Unusual set of anomalies in the lattice dynamics of lawsonite near its phase transitions

Lawsonite is a water-containing mineral occurring in the Earth's crust and mantle, exhibiting low-temperature ferroelectricity linked to water molecules. Using broad-band polarized electromagnetic spectroscopy, we studied the lattice dynamics of a single crystal between 1 Hz and 20 THz at temperatures from 20 to 500 K. We identified three anomalies associated with the two phase transitions occurring at 270 and 124K-a critical GHz relaxation, a soft THz phonon, and an overdamped sub-THz excitation in the E parallel to a, E parallel to b and E parallel to c permittivity spectra, respectively. The fact that the soft mode is polarized perpendicularly to the ferroelectric axis is very unusual.