Tuning Soft Perovskites by Alloying: Phase Boundaries, Halide Ordering, and Vacancy Migration

Perovskites with the general composition ABX3 are among the most versatile functional materials known, with applications spanning photovoltaics, solid-state electrolytes, and ferroelectrics. A unifying feature is the presence of soft lattice dynamics — low-energy structural distortions that mediate phase transitions and couple strongly to electronic, optical, and transport properties. Alloying on the A, B, or X site provides a powerful handle for tuning these dynamics, but it simultaneously introduces chemical disorder and competing structural motifs whose interplay is often poorly understood.

I will first present an analysis of the phase diagram of the mixed A-site system MA1-xFAxPbI3, modeled as a random alloy, obtained using a machine-learned interatomic potential (MLIP) based on the neuroevolution potential framework. The results reveal a morphotropic phase boundary (MPB) at approximately 27% FA content, delineating the transition between out-of-phase and in-phase octahedral tilt patterns, where the free-energy landscapes of the underlying phonon modes become nearly degenerate. Density functional theory calculations show that band edge fluctuations peak near the MPB, indicating enhanced electron–phonon coupling and dynamic disorder. By demonstrating that phonon overdamping serves as a hallmark of the MPB, this study informs design principles for stable, high-performance perovskite solar cells.

Extending this perspective to inorganic mixed halide perovskites, whose thermodynamic phase behavior is central to compositional design and device stability, I will discuss results in which halide ordering is sampled explicitly — in contrast to the random-mixing treatment above — through combined Monte Carlo and molecular dynamics simulations with MLIPs trained on density functional theory data, capturing both configurational and vibrational degrees of freedom across the CsxRb1–xPbBr3yI3-3y, CsxRb1–xPbBr3yCl3-3y, and CsxRb1–xPbCl3yI3-3y systems. All three exhibit a miscibility gap whose extent correlates with halide ion size mismatch. Outside this gap, all systems show a tendency toward layered halide ordering. In CsPbBr3yI3-3y, this ordering occurs in a device-relevant temperature regime and shifts structural transition temperatures by up to 100 K relative to randomly mixed structures, accounting for the experimentally observed two-regime composition dependence. Introducing Rb on the A-site weakens halide ordering, eliminates the two-regime behavior, and narrows the miscibility gap, establishing halide ordering as a key determinant of structural phase stability. Building on this picture, I will further show how crystal phase, halide composition, and A-site alloying jointly govern vacancy-mediated halide diffusion in these systems.