Structural effects and 4f-5d transition shifts induced by La codoping in Ce-doped yttrium aluminum garnet: First-principles study

According to first-principles calculations performed on Ce-doped and Ce, La-codoped yttrium aluminum garnet (YAG) Y3Al5O12, the effect of La codoping on the local structure around Ce defects in Ce: YAG is an anisotropic expansion in overall, in opposition to recent propositions of local lattice compression. Its effect on the lowest Ce3+ 4f -> 5d transition is found to be a redshift, in agreement with experiments. The redshift is the result of a decrease in the difference between the energy centroids of the 5d(1) and 4f(1) configurations and an increase in the effective ligand field on the Ce 5d shell associated with electronic effects of La substituting for Y. These effects are mitigated by the ligand field decrease associated with the local expansion around Ce, which gives a blueshift contribution of a smaller value. The behavior of the energy difference between the centroids of the configurations cannot be anticipated by the usual model for this quantity, in spite of its usefulness to rationalize 5d -> 4f luminescences. The second 4f -> 5d transition is found to be blueshifted upon La codoping, also in agreement with experiments.