δ-KIO3·HIO3: crystal structure, proton disorder and low temperature phase transition

Abstract

δ-KIO3·HIO3, obtained by slowly cooling a concentrated solution of KIO3 and HIO3, has been investigated by means of single crystal X-ray diffraction measurements at ambient temperature and temperature dependent powder X-ray diffraction, dielectric, calorimetric and FT–IR and FT-Raman measurements. From the single crystal X-ray measurements, the I–O bonding scheme of the iodine atoms and the hydrogen bond system were determined. The crystal structure of δ-KIO3·HIO3 was found to consist of [I3O9H3/2]3/2− ions possibly originating from occupationally disordered [I3O9H2]− and [I3O9H]2− anions. These ions are connected via hydrogen bonds to form plane grids parallel (100) with the K+ ions placed between them. Thus δ-KIO3·HIO3 should be formulated as K3/2[I3O9H3/2]. The temperature dependent FT–IR data give an evidence for a dynamic proton disorder at room temperature and for a proton ordering below 220K, which may be regarded as a structural phase transition in the proton sublattice.