The frequency and temperature dependence of dielectric properties of CH3NH3PbI3 (MAPI) crystals have been studied and analyzed in connection with temperature-dependent structural studies. The obtained results bring arguments for the existence of ferroelectricity and aim to complete the current knowledge on the thermally activated conduction mechanisms, in dark equilibrium and in the presence of a small external a.c. electric field. The study correlates the frequency-dispersive dielectric spectra with the conduction mechanisms and their relaxation processes, as well as with the different transport regimes indicated by the Nyquist plots. The different energy barriers revealed by the impedance spectroscopy highlight the dominant transport mechanisms in different frequency and temperature ranges, being associated with the bulk of the grains, their boundaries, and/or the electrodes’ interfaces.

Triglycine sulphate crystal (TGS) is a hybrid organic-inorganic ferroelectric crystal with a large electronic band-gap, transparent in visible spectra. It has large applications in electronics, as thermal IR detector. Pure and doped TGS crystals of 3-5 cm linear dimensions were grown at constant temperature in the paraelectric phase (52 oC), by slow solvent evaporation. Pure TGS samples show non-reproducible values of permittivity and losses in the ferroelectric phase after the “excursion” in the paraelectric phase. Walking up and down the crystal temperature, using a special program, the permitivity and losses were automatically recorded and analyzed. There is a continuous decrease of permittivity towards an equilibrium value during a long period of time. Previous AFM measurements from the literature have revealed peculiar aspects of domain dynamics. The relaxation process of permittivity was considered according to the equation er = A - B exp(-t/t). However, surprisingly, there is not a unique relaxation time t. For the first 500 sec (or so), the relaxation time is t » 7 minutes, while between the next time decades 1.000 - 10.000 - 100.000 sec, it is approximately 1 hour and 8 h respectively. The process is related with the ferroelectric domain’s dynamics, which are more or less strongly pined by dislocations or some other lattice defects.

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