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

Abstract

Crystals of ammonium lithium sulfate NH₄LiSO₄ in α and β modifications are studied, and conditions of their nucleation and growth are determined. The α modification of NH₄LiSO₄ and α → β 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 _{α → β} ≈ 340–350 and ≈440–450 K) in which the crystals can undergo an α → β 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 α modification of NH₄LiSO₄ at 250 K.