IR AND DTA STUDIES OF HIGH-TEMPERATURE PHASE-TRANSITION IN LITHIUM-AMMONIUM SULFATE

The temperature dependence of the IR absorption spectra of LiNH4SO4 in the region of the internal vibrational modes showed, in addition to the main peak v3(a) = 1120 cm-1, a shoulder v3(a) = 1117 cm-1. This shoulder was attributed to vibrations of the SO2-4 ions in the disordered orientations. The intensity of this shoulder was found to increase with temperature. Calculation of the energy activating the process of disorder in LiNH4SO4 at different temperatures showed that it decreases as the temperature increases. The mechanism of the high-temperature order-disorder phase transition is explained. DTA studies were found to confirm the proposed mechanism. © 1990.