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

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.