Molecular alloying drives valence change in a van der Waals antiferromagnet

© 2025 Elsevier Inc. Bespoke van der Waals (vdW) crystals provide command over the confinement and transport of charge, spin, and heat within and between two-dimensional (2D) layers. We report a novel functionality in vdW crystals by actuating valence changes through molecular alloying. The net materials Cr(pyrazine)2Br2 and Cr(pyrazine)2I2 are aliovalent, hosting Cr(III) and Cr(II), respectively, due to disparate crystal field potentials. Pressurizing and thereby strengthening of the crystal field compresses the Cr(pyrazine)2I2 layers significantly, but no Cr valence change is induced. However, alloyed Cr(pyrazine)2I2−xBrx phases exhibit hysteretic and tunable Cr(II) ⇄ Cr(III) interconversions with concomitant charge injection into the net. The valence switch manifests drastic changes to the magnetization and the electrical conductivity, which varies by up to five orders of magnitude during the valence conversion. This use of coordination chemistry addresses a gap in vdW and 2D materials science, where electronic structure engineering via valence change events has remained elusive.