Crystal Structure Determination of Metaperiodic Acid, HIO_4, with Combined X-Ray and Neutron Diffraction

摘要

According to textbooks on inorganic chemistry the system of iodine(VII) acids consists of the monomeric orthoperiodic acid, H_5IO_6, from which a series of oligomeric compounds H_(6+n)I_(2+n)O_(10+4n) can be derived by edge-sharing of the IO_6 octa-hedra, and, as a terminating link, polymeric metaperiodic acid, HIO_4. Besides the crystal structure of H_5IO_5, which is weli-established (including the hydrogen-atom positions), the proposed constitutions of iodine(VII) acids are based on insufficient experimental data. For the oligomeric periodic acids the I—O substructures of their salts are assumed to be similar to those of the free acids, and in the cases of HIO_4 and H_7I_3O_(14) polymeric or trimeric chains, respectively, of trans-edge-sharing IO_6 octahedra are proposed based on Raman spectra. Attempts to grow single crystals of HIO_4 failed until now, because the compound decomposes at normal pressure, already at 105 deg C, releasing oxygen and water. The development of X-ray and neutron diffraction techniques for ab initio crystal structure determination has encouraged us to investigate the structure of HIO_4 with these methods.