Investigation of the reconstructive phase transition between metastable (α) and stable (β) modifications of the NH4LiSO4 crystal

Crystals of ammonium lithium sulfate NH4LiSO4 in alpha and beta modifications are studied, and conditions of their nucleation and growth are determined. The alpha modification of NH4LiSO4 and alpha-->beta phase transitions are investigated using polarized light microscopy, x-ray diffraction, and differential scanning calorimetry in the temperature range 80-530 K. It is found that, depending on the conditions of growth and storage, there exist two temperature ranges (T(alpha-->beta)approximate to340-350 and approximate to440-450 K) in which the crystals can undergo an alpha-->beta reconstructive phase transition. The enthalpy of this transformation depends on the symmetry of the final phase. In the former case (340-350 K), the reconstructive phase transition leads to rapid destruction of the sample. In the latter case (440-450 K), the crystal structure undergoes a slow transformation (recrystallization) without noticeable distortions. The results obtained indicate that no structural phase transition occurs in the alpha modification of NH4LiSO4 at 250 K. (C) 2003 MAIK "Nauka/Interperiodica".