Abstract
The ferroelectric transition of (NH4)2SO4 precipitated during the freezing of aqueous ammonium sulfate droplets is investigated by means of differential scanning calorimetry (DSC). Below 223 K ammonium sulfate precipitates into ferroelectric crystals from (single) millimeter-sized and (emulsified) large micrometer-sized droplets. Upon subsequent warming, the ferro- to paraelectric (F → P) transition is observed at ∼223 K. In the case of (emulsified) small micrometer-sized droplets, (NH4)2SO4 precipitates into the paraelectric phase, and the F → P transition is absent. Since the mass sensitivity of the DSC instrument is demonstrated to be high enough to resolve the possible F → P transition of (NH4)2SO4 precipitated in small-sized droplets, this suggests that (NH4)2SO4 crystals precipitated from small micrometer-sized droplets are smaller than the critical size, below which ferroelectric ammonium sulfate does not exist. We do not know the exact critical size, but from independent measurements on (NH4)2SO4 ...
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