Thermally activated proton hopping in lawsonite, the ferroelectric transition at 125 K, and the co-elastic phase transition at 270 K

Lawsonite, CaAl2Si2O7(OH)2H2O, is a novel ferroelectric material which is dominated by ordering and disordering of protons. We report related elastic anomalies in lawsonite. A new approach to extracting acoustic data from resonant ultrasonic spectroscopy has been applied to investigate the ordering and disordering of protons. This approach is based on analysis of entire spectra, rather than the more conventional fitting of individual resonance peaks and captures three features of the transitions. Firstly, structural disorder of protons persists down to low temperatures. Secondly, the structural transition between two paraelectric phases near 270 K is mainly of the order/disorder type but is not directly coupled to the proton mobility. Thirdly, the ferroelectric transition at 125 K shows a direct link with the proton mobility with an activation energy of 0.03 eV. The latter transition occurs in the large class of ferroelectric crystals where the molecular groups are linked by hydrogen and its thermodynamic potential is almost identical to that of KDP (KH2PO4), albeit with a smaller isotope effect. While some damping is seen in small temperature intervals below the transition points, these anomalies are much smaller than those that occur at ferroelastic phase transitions