New organic and hybrid metal-organic ferroelectrics are being synthesized in the last years, which are filling the gap between the polymer and the oxide ferroelectrics in terms of piezoelectric properties, at least in one case even superior to those of PZT [1], and with advantages in terms of simplicity and cost effectiveness of preparation and flexibility of use. In fact, these materials can be prepared with purely chemical methods and deposited as films on surfaces of any shape. One major drawback is their limited thermal resistance, due to the weaker bonds and the volatility of the organic molecules at moderate temperatures. We measured the anelastic, dielectric and structural properties of the metal-free molecular perovskite MDABCO(NH4)I3, composed of corner-sharing I6 octahedra with NH4 in the centre and MDABCO molecules in the interstices among the octahedra. The MDABCO molecule is obtained from the roughly spherical DABCO = N2(C2H4)3 by attaching a methyl CH3 group to one N in order to induce an electric dipole moment. Ferroelectricity, with a Curie temperature of 448 K, is due to the alignment of these dipole moments [2]. The complex Young's modulus measured as a function of temperature in vacuum on resonating bars pressed from powder softens of 50% when heating through TC, contrary to usual ferroelectrics that are stiffer in the paraelectric phase, and starts loosing material, including iodine. Therefore, the apparently large hysteresis of 50 K in TC (400 K on cooling [2]) is rather due to the sample decomposition after having exceeded 450 K. This is demonstrated by the fact that the higher is the maximum temperature reached the lower becomes TC, though still with a considerable hysteresis of 25 K. The degraded material presents thermally activated relaxation peaks in the elastic energy loss, whose intensities increase together with the decrease of TC. Two of them, without dielectric counterpart, are tentatively assigned to hopping of free and paired iodine vacancies, which have elastic but not electric dipole in the perovskite structure. Of the peak at the lowest temperature only the tail is observed due to limitations in the experimental temperature range. The resulting mean hopping and reorientation rates would follow the Arrhenius law with ?0 ~ 10 14 s and Ea = 0.16 and 0.42 eV respectively. The peaks are much broader than pure Debye, due to the general loss of crystallinity, also apparent from X-ray diffraction. The sample degradation on exceeding TC occurs also in air, as demonstrated by the decrease of TC registered in the dielectric spectra obtained in air. References [1] W.-Q. Liao et al. "A molecular perovskite solid solution with piezoelectricity stronger than lead zirconate titanate" Science 363, 1206 (2019). [2] H.-Y. Ye et al., "Metal-free three-dimensional perovskite ferroelectrics" Science 361,151 (2018).
Thermal decomposition of the organic ferroelectric MDABCO(NH4)I3 from anelastic, dielectric and structural measurements
Francesco Cordero;Patrizia Imperatori;Anna Maria Paoletti;Gloria Zanotti
2021
Abstract
New organic and hybrid metal-organic ferroelectrics are being synthesized in the last years, which are filling the gap between the polymer and the oxide ferroelectrics in terms of piezoelectric properties, at least in one case even superior to those of PZT [1], and with advantages in terms of simplicity and cost effectiveness of preparation and flexibility of use. In fact, these materials can be prepared with purely chemical methods and deposited as films on surfaces of any shape. One major drawback is their limited thermal resistance, due to the weaker bonds and the volatility of the organic molecules at moderate temperatures. We measured the anelastic, dielectric and structural properties of the metal-free molecular perovskite MDABCO(NH4)I3, composed of corner-sharing I6 octahedra with NH4 in the centre and MDABCO molecules in the interstices among the octahedra. The MDABCO molecule is obtained from the roughly spherical DABCO = N2(C2H4)3 by attaching a methyl CH3 group to one N in order to induce an electric dipole moment. Ferroelectricity, with a Curie temperature of 448 K, is due to the alignment of these dipole moments [2]. The complex Young's modulus measured as a function of temperature in vacuum on resonating bars pressed from powder softens of 50% when heating through TC, contrary to usual ferroelectrics that are stiffer in the paraelectric phase, and starts loosing material, including iodine. Therefore, the apparently large hysteresis of 50 K in TC (400 K on cooling [2]) is rather due to the sample decomposition after having exceeded 450 K. This is demonstrated by the fact that the higher is the maximum temperature reached the lower becomes TC, though still with a considerable hysteresis of 25 K. The degraded material presents thermally activated relaxation peaks in the elastic energy loss, whose intensities increase together with the decrease of TC. Two of them, without dielectric counterpart, are tentatively assigned to hopping of free and paired iodine vacancies, which have elastic but not electric dipole in the perovskite structure. Of the peak at the lowest temperature only the tail is observed due to limitations in the experimental temperature range. The resulting mean hopping and reorientation rates would follow the Arrhenius law with ?0 ~ 10 14 s and Ea = 0.16 and 0.42 eV respectively. The peaks are much broader than pure Debye, due to the general loss of crystallinity, also apparent from X-ray diffraction. The sample degradation on exceeding TC occurs also in air, as demonstrated by the decrease of TC registered in the dielectric spectra obtained in air. References [1] W.-Q. Liao et al. "A molecular perovskite solid solution with piezoelectricity stronger than lead zirconate titanate" Science 363, 1206 (2019). [2] H.-Y. Ye et al., "Metal-free three-dimensional perovskite ferroelectrics" Science 361,151 (2018).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
