To solve the long-standing controversy over the number of ferroelectric phase transitions in ${\left({\mathrm{CH}}_3{\mathrm{NHCH}}_2\mathrm{COOH}\right)}_3{\mathrm{CaCl}}_2$, we performed a comprehensive series of new dielectric, second-harmonic generation (SHG), calorimetric, infrared, and Raman measurements down to 10 K. Detailed analysis of the data excludes other ferroelectric transitions than the one at $T_{\mathrm{C}}=130$ K from the $Pnma$ to $Pn{2}_{1}a$ orthorhombic space groups. The observed pseudoproper ferroelectric (possibly ferrielectric) phase transition is driven by a terahertz soft mode in the $E\parallel b$ spectra, which couples to a critical microwave relaxation (central mode). Below $T_{\mathrm{C}}$, additional phonons are activated in the infrared and Raman spectra due to the change of selection rules, but all the observed modes can be assigned to orthorhombic $Pn{2}_{1}a$ structure without further reduction of the crystal symmetry. We found a weak and strongly frequency-dependent dielectric anomaly in the radiofrequency permittivity ${\varepsilon }_b^{\prime }(T$) close to 65 K, where the second phase transition was previously suggested. This behavior is assigned to the domain wall dynamics. Temperature dependences of the spontaneous polarization, heat capacity, and SHG signal show no indication of any further phase transition below $T_{\mathrm{C}}$.

• Received 23 September 2023
• Revised 6 November 2023
• Accepted 13 November 2023

DOI:https://doi.org/10.1103/PhysRevB.108.224101