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 k 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 (epsilon) over cap (c) near T-c1, which is due to an overdamped polar excitation in the E parallel to 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, (epsilon) over cap (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 (epsilon) over cap (b). Such anomaly, consisting in a soft mode polarized perpendicularly to the ferroelectric axis, is unusual in ferroelectrics.