Highlights on the reversible nonpolar-to-polar P 3 1 21- P 3 1 phase transition at low temperature in NaLa(SO 4 ) 2 ·H 2 O: mechanism and piezoelectric properties.

We report the existence in NaLa(SO 4 ) 2 ·H 2 O of a displacive phase transition under 200 K from the nonpolar P 3 1 21 to the polar P 3 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 P 3 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.