Coupled structural and electronic evolution under pressure in CuIr2Se4, CuRh2S4, and CuRh2Se4

Deep within the world of solid materials, there exists a family of crystals known as spinels. These are not just static rocks; they are dynamic stages where atoms arrange themselves in complex, three-dimensional lattices that dictate how electricity flows. In these structures, certain metal atoms sit at specific points, forming a network that can be frustrated, meaning the atoms struggle to settle into a single, comfortable arrangement because of competing forces. This struggle gives rise to a fascinating variety of behaviors: the material might conduct electricity like a metal, act as an insulator that blocks current, or even become a superconductor, carrying electricity with zero resistance. Scientists are particularly interested in how these materials behave when squeezed. Usually, squeezing a material brings its atoms closer together, which helps electrons move more freely and makes the material more metallic. However, in some of these spinel crystals, the opposite happens. When pressure is applied, they suddenly stop conducting electricity and become insulators, suggesting that the atoms are rearranging themselves into a new, ordered pattern that traps the electrons.

A team of researchers set out to explore this counterintuitive behavior in three closely related crystals: copper-iridium-selenium, copper-rhodium-sulfur, and copper-rhodium-selenium. These compounds share the same basic architectural blueprint, but they differ in the specific metal and non-metal atoms they contain. The scientists wanted to understand how changing these ingredients affects the way the crystals respond to pressure. They used powerful X-rays from a large particle accelerator to watch the atomic structure of these materials change in real time as they were squeezed, while simultaneously measuring how easily electricity could pass through them. Their goal was to see if the structural changes were the direct cause of the electrical changes and to determine if the specific atoms used could tune the material to become a superconductor or an insulator.

The researchers began by confirming that at normal pressure and low temperatures, the copper-rhodium-sulfur and copper-rhodium-selenium samples were indeed superconductors, meaning they could carry electricity without any loss. However, there was a surprise waiting in the copper-iridium-selenium sample. While previous studies had failed to find superconductivity in this specific compound down to very low temperatures, this team discovered that it does, in fact, become a superconductor at a temperature of 0.29 Kelvin. This is just a fraction of a degree above absolute zero, but the signal was strong and clear, showing that the entire bulk of the material was participating in this state, not just a tiny surface layer. This finding corrected the historical record for this material, establishing it as a true superconductor, albeit one that operates at a much lower temperature than its cousins.

When the team started applying pressure, the story became more dramatic. As they squeezed the crystals, the atoms began to shift from their original, highly symmetric cubic arrangement into a new, slightly distorted shape known as a monoclinic structure. This transformation was not a smooth slide; it was a distinct phase change where the old cubic pattern and the new monoclinic pattern existed side-by-side for a while before the new one took over. Using advanced computer simulations to guide their interpretation of the X-ray data, the researchers found that in the copper-iridium-selenium and copper-rhodium-sulfur samples, this new structure involved a specific type of atomic ordering. In this new arrangement, some of the metal atoms formed unusually short bonds with their neighbors, while others remained further apart. This created a pattern where the atoms effectively split into two groups based on how tightly they were holding hands, a phenomenon the researchers describe as bond disproportionation.

The electrical measurements revealed a tight link between this atomic rearrangement and the flow of electricity. As the new monoclinic structure began to form, the materials stopped acting like metals and started behaving like insulators. The electrons, which were previously free to roam, became trapped in the new atomic pattern. However, the speed and pressure at which this happened depended heavily on which atoms were in the crystal. In the copper-iridium-selenium sample, the transition to an insulating state happened quickly and at a relatively low pressure. Once the new structure formed, the material became a strong insulator even at room temperature. In contrast, the rhodium-based compounds required much higher pressures to undergo the same structural change, and their shift toward insulating behavior was more gradual, taking place over a wider range of pressures.

The competition between superconductivity and this new insulating state was also stark. In the copper-iridium-selenium sample, the superconductivity vanished abruptly as soon as the insulating structure began to appear. The two states seemed to be mutually exclusive; as one took hold, the other was immediately crushed. In the copper-rhodium-sulfur sample, the relationship was more forgiving. The superconductivity persisted for a while even as the pressure increased and the insulating structure started to form, only fading away slowly over a broader pressure range. This suggests that the specific choice of metal and non-metal atoms acts like a tuning knob, adjusting the pressure required to trigger these changes and determining how fiercely the superconducting and insulating states fight for dominance.

For the copper-rhodium-selenium sample, the picture was slightly less clear. The researchers could confirm that it also transformed into the same type of monoclinic supercell as the others, but the X-ray data was too messy to pinpoint the exact atomic arrangement inside. Despite this uncertainty, the electrical behavior still followed the same general trend, shifting toward insulation as the pressure increased, though it did so more slowly than the other two. This suggests that while the exact atomic details might vary, the fundamental drive toward this new structural state is a shared property of this family of crystals.

Ultimately, the study reveals that the path a material takes under pressure is not random but is deeply rooted in the specific chemistry of its ingredients. The size of the electron clouds around the metal atoms and the distance between them, which are controlled by whether sulfur or selenium is present, dictate how easily the atoms can rearrange themselves. By swapping these elements, scientists can effectively tune the material, deciding whether it will become a superconductor, an insulator, or a battleground where the two states compete. This work provides a unified view of how structure and electricity are intertwined in these complex crystals, showing that even small changes in composition can lead to vastly different physical realities under the extreme conditions of high pressure.