Structural and vibrational studies of Li[Kx(NH4)1-x]SO4 and Li2KNH4(SO4)2 mixed crystals

Mixed crystals of Li[K-x(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: Li[K-x(NH4)(1-x)]SO4 with xgreater than or equal to0.94 and Li2KNH4(SO4)(2). Different phases of Li[K-x(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 P6(3) 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 Li[K-x(NH4)(1-x)]SO4 xgreater than or equal to0.94 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 P6(3)/mmc with the cell a-parameter double that of LiKSO4. The phase transition is at 471.9 K. (C) 2003 Elsevier Science (USA). All rights reserved.