How perovskites twist

Perovskites are a popular substance in materials science, particularly in solar panel research. They’re flexible, they can be made from a number of elements, such as calcium, lead, titanium, and halogens, and they’re much more efficient at producing electricity than traditional silicon solar panels.

But there are still a few problems to solve before they can be installed on roofs. While the components of perovskites are well known, scientists still weren’t sure on the way atoms are arranged in the crystals – and particularly, how the atoms moved within this structure.

It can be very difficult to figure out the arrangement of atoms in a crystal structure. Many analytical techniques involve breaking up or dissolving molecules, which can’t be done to a solid crystal.

“These structural motions are notoriously difficult to pin down experimentally,” says Volker Blum, a professor of mechanical engineering and material science at Duke University in the US. “The technique of choice is neutron scattering, which comes with immense instrument and data analysis effort.”

Some of Blum’s colleagues have published a paper in Nature Materials, which reveals a clearer picture of one of the most common perovskite crystals.

The researchers used X-Ray scattering and neutron scattering to examine the structure of CsPbBr3, a simple halide perovskite. They measured the perovskite crystal from a range of angles and over a series of different time intervals, so that they could get an idea of how the atoms moved within the crystal structure.

The researchers found that the atoms were arranged in octahedral structures, with bromine atoms acting as a sort of hinge. The bromine atoms allowed other parts of the perovskite crystal lattice to move and rotate around each other. This explains some of the “softness” of perovskite crystals.

“Because of the way the atoms are arranged with octahedral motifs sharing bromine atoms as joints, they’re free to have these rotations and bends,” says Olivier Delaire, associate professor of mechanical engineering and materials science at Duke and corresponding author on the paper.

“But we discovered that these halide perovskites in particular are much more ‘floppy’ than some other recipes. Rather than immediately springing back into shape, they return very slowly, almost more like Jell-O or a liquid than a conventional solid crystal.”

The bromine joints are important for explaining many of the other desirable properties of halide perovskites, according to Delaire. They’re confident that the structure they’ve determined will be similar to the way more complicated perovskites work.

“This study shows why this perovskite framework is special even in the simplest of cases,” says Delaire. “These findings very likely extend to much more complicated recipes, which many scientists throughout the world are currently researching. As they screen enormous computational databases, the dynamics we’ve uncovered could help decide which perovskites to pursue.”

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