Cordero, F.; Craciun, F.; Imperatori, P.; Raglione, V.; Zanotti, G.; Moldovan, A.; Dinescu, M. Phase Transition and Point Defects in the Ferroelectric Molecular Perovskite (MDABCO)(NH4)I3. Materials2023, 16, 7323.
Cordero, F.; Craciun, F.; Imperatori, P.; Raglione, V.; Zanotti, G.; Moldovan, A.; Dinescu, M. Phase Transition and Point Defects in the Ferroelectric Molecular Perovskite (MDABCO)(NH4)I3. Materials 2023, 16, 7323.
Cordero, F.; Craciun, F.; Imperatori, P.; Raglione, V.; Zanotti, G.; Moldovan, A.; Dinescu, M. Phase Transition and Point Defects in the Ferroelectric Molecular Perovskite (MDABCO)(NH4)I3. Materials2023, 16, 7323.
Cordero, F.; Craciun, F.; Imperatori, P.; Raglione, V.; Zanotti, G.; Moldovan, A.; Dinescu, M. Phase Transition and Point Defects in the Ferroelectric Molecular Perovskite (MDABCO)(NH4)I3. Materials 2023, 16, 7323.
Abstract
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.
Keywords
molecular ferroelectrics; organic perovskites; anelasticity; point defects complexes
Subject
Physical Sciences, Condensed Matter Physics
Copyright:
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