Lithium ammonium sulphate, a polar ferroelastic which is not simultaneously ferroelectric

Lithium ammonium sulphate LiNH4SO4 has been shown to be the first of a new family of polar ferroelastic crystals which are not simultaneously ferroelectric. Stress-induced shear twin motion in the ferroic phase below 10 °C verifies ferroelasticity, and the symmetry constraints are such that the prototypic polar axis does not change. From symmetry it is shown that only seven species of fully ferroelastic nonferroelectric polar ferroics are possible. Other crystals which may show this phenomenon are discussed.