Mn2+EPR study of the phase transition in an ammonium sulfate single crystal: Existence of two inequivalent sublattices

Double Freezing of (NH4)2SO4/H2O Droplets below the Eutectic Point and the Crystallization of (NH4)2SO4 to the Ferroelectric Phase

“…The fact that below the T c ammonium sulfate crystallizes directly to the ferroelectric phase may find implication in the theory of the ferroelectric transition which despite a long study is not fully understood yet. 30,40,44 Conclusions This paper has provided for the first time experimental evidence on the phase separation into ice and a residual freezeconcentrated solution that occurs during the freezing of millimeter-scaled (NH 4 ) 2 SO 4 /H 2 O droplets below the eutectic temperature of T e ≈ 254.5 K. The finding is contradictory to the behavior of a bulk solution, which below the T e exists as a solid mixture of ice/(NH 4 ) 2 SO 4 . The residual solution is formed by the expulsion of NH 4 + and SO 4 2-from the ice lattice and always freezes on further cooling, producing in such a way a second freezing event.…”

“…In the lower warming thermograms (red lines), the two endothermic peaks are due to the heat of fusion absorbed during the melting of the eutectic mixture of ice/(NH 4 ) 2 SO 4 and ice, respectively. The temperature of ∼254.5 K is the eutectic melting temperature of ice/(NH 4 ) 2 SO 4 (see text for details) The magnified parts show a ferroelectric-to-paraelectric (F → P) transition at ∼223 K in crystalline (NH 4 ) 2 SO 4 . The weight of droplets is 6.52 and ∼9.2 mg.…”

“…It is known that, as crystalline (NH 4 ) 2 SO 4 is cooled, a transition occurs from the paraelectric phase with the space group of Pnam ( D 2 h 16 ) to the ferroelectric phase with the space group of Pna 2 1 ( C 2 v 9 ) at the ferroelectric “Curie” temperature of T c ≈ 223 K. ,− On warming, a reverse ferroelectric-to-paraelectric transition (F → P) occurs at the same temperature. Figure demonstrates an example of paraelectric-to-ferroelectric (P → F) and F → P transitions that occur in a single (NH 4 ) 2 SO 4 crystal.…”

Ferroelectric Transition Vanishes in (NH4)2SO4 Precipitated in Small-Sized Aqueous Droplets

“…Ammonium sulfate, (NH 4 ) 2 SO 4 , is also ferroelectric with T c ≈ 223 K. − However, the finite size effects in (NH 4 ) 2 SO 4 have not been studied, perhaps because (NH 4 ) 2 SO 4 is of no technological importance. Besides scientific curiosity, the study of the finite size effects in (NH 4 ) 2 SO 4 can cast additional light on the important problems of nanoscience such as the size effects on the structure and properties of small particles.…”

“…It can be also useful for the theory of ferroelectric transition of (NH 4 ) 2 SO 4 and atmospheric physics and chemistry. Despite many attempts at investigating with many experimental techniques, − the mechanism of the ferroelectric transition of (NH 4 ) 2 SO 4 remains not fully understood. A number of mechanisms and theories of ferroelectric transition have been proposed which include sudden distortions in the SO 4 2– ions, a so-called point charge model, an order–disorder mechanism, the soft mode theory (however, the soft B 1u mode, which is the main hypothesis of this theory, was not detected in the IR spectrum), etc.…”