Highlights on the reversible nonpolar-to-polar P3₁21-P3₁ phase transition at low temperature in NaLa(SO₄)₂?H₂O: mechanism and piezoelectric properties

We report the existence in NaLa(SO4)(2)center dot H2O of a displacive phase transition under 200 K from the nonpolar P3(1)21 to the polar P3(1) space group. This phase transition was predicted by density functional theory based calculations and experimentally confirmed from infrared spectroscopy and X-ray diffraction. The A(2) polar irreducible representation is the primary order parameter. The structural water and hydrogen bonding are the mechanism driving the phase transition. The piezoelectric properties of this new P3(1) phase have been investigated by first principles based calculations. The highest piezoelectric-strain constants in the zero Kelvin limit are predicted for the d(12) and d(41) elements with values about 3.4 pC N-1. This compound could be interesting as piezoelectric actuator for cryogenic applications.