Untersuchungen der Struktur von „antiferroelektrischem”(NH4)2H3IO6 mit Kernresonanzmethoden

The orientations and asymmetry parameters of the EFG tensors at the different lattice sites of I¹²⁷ in (NH₄)₂H₃IO₆ have been determined by proton-iodine crossover relaxation at room temperature and at 94°K. Based on these results it was possible to prove the existence of superstructure for the hexagonala- andc-directions and to reduce the number of allowed space groups of the low temperature phase to two: <img src="/fulltext-image.asp?format=htmlnonpaginated&src=G621HM2203T5L1T3_html\10051_2006_Article_BF02422585_TeX2GIFIE1.gif" border="0" alt=" $$R\overline 3 c$$ " /> andR32. In both space groups half of the hydrogen bonds remain symmetric and only half of them become asymmetric (ordered) on lowering the temperature through the phase transition, but only inR32 there are antipolar sublattices. The transition entropies calculated for this model of the hydrogen arrangement agree fairly well with the measured value. With the evidence available it is not possible to rule out one of the two space groups.