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Updated: Jul 9, 2025
08:12
Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films
Published on: September 8, 2017
9.6K
Phase Transition and Point Defects in the Ferroelectric Molecular Perovskite (MDABCO)(NH4)I3
Francesco Cordero1, Floriana Craciun1, Patrizia Imperatori1
1Istituto di Struttura della Materia-CNR (ISM-CNR), Area della Ricerca di Roma-Tor Vergata, Via del Fosso del Cavaliere 100, I-00133 Rome, Italy.
Materials (Basel, Switzerland)
|December 9, 2023
Summary
This study reveals that metal-free perovskites degrade above their ferroelectric transition, introducing defects that lower the transition temperature and alter elastic properties. These findings suggest improper ferroelectricity driven by molecular order, not just polarization.
Area of Science:
- Materials Science
- Solid State Physics
- Crystallography
Background:
- Metal-free molecular perovskites offer unique ferroelectric properties.
- The ferroelectric transition in (MDABCO)(NH4)I3 occurs at 448 K.
- Understanding material degradation is crucial for device stability.
Purpose of the Study:
- To investigate the anelastic, dielectric, and structural properties of (MDABCO)(NH4)I3.
- To understand the impact of degradation on ferroelectric properties.
- To identify defect types and their influence on material behavior.
Main Methods:
- Anelastic spectroscopy (complex Young's modulus).
- Dielectric permittivity measurements.
- X-ray diffraction (XRD).
Main Results:
- Material degradation and mass loss observed above the ferroelectric transition temperature (TC).
- Significant softening of the elastic modulus (50%) and a decrease in TC due to defects.
- Broad relaxation peaks in elastic energy loss attributed to point defects, likely Schottky-type vacancies.
- Anelastic relaxation process around 200 K linked to iodide and MDABCO vacancies, following Arrhenius behavior.
Conclusions:
- The ferroelectricity is likely improper, driven by molecular reorientation rather than polarization.
- Lattice disorder and defect interactions significantly influence material properties.
- A method is proposed to quantify different types of iodide vacancies based on their interactions with cation vacancies.
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