Abstract
Abstract The structural phase transformation between the ferroelectric and paraelectric form of NH4IO, near 83°C is dominated by tilt motions of IO6-octahedra within a perovskite-like framework. Simultaneously, small displacements of NH4 along the polar axis give rise to a switchable spontaneous polarization at low temperatures. As the transition mechanism is governed by the tilting of octahedra and not by the appearance of ferroelectricity, NH4IO3 is characterised as an improper ferroelectric material. Although the space group of the high-temperature β-phase is polar, the polarization moment is zero. Raman-active thermal soft modes appear in the ferroelectric phase. The phase transition is first order with anomalies of dielectric, piezoelectric and elastic constants.
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