Unusual features of lattice dynamics in lawsonite near its phase transitions

Lattice dynamics of a single crystal of lawsonite were studied over a broad range of frequencies (1 Hz to 20 THz) using impedance, THz time-domain and infrared spectroscopies. Based on polarized spectra of complex permittivity ε̂ measured as a function of temperature between 10 K and 500 K, we analyzed the properties of the two known phase transitions—an antiferrodistortive one near T_c1=270 K and a ferroelectric one, occurring at T_c2=124 K . The former one is accompanied by a flat maximum in the THz-range permittivity ε̂_c near T_c1 , which is due to an overdamped polar excitation in the 𝐄∥ c spectra reflecting the dynamics of water and hydroxyl groups. The strength of this mode decreases on cooling below T_c1 , and the mode vanishes below T_c2 due to hydrogen ordering. At the pseudoproper ferroelectric phase transition, two independent anomalies in permittivity were observed. First, ε̂_a exhibits a peak at T_c2=124 K due to critical slowing down of a relaxation in the GHz range. Second, infrared and THz spectra revealed an optical phonon softening towards T_c2 which causes a smaller but pronounced maximum in ε̂_b . Such anomaly, consisting in a soft mode polarized perpendicularly to the ferroelectric axis, is unusual in ferroelectrics.