Optically Active Metals for the Technologies of Tomorrow

Dr. Yuqing Yin working with a diamond anvil cell in the University laboratory. © Yuqing Yin

Recent findings by an international team led by the University of Bayreuth show that the compound magnesium chloride (Mg₃Cl₇) defies conventional rules of metallic behaviour. While typical metals conduct electricity via a “sea” of free electrons surrounding their atoms, conductivity in magnesium chloride occurs through electrons provided by chloride ions, making it an anionic metal. This mechanism weakens the usual electrical screening found in metals and allows the compound to maintain a permanent internal separation of charges – a property known as polarity. Remarkably, this polar metal not only conducts electricity: when exposed to light, it emits light at twice the frequency. This rare combination of electrical conductivity, polarity, and optical frequency-doubling is not only unusual but also highly valuable for applications in electronics, sensors, and energy systems.

“It is very exciting that we have discovered a metal that not only conducts electricity but also emits light in unexpected ways,” says Dr Yuqing Yin, post-doctoral researcher in the Group of Material Physics and Technology at Extreme Conditions at the University of Bayreuth and lead author of the study. “Such a combination is extremely unusual in nature and opens up entirely new perspectives for the design of multifunctional materials.”

The discovery was made under high pressure using a diamond anvil cell – an instrument capable of generating pressures comparable to those found deep inside planets. Using intense synchrotron X-ray beams, the team was able to determine the crystal structure of magnesium chloride in situ, as the material only exists under extreme conditions. Although it cannot yet be produced in industrial quantities, the discovery opens the door to a new class of materials that combine metallic conductivity with valuable optical properties.

Schematic illustration of the discovery: a new magnesium chloride phase with unusual composition, Mg₃Cl₇, synthesized in a laser-heated diamond anvil cell under high pressure and temperature, was identified as a polar metal exhibiting second-harmonic generation (SHG). Figure courtesy: Yuqing Lin

“We are only at the beginning,” notes Professor Dr. Leonid Dubrovinsky, researcher at the Bavarian Geoinstitute (BGI) at the University of Bayreuth and senior co-author of the publication. “The principles we have uncovered offer new ways of thinking about chemistry and materials design. Our work shows that even very simple elements like magnesium and chlorine can, under the right conditions, form completely unexpected structures with unique properties.”

The study highlights how high-pressure research continues to reveal surprising behaviours in seemingly ordinary elements and compounds. By pushing materials beyond the boundaries of everyday chemistry, scientists are uncovering new rules – and new possibilities – for designing the functional materials of the future.