Hydration as a Structural Switch: Controlling Magnetic Coupling and CO2 Photoreduction in Transition-Metal Halide Perovskites

Abstract

Transition-metal halides are of particular interest because their partially filled d-orbitals, containing unpaired electrons, give rise to distinctive magnetic and electronic properties. Notably, hydration can act as a structural switch, inducing substantial modifications in their magnetic interactions and electronic behavior. Thus, elucidating the role of hydration and the corresponding transition-metal dynamics is essential for unraveling the rich photophysical characteristics and functional potential of these materials. In this study, we synthesized CsMnCl3·2H2O crystals and investigated the impact of structural water on their low-temperature optical, magnetic, and catalytic properties. We observed that solvent-induced crystal-to-crystal phase transformations, driven by the loss and regaining of water of crystallization, alter the coordination number of Mn2+ ions. Optical studies revealed distinct emission behaviors: CsMnCl3·2H2O exhibited superlinear emission behavior in the excitation power range of 0.1 mW to 1 mW, while CsMnCl3 displayed sublinear behavior. Magnetic characterization indicated a higher Néel temperature for CsMnCl3 compared to CsMnCl3·2H2O, reflecting stronger Mn2+–Mn2+ interactions and enhanced antiferromagnetic coupling in the anhydrous phase. Catalytic measurements further established that CsMnCl3 exhibits superior performance in CO2 photoreduction, particularly in selective methane (CH4) formation, due to improved charge-carrier separation in synergy with stronger magnetic coupling. Collectively, these findings highlight the pivotal role of hydration as a structural switch in modulating the optical, magnetic, and catalytic responses of lead-free transition-metal halide perovskites, offering design principles for next-generation optoelectronic and photocatalytic materials.