Controlling Crystal Structure Through 'Negative Pressure'

When scientists study how materials behave under extreme conditions, they typically examine what happens under compression. But what occurs when you pull matter apart in all directions simultaneously? This phenomenon, known as "negative pressure," has largely remained theoretical - until now.

A team of researchers at Drexel University has developed a method to achieve and precisely control negative pressure in crystals, opening new possibilities for engineering materials with unique properties. The research, published in the Proceedings of the National Academy of Sciences , demonstrates the first experimental platform for studying how crystal structures respond when systematically and uniformly stretched rather than compressed.

"Externally applied pressure is a powerful means to control the crystal structure of solids," said Aaron Fafarman, PhD , associate professor of chemical and biological engineering. "Until now, researchers have been limited to testing how crystal structures vary under greater and greater pressure, but there has not been a way to perform the opposite experiment: to continuously stretch the material equally in three dimensions, rather than compressing it."

The researchers achieved this by synthesizing perovskite crystals within the nanoscopic pores of a rigid aluminum oxide scaffold at high temperatures. Fafarman explains the process using a familiar analogy: "It's similar to cooking a hard-boiled egg. The egg whites solidify in a thermally expanded state, bonding with the hard, unmoving shell at the cooking temperature. Once the egg returns room temperature, that bond provides a force that keeps the whites expanded. If you were to crack the egg, the force (negative pressure) exerted by the shell would be released, causing the shell to collapse inward as the whites contract. The key difference is that we do not crack the shell (/scaffold), thus preserving the negative pressure acting on the perovskite crystals."

In experiments primarily performed by Dr. Arkita Chakrabarti, then a PhD student in Fafarman’s lab, it was found that this method could generate negative pressures of hundreds of megapascals (with one megapascal being ten times atmospheric pressure) simply by varying the synthesis temperature.

When confined within pores smaller than 40 nanometers, the crystals experienced uniform negative pressure that could be precisely controlled. This threshold is crucial, Fafarman notes: "Forces exerted by the walls will necessarily dissipate as you move away from the interface between scaffold and guest material. If the pore size is large, most of the guest material is far away from the interface and not experiencing a large force, if any."

The research team worked specifically with cesium lead iodide (CsPbI₃), which Fafarman describes as having "ideal properties to demonstrate this effect and is a technologically intriguing photovoltaic material in its own right.” Under the conditions studied, the crystal structure became more symmetrical and organized - a finding that could have significant implications.

"Most mechanical perturbations are symmetry reducing," Fafarman explained. "Moreover, there are many desirable phenomena in this materials class and others that accompany higher symmetry, such as absorbing more of the wavelengths of light in the solar spectrum." However, he suggests the concept could be applied to other materials with similar properties, particularly other candidates for photovoltaic applications.

Looking ahead, this breakthrough could open new avenues in materials science. "Tunable negative pressure is a new handle for exploring fundamental properties of materials," Fafarman said. "Negative pressure reduces the interatomic forces outside their familiar range, which could lead to very different dynamics of atomic motion. For example, facile atomic motion could enable a phenomenon known as “ferroelectric switching,” with applications in advanced digital information storage and processing."

The work represents a significant step forward in materials science, providing the first experimental platform for studying matter under controlled negative pressure. This fundamental advance could ultimately lead to new ways of engineering materials with enhanced properties for applications in electronics, energy conversion, and other fields.