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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    $$\hat{\varepsilon }$$  ε ^ 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    $$T_{\mathrm{c}1}=270\,\mathrm{K}$$  T c 1 = 270 K and a ferroelectric one, occurring at    $$T_{\mathrm{c}2}=124\,\mathrm{K}$$  T c 2 = 124 K . The former one is accompanied by a flat maximum in the THz-range permittivity    $$\hat{\varepsilon }_{\mathrm{c}}$$  ε ^ c near    $$T_{\mathrm{c}1}$$  T c 1 , which is due to an overdamped polar excitation in the    $$\mathbf {E} \parallel c$$  E ‖ c spectra reflecting the dynamics of water and hydroxyl groups. The strength of this mode decreases on cooling below    $$T_{\mathrm{c}1}$$  T c 1 , and the mode vanishes below    $$T_{\mathrm{c}2}$$  T c 2 due to hydrogen ordering. At the pseudoproper ferroelectric phase transition, two independent anomalies in permittivity were observed. First,    $$\hat{\varepsilon }_a$$  ε ^ a exhibits a peak at    $$T_{\mathrm{c}2}=124\,\mathrm{K}$$  T c 2 = 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    $$T_{\mathrm{c}2}$$  T c 2 which causes a smaller but pronounced maximum in    $$\hat{\varepsilon }_b$$  ε ^ b . Such anomaly, consisting in a soft mode polarized perpendicularly to the ferroelectric axis, is unusual in ferroelectrics.

Unusual features of lattice dynamics in lawsonite near its phase transitions