Abstract
The study of temperature dependency of the electric field gradient (EFG) tensor asymmetry parameter η(T) from 127I nuclear quadrupole spectra (NQR) for NH4IO3 crystals in the temperature range of 100–400K including the phase transition temperature are presented. It is shown that the phase transition in the NH4IO3 crystal has the properties of phase transitions of both first and second orders. It is shown that some decrease in the barrier energy for reorientation vibrations of IO3− ions (U=24kcal/mol) can be explained by the anomalous expansion of the crystal in the pretransition temperature range of 335–367K and by some weakness of the hydrogen bond subsystem.
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