Antisite defects in Ce-doped YAG (Y3Al5O12): First-principles study on structures and 4f-5d transitions

The interactions between Ce 3+ and antisite defects (AD) in YAG (Y 3Al 5O 12) are studied by means of first-principles calculations: periodic-boundary-conditions density-functional- theory for a 160 atom YAG unit cell with one Ce 3+ and one or two ADs, and complete-active-space second-order perturbation theory for the 4f 1, 5d 1, and 6s 1 electronic manifolds of the (CeO 8Al 2O 4) 15- embedded cluster. Attractive interactions are found between Ce 3+ and the ADs. The formation of one AD is more favorable in Ce:YAG than in YAG, but the formation of a second AD is less favorable, which means that the presence of Ce tends to lower the concentration of antisite defects in YAG. The interaction between Ce 3+ and antisite defects blueshifts the two lowest Ce 3+ 4f → 5d transitions. This result rules out the involvement of antisite defects in the recently reported excitation of the lowest 5d → 4f emission with photons below the zero-phonon line and leaves other distorted Cerium centers for consideration, like Ce 3+ interacting with interstitial non-stoichiometric Yttrium or with vacancies. The reasons behind the blueshifts are analyzed in detail: they are dominated by a decrease in the effective ligand-field splitting of the 5d 1 manifold, almost entirely due to the structural changes of short- and long-range and with almost negligible electronic effects from the Y and Al site exchanges.