Theoretical Insights into Defect Chemistry and Luminescence of Ag– and Intrinsic Defects in Cesium Halides

Abstract This work elucidates the defect chemistry and luminescence of Ag-doped centers in cesium halides using first-principles calculations. Under Cs-rich conditions, Ag preferentially occupies the halogen site as Ag– in CsX (X = Cl, Br, I). The calculated energies of the A-band (3P1 ↔ 1S0) and C-band (1P1 ↔ 1S0) of Ag– deviate from experimental values by merely 0.2–0.3 eV and correctly reproduce the experimental red shift from CsCl to CsI. The small Stokes shift (0.2–0.3 eV) indicates minimal geometric relaxation. The difference between the equilibrium geometries of the triplet and singlet excited states of Ag– is below the numerical resolution of the relaxations and is therefore not resolvable in the present calculations. The self-trapped exciton (STE) exhibits strong lattice relaxation, with calculated emission energies of 4.44, 3.87, and 3.55 eV for CsX (X = Cl, Br, I), respectively, in reasonable agreement with experiment. Halogen vacancies emit at 5.30, 4.73, and 4.02 eV. Based on these calculations, we re-examine the characteristic emission bands of pure hosts: notably, the 3.6 eV band in CsI is attributed to the STE, and an iodine-vacancy origin is proposed for the 4.2 eV band. This study provides an inorganic chemical perspective on Ag– center formation and intrinsic luminescence in cesium halides, offering insights that may inform the rational design of Ag-doped scintillator materials.