delta-KIO3 center dot HIO3: crystal structure, proton disorder and low temperature phase transition

delta-KIO3.HIO3, obtained by slowly cooling a concentrated solution of KIO3 and HIO3, has been investigated by means of single crystal X-ray diffracti on measurements at ambient temperature and temperature dependent powder X-r ay diffraction, dielectric, calorimetric and FT-IR and FT-Raman measurement s. From the single crystal X-ray measurements, the I-O bonding scheme of th e iodine atoms and the hydrogen bond system were determined. The crystal st ructure of delta-KIO3.HIO3 was found to consist of [I3O9H3/2](3/2-) ions po ssibly 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 delta-KIO3.HIO3 s hould be formulated as K-3/2[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 220 K, which may be regarded as a structural phase transition in the proton sublattice. (C) 2000 Elsevier Science B.V. All ri ghts reserved.