Speaker
Description
Lawsonite [CaAl2Si2O7(OH)2·H2O] is an uncommon mineral found in metamorphic rocks,
occurring at subduction zones, supposedly at depths of up to 250 km. It contains 11.5 wt%
water in its crystal structure. At room temperature, it is orthorhombic, crystallizing in the
Cmcm space group. Its structure contains a silicate tetrahedra framework with four formulas
in the unit cell. Each chemical formula comprises one water molecule embedded in a
structural cavity; these cavities form channels parallel with the c axis. The protons form O-H
bonds, and they may diffuse along the channels. The static and dynamic orientations of the
water molecules plays a key role in two low-temperature phase transitions—a structural one at
270 K, and a ferroelectric one at 124 K. We studied the lattice dynamics in a single crystal of
lawsonite using infrared, Raman and THz time-domain spectroscopies. We have found a
strong soft phonon mode linked to the ferroelectric phase transition. The observed unusual
hardening of other phonons with heating is linked to anomalous temperature dependence of a
unit cell parameter. In view of a dielectric anomaly identified earlier, our results show that the
ferroelectric phase transition is of mixed displacive / order-disorder type.
