Improper ferroelectricity in lawsonite CaAl ₂\ud Si ₂ O ₇(OH)₂·H₂ O: hysteresis and hydrogen ordering

Ferroelectric hysteresis measurements on ceramic lawsonite show a temperature dependence of the remanent polarization Pr = PoΘs(cothΘs/T − cothΘs/Tc) ~ Q2, Θs = 26 K, where Q is the thermodynamic order parameter of the phase transition Pmcn–P 21cn. This almost linear temperature evolution of Pr proves the improper nature of ferroelectricity in lawsonite. The Curie temperature is Tc = 124 K. The phase transition is strictly continuous, with a weak conjugated field near the transition point, and hydrogen ordering is discussed as the primary driving mechanism