How halide atoms are arranged shifts a perovskite's phase transition by up to 100 K.

PerovskitesPerovskitesA family of crystals with a specific cage-like atomic arrangement; mixing different halogens in the cage tunes how they absorb and emit light. are materials used to build solar cells and light-emitting diodes, but they can be unstable. To improve them, scientists mix different halide atoms into the material. The problem is that we do not fully understand how these mixtures arrange their atoms at different temperatures. Researchers built computer models trained on quantum mechanical data to simulate large groups of these atoms as they move and vibrate. They found that the mixed halides often separate into different phases instead of mixing smoothly. When heated enough to mix, the halides do not mix randomly. Instead, they organize into specific layered patterns inside the crystal. In a mixture of bromine and iodine, this internal ordering changes the temperature where the crystal structure shifts by up to 100 K. Adding rubidium weakens this ordering and shrinks the separation zones, showing that internal atom patterns strongly control material stability.

Why it matters

Mixed halide perovskites are promising for solar cells, LEDs, and photodetectors, but their stability depends on temperature-driven structural changes. This work shows that the arrangement of halide atoms, not just their proportions, can shift a key phase transition by up to 100 K, changing the temperature window in which a device stays stable. Materials designers can use this to pick compositions that hold their structure in real-world use.

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