Strong Impact of Halide Ordering on Structural Phase Transitions in Mixed Perovskites

Solar cells and light-emitting diodes that are cheap, efficient, and easy to manufacture often rely on a specific family of materials known as perovskites. These crystals are built from a repeating pattern of atoms that can be easily tweaked by swapping one type of atom for another, allowing scientists to tune how the material interacts with light. However, when researchers mix different types of atoms together to create these custom materials, the atoms do not always stay perfectly mixed. Instead, they can separate into distinct regions or arrange themselves in unexpected patterns, which can destabilize the material and ruin the performance of a device. Understanding exactly how these atoms behave when mixed, and how they arrange themselves as the temperature changes, is essential for building stable, long-lasting electronics.

A team of researchers has now used advanced computer simulations to uncover a hidden rule that governs how these mixed crystals behave. By creating a highly accurate digital model of the atoms, they were able to watch how the materials change over time and temperature in a way that is difficult to achieve in a physical laboratory. They focused on crystals made from lead combined with three different halide elements: bromine, chlorine, and iodine. While it was known that these elements sometimes refuse to mix, the researchers discovered that even when they do mix, they do not stay random. Instead, the atoms prefer to line up in specific layers, with certain elements clustering in the top and bottom positions of the crystal structure while others occupy the sides.

The researchers trained a powerful computer program using data from quantum physics calculations to act as a virtual laboratory. This program, known as a machine-learned interatomic potential, allowed them to simulate millions of atoms moving and shifting over time. They tested three different combinations of halides to see how the atoms arranged themselves as the temperature rose and fell. The simulations revealed that in all three systems, the atoms tend to separate into layers based on their size. In some mixtures, the larger atoms prefer the top and bottom spots, while in others, they prefer the middle. This layering happens even when the atoms are globally mixed throughout the crystal, creating a subtle but powerful internal order.

This internal ordering has a dramatic effect on when the material changes its shape. Crystals often shift from one geometric structure to another as they heat up, much like ice melting into water. The researchers found that when the atoms are allowed to settle into their preferred layered arrangement, the temperature at which these shape changes occur can shift by as much as 100 degrees. In one specific mixture of bromine and iodine, this effect is particularly strong and happens at temperatures relevant to real-world devices. The simulations showed that if the atoms are forced to stay in a random, mixed-up state, the material changes shape at a different temperature than if they are allowed to organize themselves. This explains why real-world experiments sometimes show a complex, non-linear relationship between the mix of ingredients and the temperature at which the material changes. The researchers suggest that the atoms in real devices are likely caught in a middle ground, partially ordered but not fully settled, which creates the unique behavior seen in the lab.

The study also explored what happens when a second type of metal atom is added to the mix to replace some of the original metal. When the researchers introduced a smaller metal atom into the structure, it disrupted the tendency of the halide atoms to form layers. This disruption made the material more stable and reduced the temperature range where the atoms would separate into different regions. The smaller metal atom effectively forced the halide atoms to mix more evenly, preventing the formation of the distinct layers that had been shifting the transition temperatures. This finding suggests that by carefully choosing which metal atoms to include, scientists can control how the halide atoms arrange themselves, offering a new way to stabilize these materials for use in solar panels and LEDs.

The researchers confirmed that the size difference between the halide atoms is the primary driver for whether they will mix or separate. When the atoms are very different in size, they struggle to coexist in the same space, leading to a wider range of temperatures where they separate. When the sizes are more similar, they mix more easily. The simulations showed that the gap where separation occurs is widest for the combination of chlorine and iodine, which have the largest size difference, and narrowest for bromine and chlorine. By adding the smaller metal atom, the researchers were able to shrink this separation gap for two of the three mixtures, making the materials more robust.

Ultimately, the work highlights that the stability of these promising materials depends not just on what ingredients are used, but on how those ingredients arrange themselves at the atomic level. The tendency for atoms to form layers is a key factor that determines when the material changes its structure and how well it holds together. By understanding and controlling this ordering, scientists can design better materials that are less likely to degrade over time. The study provides a clear roadmap for why certain mixtures behave the way they do and offers a method for predicting how new combinations will perform, paving the way for more reliable and efficient optoelectronic devices.