Finnish Chemists Made a Mineral That Changes Color Where No Human Can See It

The useful thing about the new material from the University of Turku is that you cannot tell it is doing anything. Put it under ultraviolet light and it changes color. Look at it and nothing has happened. Point a near-infrared camera or a spectrometer at it and the change is obvious.

The material is called davyne, a member of the cancrinite mineral family, and it came out of work on something else. Turku's Intelligent Materials Chemistry Group has spent years on hackmanite, a natural mineral that can also be made synthetically and that is photochromic — it turns from white to pink or violet under UV exposure and reverts when left in white light or heated to about 100°C. The group has built radiation dosimeters and other sensors on that behavior.

"The development of this new material began with a thesis project carried out by a student in our research group," said Professor Mika Lastusaari, the principal investigator. The project swapped calcium into the structure in place of sodium, and the color change went yellow instead of violet. Then something odder turned up. "We also noticed that some of the material samples did not change color visibly, but similar color changes occurred in the near-infrared region, which is invisible to the human eye," Lastusaari said.

Those samples were not calcium hackmanite. They were davyne, forming as a byproduct of the calcium hackmanite synthesis. The two compounds share a chemical formula and differ in how the atoms are arranged, and the equilibrium of the reaction strongly favored the hackmanite, which made pure davyne hard to obtain. The group worked with materials-engineering researchers and used machine-learning methods to optimize the synthesis until they could produce davyne in as pure a form as possible.

Then they had to explain it. In hackmanite the color change comes from an electron moving from a particular ion into a chlorine vacancy in the crystal, and the size of that vacancy sets the resulting color. Davyne has no such cavities. "Davyne's structure is different and the material has long, empty tunnels instead of the cavities that are typical of hackmanite," Lastusaari said. The team was nonetheless able to show that the mechanism is the same kind of electron transfer, operating in a different geometry — which is why the switch lands in the near-infrared instead of the visible.

The obvious application is the one the researchers tested: an anti-counterfeiting tag. A mark printed in davyne is not merely hard to copy, it is hard to know about. It sits on a package or a document looking like nothing, and it authenticates only when someone shines UV on it and reads the result through infrared optics. The findings are published in Angewandte Chemie International Edition.