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Chemical Origin of Exciton Self-trapping in Cs$_3$Cu$_2$X$_5$ Cesium Copper Halides

Copper halides Cs3Cu2X5 (X=Cl, Br, I) are promising materials for optoelectronic applications due to their high photoluminescence efficiency, stability, and large Stokes shifts. In this work, we uncover the chemical bonding origin of the Stokes shift in these materials using density functional theory calculations. Upon excitation, one [Cu2X5]3- anion undergoes sizeable local distortions, driven by Cu-X and Cu-Cu bond formation. These structural changes coincide with the formation of a self-trapped exciton, where particularly the hole is strongly localized on one anion. Analysis of the electronic structure and bonding reveals reduced antibonding interactions and enhanced bonding character in the excited state, stabilizing the distorted geometry. Our results establish a direct link between orbital-specific hole localization and bond formation. It provides a fundamental understanding of the excitation mechanism in Cs3Cu2X5 and offers design principles to tune optical properties in 0D copper halides.