Molecular alloying drives valence change in a van der Waals antiferromagnet

Bespoke van der Waals (vdW) crystals provide control over the generation, confinement, and transport of charge, spin, light, and heat within and between atomically precise two-dimensional (2D) layers. We report a novel functionality in vdW crystals by actuating valence changes in a metal-organic antiferromagnet through molecular alloying. The quadratic net materials Cr(pyz)2Br2 and Cr(pyz)2I2 (pyz = pyrazine) are aliovalent, with different Cr(III) and Cr(II) oxidation states due to disparate crystal field potentials induced by I– and Br–. Applying isotropic pressure compresses the layers in Cr(pyz)2I2 significantly (16% at 1.5 GPa), but no Cr valence change is induced by mechanical strengthening of the axial crystal field. However, the alloyed, solid solutions Cr(pyz)2I2–xBrx exhibit quantitative, hysteretic, and tunable Cr(II) ⇄ Cr(III) interconversions with concomitant charge injection into the organic scaffold. This valence change, driven by the larger chemical pressure exerted by Br– over I–, manifests drastic changes to the magnetization and electrical conductivity, which varies by up to five orders of magnitude across the transition. The use of reticular coordination chemistry addresses a current gap in vdW and 2D materials science, where electronic structure engineering via valence change events has remained elusive. The concept of molecular alloying in vdW crystals expands the functionalities for future magnetoelectronics with drastically different electronic and magnetic states interchangeable by mild external stimuli.