Structure and Lattice Dynamics of NH4IO3 . 2HIO3 Crystal as Derived from IR, Raman and 127 I NQR Data

The IR absorption and Raman spectra of NH4IO3_2HIO3 crystal (AIH) were reinvestigated in a wide temperature range (13-300 K), including the phase transition temperature Tc = 213 K. The pressure dependences of the quadrupole coupling constant e2Qqzz and asymmetry parameter of electric field gradient tensor (EFGT) were obtained from the analysis of 127I nuclear quadrupole resonance (NQR) spectra at 77 K. It was shown that the phase transition of the crystal can be described as an isostructural phase transition with ordering of protons in bifurcated hydrogen bonds and duplication of the unit cell.