Improper ferroelectricity in lawsonite CaAl2Si2O7(OH)2•H2O: hysteresis and hydrogen ordering

Ferroelectric hysteresis measurements on ceramic lawsonite show a temperature dependence of the remanent polarization P-r = Po Theta(s)(coth Theta(s)/T - coth Theta(s)/T-c) similar to Q(2), Theta(s) = 26 K, where Q is the thermodynamic order parameter of the phase transition Pmcn-P2(1)cn. This almost linear temperature evolution of P-r proves the improper nature of ferroelectricity in lawsonite. The Curie temperature is T-c = 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.