Hypothetical phase transition, structural disorder and spontaneous polarization in ammonium sulfate crystals

EPR and Uniaxial Pressure Measurements of the Phase Transition in (NH 4 ) 2 SO 4 Crystal

“…3) is observed in a large temperature interval, between the superionic phase and the ferroelectric phase transition. It was estimated that the distinction of two SeO − 3 radicals I and II would disapear at 585 K, which was interpreted as the hypothetical improper ferroelastic phase transition [14]. Above 585 K two ammonium groups I and II would become equivalent and the crystal symmetry would change to P6 3 /mmc [13].…”

“…Two magnetically nonequivalent SeO − 3 radicals of the same orientation, designated I and II were detected and interpreted in terms of different coupling of a single SO 4 group to two nonequivalent ammonium groups: NH 4 (I) and NH 4 (II) [7,14]. The phase transition temperature T c was monitored by observing splitting of EPR resonance lines.…”

“…Large separation of the EPR lines due to very large isotropic and anisotropic hyperfine constants help to study phase transition very precisely. Details of the EPR spectra due to SeO − 3 radicals in AS were published previously [7,14]. AS crystals at room temperature are ferroelastic.…”

“…The term "hypothetical" means that the phase transition would occur above the melting point. The hypothetical phase transition was estimated to occur at approximately 585 K [14].…”

Acoustic Shock‐Wave‐Induced Crystallographically Order–Disorder Switchable Phase Transition of Ammonium Sulfate Crystal: X‐Ray Diffraction and Raman Spectroscopic Approach

“…[25] Following this work, quite a few types of switchable magnetic states have been demonstrated for technologically important nanocrystalline materials such as Co 3 O 4 , α-MnO 2 , and ZnFe 2 O 4 . [26][27][28] Note that sulfate anionic materials such as potassium sulfate, [29,30] sodium sulfate, [31,32] lithium sulfate, [33,34] and ammonium sulfate [35,36] are highly sensitive to pressure and temperature such that they undergo phase transitions based on the order-disorder of SO 4 ions at high-temperature which have been very well documented. [37][38][39][40] Reports prove that dynamic shock waves trigger the switchable crystallographic phase transition of potassium sulfate, which has paved the way for finding the phase stability of sodium sulfate and lithium sulfate single crystals such that under shocked conditions, the reversible phase transitions of crystallographic nature in accordance to the counts of exposed shocks have been attained and the observed results have been reported.…”