Thermal Analysis of Li[K_x(NH_4)_(1-x)]SO_4 Mixed Crystals

摘要

Pure LiNH_4SO_4 (LAS) and LiKSO_4 (LKS) and mixed (Li[K_x(NH_4)_(1-x)]SO_4) single crystals were synthesized and grown by the slow evaporation technique of aqueous solutions at constant temperature (309 K). The atomic absorption analysis confirmed that the obtained mixed crystals were rich in potassium. In this work, we studied the effect of replacing NH_4~(+2) ions with K~(+2) ions (increasing x value) in Li[K_x(NH_4)_(1-x)]SO_4 mixed crystals on the structural phase transition and the decomposition process. Thermal analysis showed that LAS crystal has a structural phase transition at 457 K. By replacing NH_4~(+2) ions with K~(+2) ions (increasing x value), this phase shifts to higher temperatures until it reaches to 709 K for pure LKS crystal. Increasing the x value leads to increasing both the enthalpy and entropy of this phase transition. Thermal analysis showed, also, that the decomposition process for LAS crystal consists of two stages between 550 K and 773 K. The total mass loss associated with the two stages is 53.78%. This confirms that the evolved molecules are SO_3, H_2O, and NH_3. By replacing some NH_4~(+2) ions by K~(+2) ions (increasing x value), the temperatures range of the two stages and the total mass loss showed a noticeable decrease of the evolved molecules with increasing K~(+2) ions. When NH_4~(+2) ions completely replaced with K~(+2) ions (x = 1), the decomposition process does not occur. This behavior is due to the fact that LKS pure crystal (x = 1) is stable in the measured temperature range.