Structural and vibrational studies of Li[Kx(NH4)1−x]SO4 and Li2KNH4(SO4)2 mixed crystals |
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Authors: | Jorge Mata Judit Molera |
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Institution: | Departament de Cristal·lografia, Universitat de Barcelona, Martin i Franquès s/n, E-08028 Barcelona, Spain |
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Abstract: | Mixed crystals of LiKx(NH4)1−x]SO4 have been obtained by evaporation from aqueous solution at 313 K using different molar ratios of mixtures of LiKSO4 and LiNH4SO4. The crystals were characterized by Raman scattering and single-crystal and powder X-ray diffraction. Two types of compound were obtained: LiKx(NH4)1−x]SO4 with x?0.94 and Li2KNH4(SO4)2. Different phases of LiKx(NH4)1−x]SO4 were yielded according to the molar ratio used in the preparation. The first phase is isostructural to the room-temperature phase of LiKSO4. The second phase is the enantiomorph of the first, which is not observed in pure LiKSO4, and the last is a disordered phase, which was also observed in LiKSO4, and can be assumed as a mixture of domains of two preceding phases. In the second type of compound with formula Li2KNH4(SO4)2, the room-temperature phase is hexagonal, symmetry space group P63 with cell-volume nine times that of LiKSO4. In this phase, some cavities are occupied by K+ ions only, and others are occupied by either K+ or NH4+ at random. Thermal analyses of both types of compounds were performed by DSC, ATD, TG and powder X-ray diffraction. The phase transition temperatures for LiKx(NH4)1−x]SO4x?0.94 were affected by the random presence of the ammonium ion in this disordered system. The high-temperature phase of Li2KNH4(SO4)2 is also hexagonal, space group P63/mmc with the cell a-parameter double that of LiKSO4. The phase transition is at 471.9 K. |
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Keywords: | Phase transition Ferroelectric material X-ray diffraction Raman scattering Thermal analysis Crystal structure Mixed crystals Lithium sulfate compounds Ammonium Potassium |
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