Analysis of Energy-Structure Relationship for Ce3+ in Garnet-Type Oxides Based on First-Principles Calculations

Abstract

The 4f-5d transitions of Ce3+ in crystals are utilized for various optical materials such as solid-state lasers, scintillators, and phosphors. Since the 4f-5d transition energy of Ce3+ in crystals strongly depends on the local structure around Ce3+, it is important to understand the relationship between the energy levels and the local structure for theoretical design of novel optical materials based on Ce3+. In order to control the local structure, garnet-type oxides are useful as host crystals since there are three types of cation sites and the local structure around the impurity Ce3+ ion can be modified widely by cation substitution. In garnet-type oxides, Ce3+ usually occupies the dodecahedral site with D2 symmetry. The arrangement of eight nearest neighbor O2- ions can be understood as interpenetrating two tetrahedrons. Therefore, the geometry of CeO8 cluster can be specified by six structural parameters corresponding to the coordinates of two representative ligand positions. However, investigation of the energy-structure relationship depending on six structural parameters is not straightforward. In this work, in order to visualize the energy-structure relationship of Ce3+ occupying the dodecahedral site in garnet-type oxides, two-dimensional energy maps were constructed by performing first-principles relativistic molecular orbital calculations for a series of CeO8 clusters constructed by changing two selected structural parameters. The 4f-5d transition energies of Ce3+ for these clusters were calculated based on the Slater’s transition state method. Then the relationship between the 4f-5d transition energy and the local structure depending on the six structural parameters was clarified by the combination of these energy maps.