A morphotropic phase boundary in MA_1-xFA_xPbI₃: linking structure, dynamics, and electronic properties

Area of Science:

• Materials Science
• Solid-State Physics
• Computational Chemistry

Background:

• Mixed-cation halide perovskites are crucial for optoelectronic applications.
• Understanding their phase behavior is key to improving stability and performance.
• Existing experimental data on MA1-xFAxPbI3 phase diagrams are incomplete and sometimes conflicting.

Purpose of the Study:

• To systematically map the phase diagram of MA1-xFAxPbI3.
• To identify the structural and electronic origins of phase transitions.
• To provide insights for designing stable and high-performance perovskite materials.

Main Methods:

• Machine-learned interatomic potential for molecular dynamics simulations.
• Density functional theory (DFT) calculations.
• Phonon mode projections and analysis.

Main Results:

• Identified a morphotropic phase boundary (MPB) at ~27% FA content in MA1-xFAxPbI3.
• Discovered a crossover in phonon mode degeneracy at the MPB, leading to nanoscale layered structures.
• Observed peak band edge fluctuations and enhanced electron-phonon coupling near the MPB.

Conclusions:

• Phonon dynamics directly influence the phase behavior and electronic structure of mixed-cation perovskites.
• Phonon overdamping is a hallmark of the MPB, offering a design principle for perovskite solar cells.
• The findings provide a unified description of the MA1-xFAxPbI3 system, aiding material optimization.