Thermal contraction in NH₄IO₃ and its significance with respect to the phase transition

NH₄IO₃ shows an abrupt contraction in the direction of spontaneous polarization at 82°C. This anomaly is accompanied by changes in the optical properties and phonon spectra, which were also studied at different pressures. On the basis of the lattice parameters and Raman spectra a probable mechanism is suggested for the phase transition.