Host effects on the optically active 4f and 5d levels of Ce3+ in garnets

When Ce3+ is doped in a garnet, it substitutes for some cations. The effect of the substitution on the optically active 4f and 5d levels of Ce3+ can be analyzed in terms of an undistorted substitution followed by a structural relaxation, but, whereas the contribution of the undistorted substitution can be predicted/calculated using the crystallographic structure of the pure garnet, which is at hand, that of the structural relaxation demands the detailed local structure around the Ce3+ impurity, which is hard to know. Hence the importance of knowing the role of the undistorted substitution. What we study in this paper is the role of the unrelaxed host effects on the 4f and 5d levels of Ce3+-doped garnets (i. e. the interactions between Ce and the second and more distant neighbors). When they are added to the (previously studied) unrelaxed first-neighbor effects, they give the contributions of the undistorted substitutions. We performed spin-orbit coupling, relativistic, embedded cluster, wave function based ab initio calculations on the (CeO8)(13-) cluster under the effects of the embedding potentials of 21 selected garnets of Si, Al, and Ga, which have been obtained in this work, using experimental unrelaxed structures of the pure garnets. The calculations reveal that the unrelaxed host effect: (1) produces a red shift of the first 4f - 5d transition and (2) is an important contribution to this transition which plays a very important role in differentiating the values it has in different garnet families. The unrelaxed host effect is found to be responsible for the low value of the first 4f - 5d transition in Ce3+-doped Lu2CaMg2Si3O12. It is analyzed in terms of 5d and 4f centroid energy contributions and crystal field splitting contributions in the 21 doped garnets. The effect on the splittings of the 4f and 5d levels is also discussed. The undistorted host approximation is found to give reasonable comparison with experiments, so that it represents a relatively fast way to provide reliable ab initio information on the 4f and 5d levels of Ce3+ in garnets.