - 2.795(7) A). The one structural difference that separates the β phase from the α and γ phases is that the hydroxyl group is bridging between two I(V) atoms, resulting in a smaller hydrogen bonding distance (O-O distance: 2.559 A (β), 2.665 A (γ) and 2.696 A (α)) and presumably a different crystalline energy. Similar to γ-HIO_3, β-HIO_3 is metastable and slowly converts to α-HIO_3. It is hypothesized that β-HIO_3 is a transition step in the formation of α-HIO_3 and β-HIO_3 is a result of trapped water inside particles during crystallization.