Unusual ferroelectric phase transition in lawsonite triggered by the antiferrodistortive order parameter

Lawsonite [CaAl₂Si₂O₇(OH)₂·H₂O] is a rare mineral found in metamorphic rocks. Its crystal structure contains 11.5 wt% of water. At room temperature, it is orthorhombic, crystallizing in the Cmcm space group. The structure contains a silicate tetrahedra framework with four chemical formulas in the unit cell, each of them comprising one water molecule embedded in a structural cavity. The static and dynamic orientations of the water molecules play a key role in two low-temperature phase transitions—an antiferrodistortive one at T_c1=270 K (Pmcn space group), and a ferroelectric one at T_c2=124 K (P2₁cn). We studied the lattice dynamics in a single crystal of lawsonite using polarized THz time-domain transmittance and infrared reflectance spectroscopies. In addition, dielectric properties between 1 Hz and 1 GHz were measured, and static polarization was obtained from pyrocurrent measurements. [1]

The ferroelectric order appears below T_c2 in the crystallographic direction a. Low-frequency permittivity reaches a value of up to ε`~200. The dielectric anomaly weakens in the microwave region, and it is not observable in the THz range. Also, in the b direction, a rise in permittivity ε` up to ~250 was observed nearT_c2, extending in frequency up to the THz and infrared ranges, where a partially softening phonon mode was observed. For explanation of both phase transitions we used the Landau theory. The ferroelectric phase transition appears to be triggered by the non-polar soft mode driving the higher-temperature antiferrodistortive phase transition. The soft phonon originates in a biquadratic coupling between the equilibrium order parameter of the antiferrodistortive phase below T_c1 and the polarization component P_b.

[1] F. Kadlec et al.: Unusual features of lattice dynamics in lawsonite near its phase transitions. Scientific reports 12, 6157 (2022).