Phase Transition and Dynamics of Defects in the Molecular Piezoelectric TMCM-MnCl3 and the Effect of Partial Substitutions of Mn

We present dielectric and anelastic spectroscopy measurements of the molecular piezoelectricTMCM-MnCl3 and TMCM-Mn0.95M0.05Cl3 (M = Cu, Fe, Ni; TMCM = trimethylchlorometylammonium),whose powders were pressed into discs and bars and deposited as films on Si byMatrix-Assisted Pulsed Laser Evaporation (MAPLE). As in other molecular ferroelectrics, the dielectricpermittivity e0 drops at the structural transition temperature TC, below which the numberof directions that the polar TMCM molecules visit is reduced, with the formation of ferroelectricdomains. Concomitantly, the Young's modulus E starts increasing and the elastic energy loss hasa step-like increase, attributable to the motion of the domain walls. Both the dielectric and elasticanomalies indicate the improper character of the ferroelectric transition, where the ordering of themolecular orientations is not driven by the cooperative interaction of their electric dipoles. Belowroom temperature, at least two thermally activated relaxation processes appear both in the dielectricand anelastic spectra, whose real and imaginary parts measured at several frequencies can be fit withthe Havriliak-Negami formula. The microscopic parameters so-obtained indicate that they are due topoint defects, and it is argued that they are Cl vacancies and their complexes with TMCM vacancies.The considerable width of these relaxation maxima is explained by the geometry of the hexagonalperovskite structure. The partial substitution of Mn with 5% Ni has little effect on the anelastic anddielectric spectra, while Cu and, especially, Fe cause a large enhancement of the losses attributed todomain wall relaxation, with substantial contributions also above TC. The condensation of waterfrom the humidity in the powders compacted by cold pressing was observed and discussed. Thepiezoelectric activity of the films was assessed by PFM.