The nature of spontaneous twisting of (NH4)2SO4 crystals at the curie point

Spontaneous twisting of (NH₄)₂SO₄ crystals at the phase transition from the paraelectric phase Pnam to the ferroelectric phase Pna2 (T _C = 223 K) was studied using the method of low-frequency torsion pendulum. It is shown that the macroscopic twisting of samples is caused by the rearrangement of ferroelastic twins with the {011} and {031} twinning planes existing in both the paraelectric and ferroelectric phases. A model interpreting the effect of the spontaneous twisting below the ferroelectric Curie point is proposed.