Nuclear magnetic resonance study of the phase transitions and local environments of α-alum NH4Al(SO4)2·12H2O single crystals

Reflectance Spectroscopy of Ammonium Salts: Implications for Planetary Surface Composition

Recent discoveries have demonstrated that the surfaces of Mars, Ceres and other celestial bodies, as well as asteroids and comets, are characterized by the presence of ammonium-bearing minerals. A careful study of remote data compared with the analyses of more accurate laboratory data might allow a better remote characterization of planetary bodies. In this paper, the reflectance spectra of some ammoniated hydrous and anhydrous salts, namely sal-ammoniac NH4Cl, larderellite (NH4)B5O7(OH)2·H2O, mascagnite (NH4)SO4, struvite (NH4)MgPO4·6H2O and tschermigite (NH4)Al(SO4)2·12H2O, were collected at 293 and at 193 K. The aim is to detect how the NH4 vibrational features are affected by the chemical and structural environment. All samples were recovered after cooling cycles and were characterized by X-ray powder diffraction. Reflectance spectra of the studied minerals show absorption features around 1.3, 1.6, 2.06, 2.14, 3.23, 5.8 and 7.27 μm, related to the ammonium group. Between them, the 2ν3 at ~1.56 μm and the ν3 + ν4 at ~2.13 μm are the most affected modes by crystal structure type, with their position being strictly related to both anionic group and the strength of the hydrogen bonds. The reflectance spectra of water-rich samples [struvite (NH4)MgPO4·6(H2O) and tschermigite (NH4)Al(SO4)2·12(H2O)] show only H2O fundamental absorption features in the area from 2 to 2.8 μm and a band from hygroscopic water at 3 μm. Thermal analyses (TA), thermal gravimetry (TG) and differential scanning calorimetry (DSC) allowed to evaluate the dehydration temperatures and the occurring phase transitions and decompositions in the analyzed samples. In almost all samples, endothermic peaks at distinct temperatures were registered associated to loss of water molecules, differently linked to the structures. Moreover, an endothermic peak at 465 K in sal-ammoniac was associated to the phase transition from CsCl to NaCl structure type.

Behavior of H2O surrounding NH4+ and Al3+ in NH4Al(SO4)2·12H2O by 1H MAS NMR, 14N NMR, and 27Al NMR

In order to understand the thermodynamic properties and structural geometry of NH₄Al(SO₄)₂·12H₂O, we studied the magic angle spinning nuclear magnetic resonance (MAS NMR) and static NMR as a function of temperature.

Electrolytic coloration below 0°C and spectral properties of ammonium alum crystals using a pointed anode

Ammonium alum crystals are colored electrolytically below 0°C and under various voltages using a pointed anode and a flat cathode. SO2(-), O3(-), O2(-), O(-) hole-trapped centers and O(0), O(2-), H(+) radicals are produced in colored ammonium alum crystals. The absorption bands of the SO2(-) and O3(-) hole-trapped centers are observed in the absorption spectra of the colored ammonium alum crystals. The hole-trapped centers and radicals come mainly from the electric-field-induced decomposition of the SO4(2-) radicals and crystalline water molecules. Current is measured during electrolytic coloration. Electron exchanges from electrons and small charged radicals to electrodes induce complete current.