Molecular alloying: when mixing anions brings new functionality
20 Jul 2026 - Rosie de Laune
Have you ever known a child who has a talent that neither of their parents has? A research collaboration, led by a group from TU Denmark, have discovered an equivalent phenomenon in a layered metal organic material. They created a new molecular alloy that can undergo a significant shift in electronic and magnetic behaviour when its temperature changes – a property neither of its parent materials had.
Van der Waals crystals are materials made of strongly bonded layers, chains, or ribbons that are bound together by weak intermolecular forces. In this study, published in Chem, a research team have been able to enhance their versatility by introducing the capability for substantial and reversible shifts in properties like electrical conductivity and magnetisation. These shifts, which can be driven by mild external stimuli such as a change in temperature, could lead to a new era of responsive 2D materials, and devices with energy-efficient, on-demand switching capabilities in fields like data storage, sensors, and spintronics.
They began by studying the two metal-organic van der Waals crystals Cr(pyrazine)2I2 and Cr(pyrazine)2Br2 individually, finding that they could not induce a valance change in either material. However, when combining them to produce a molecular alloy, they were able to switch between the two chromium states Cr(II) and Cr(III) by changing the temperature.
Working with ISIS instrument scientists Duc Le and Helen Walker, they used Inelastic Neutron Scattering on MARI to determine the valence state of chromium, and the magnetic behaviour, in Cr(pyrazine)2I2. Unexpectedly, they did not see any magnetic excitations in the bromide or chloride equivalents. Using X-ray absorption spectroscopy data measured at the ESRF, and bulk susceptibility measurements, the group were able to show the valence change in the alloyed material caused by changing the temperature.
The combined data univocally support the existence of a temperature driven valence change in the Cr(pyz)2X2 alloys, but the complete absence of any transition in the parent materials. The valence change causes drastic changes to the magnetisation and electrical conductivity, which varies by up to five orders of magnitude across the transition.
Our successful demonstration of stimuli-responsive valence changes in these crystals represents a foundational advancement, potentially enabling the integration of such materials into layered architectures for multifunctional, reconfigurable technologies.
Kasper Pedersen, TU Denmark
“Our successful demonstration of stimuli-responsive valence changes in these crystals represents a foundational advancement, potentially enabling the integration of such materials into layered architectures for multifunctional, reconfigurable technologies,” explains corresponding author Kasper Pedersen, from TU Denmark.
“This work establishes a new paradigm in the manipulation of (metal-organic) van der Waals crystals, where molecular alloying can drive complete and temperature-tuneable valence change transitions from parent materials that do not display any valence change transition themselves.”
The study also featured on the journal front cover, where the art represents the valence change as lightning bolts of electrons surging through the lattice, symbolising how tiny molecular edits can ignite dramatic transformations across the whole crystal.
The concept of molecular alloying could also be transferable to entirely inorganic van der Waals crystals, heralding a new era of responsive 2D materials. However, molecule-based frameworks such as the one in this study have significantly greater and are therefore anticipated to produce substantial responses to mechanical stress. This implies great potential for next-generation electronic and spintronic applications, where precise control over charge and spin is paramount.
The full paper can be found at DOI: 10.1016/j.chempr.2025.102557
