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presents the anelastic spectra of all the TMCM-Mn 0.95 M 0.05 Cl 3 compounds measured during cooling, exciting the fundamental resonance. The curves show that R2, R3, and R4 are robust features of these materials, and the substitution of Mn with 5% Ni, Cu, and Fe only changed their amplitudes with little effect on the peak temperatures.
Source publication
We present dielectric and anelastic spectroscopy measurements of the molecular piezoelectric TMCM-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 by Matrix-Assisted Pulsed Laser Evaporation (MAPLE). As in other molecular ferroelectrics,...
Citations
... From this point of view, the step in the modulus is nothing other than the softening from the coupling between strain and the molecular rotation mode, which is frozen below T C . Order-disorder transitions of the molecular orientations of this type are found in the metal-organic perovskites NH 4 -Zn(HCOO) 3 [29] and TMCM-MnCl 3 [30,31], also improper ferroelectric, and MAPbI 3 [32] and FAPbI 3 [33] (tetragonal-to-orthorhombic transitions). As discussed in the latter cases, part of the stiffening in the low-temperature phase may be due to the formation of stronger H bonds of the ordered molecules with the surrounding halide octahedra. ...
... Even though peaks P1-P3 are considerably broader than Debye relaxations, they are clearly caused by well-defined defects with quite different activation barriers E. It would be tempting to make parallels with the anelastic relaxation spectra of other defective perovskites, like O deficient SrTiO 3−δ [38] and partially decomposed (TMCM)MnCl 3 [31], but the present situation is different. Oxide perovskites are quite stable compounds and may loose only O atoms at high temperature in a reducing atmosphere. ...
... In halide perovskites heated close to the decomposition temperature, it is unlikely that only anion vacancies are formed, since the organic cation is volatile and the prevalent mechanism of decomposition is expected to be the formation of Schottky defects, namely neutral pairs or complexes of cation and anion vacancies. In (TMCM)MnCl 3 , it was assumed that an equal concentration of TMCM + and Cl − was formed [31] for two reasons: (i) A = TMCM is organic and more volatile than inorganic B = Mn; (ii) vacancies on the B-site of perovskites are rare, though they may be created under particular circumstances [40][41][42]. A reason for the tendency to lose A rather than B ions is that the BX 6 octahedra are clearly the stable backbone of the lattice, because the B-X bonds are shorter than the A-X ones and are therefore stronger, especially when they involve a greater charge (B 4+ and A 2+ or B 2+ and A + ). ...
We measured the anelastic, dielectric and structural properties of the metal-free molecular perovskite (ABX3) (MDABCO)(NH4)I3, which has already been demonstrated to become ferroelectric below TC= 448 K. Both the dielectric permittivity measured in air on discs pressed from powder and the complex Young’s modulus measured on resonating bars in a vacuum show that the material starts to deteriorate with a loss of mass just above TC, introducing defects and markedly lowering TC. The elastic modulus softens by 50% when heating through the initial TC, contrary to usual ferroelectrics, which are stiffer in the paraelectric phase. This is indicative of improper ferroelectricity, in which the primary order parameter of the transition is not the electric polarization, but the orientational order of the MDABCO molecules. The degraded material presents thermally activated relaxation peaks in the elastic energy loss, whose intensities increase together with the decrease in TC. The peaks are much broader than pure Debye due to the general loss of crystallinity. This is also apparent from X-ray diffraction, but their relaxation times have parameters typical of point defects. It is argued that the major defects should be of the Schottky type, mainly due to the loss of (MDABCO)2+ and I−, leaving charge neutrality, and possibly (NH4)+ vacancies. The focus is on an anelastic relaxation process peaked around 200 K at ∼1 kHz, whose relaxation time follows the Arrhenius law with τ0 ∼ 10−13 s and E≃0.4 eV. This peak is attributed to I vacancies (VX) hopping around MDABCO vacancies (VA), and its intensity presents a peculiar dependence on the temperature and content of defects. The phenomenology is thoroughly discussed in terms of lattice disorder introduced by defects and partition of VX among sites that are far from and close to the cation vacancies. A method is proposed for calculating the relative concentrations of VX, that are untrapped, paired with VA or forming VX–VA–VX complexes.
... From this point of view, the step in the modulus is nothing other than the softening from the coupling between strain and the molecular rotation mode, which is frozen below T C . The transition is then similar to the other order-disorder transitions of the molecular orientations in the metal-organic perovskites NH 4 -Zn(HCOO) 3 [16] and TMCM-MnCl 3 [17,18], also improper ferroelectric, and to the tetragonal-to-orthorhombic transitions in MAPbI 3 [19] and FAPbI 3 [20]. As discussed in those previous cases, part of the stiffening in the low-temperature phase can be due to the formation of stronger H bonds of the ordered molecules with the surrounding halide octahedra. ...
... Notice that isolated V X do not have an electric dipole and therefore do not cause dielectric relaxation, while pairs of cation and anion vacancies have both elastic and electric dipoles. Even though peaks P1-P3 are considerably broader than Debye relaxations, they are clearly due to well defined defects with quite different activation barriers E. It would be tempting to make parallels with the anelastic relaxation spectra of other defective perovskites, like O deficient SrTiO 3−δ [26] and 10 of 19 partially decomposed (TMCM)MnCl 3 [18], but the present situation is different. Oxide perovskites are quite stable compounds and may loose only O atoms at high temperature in reducing atmosphere. ...
... In these halide perovskites heated close to the decomposition temperature it is unlikely that only anion vacancies are formed, since the organic cation is volatile and the prevalent mechanism of decomposition is expected to be the formation of Schottky defects, namely neutral pairs or complexes of cation and anion vacancies. In (TMCM)MnCl 3 it was assumed that an equal concentration of TMCM + and Cl − was formed [18] for two reasons: i) A = TMCM is organic and more volatile than inorganic B = Mn; ii) vacancies on the B-site of perovskites are rare, though they may be created under particular circumstances [28][29][30]. A reason for the tendency to loose A rather than B ions is that the BX 6 octahedra are clearly the stable backbone of the lattice, because the B-X bonds are shorter than the A-X ones and therefore stronger, especially when they involve larger charge (B 4+ and A 2+ or B 2+ and A + ). ...
We measured the anelastic, dielectric and structural properties of the metal-free molecular perovskite (ABX$_{3}$) (MDABCO)(NH$_{4}$)I$_{3}$, which has already been demonstrated to become ferroelectric below $T_{\text{C}}=$ 448~K. Both the dielectric permittivity measured in air on discs pressed from powder and the complex Young's modulus measured on resonating bars in vacuum show that the material starts deteriorating with loss of mass just above $T_{\text{C}}$, introducing defects and markedly lowering $T_{\text{C}}$. The elastic modulus softens of 50\% when heating through the initial $T_{\mathrm{C}}$, contrary to usual ferroelectrics, which are stiffer in the paraelectric phase. This suggests improper ferroelectricity, where the primary order parameter of the transition is not the electric polarization, but the orientational order of the MDABCO molecules. The degraded material presents thermally activated relaxation peaks in the elastic energy loss, whose intensities increase together with the decrease of ~$T_{\mathrm{C}}$. The peaks are much broader than pure Debye, due to the general loss of crystallinity, also apparent from X-ray diffraction, but their relaxation times have parameters typical of point defects. It is argued that the major defects should be of the Schottky type, mainly due to the loss of (MDABCO)$^{2+} $ and I$^{-}$, leaving charge neutrality, and possibly also (NH$_{4}$)$^{+}$ vacancies. The focus is on an anelastic relaxation process peaked around 200~K at $\sim 1$~kHz, whose relaxation time follows the Arrhenius law with $\tau $$_{0}$~$\sim $\ $10^{-13}$\ s and $E\simeq 0.4$~eV. This peak is attributed to I vacancies (V$_{\text{X}}$) hopping around MDABCO vacancies (V$_{\text{A}}$) and its intensity presents a peculiar dependence on temperature and content of defects. The phenomenology is thoroughly discussed in terms of lattice disorder introduced by defects and of partition of V$_{\text{X}}$ among sites that are far from and close to the cation vacancies. A method is proposed for calculating the relative concentrations of V$_{\text{X}}$, that are untrapped, paired with V$_{\text{A}}$ or forming V$_{\text{X}}$--V$_{\text{A}}$--V$_{\text{X}}$ complexes.
