Evidence of multicenter structure of cerium ions in gadolinium gallium garnet crystals studied by infrared absorption spectroscopy

Low temperature, infrared absorption spectra of gadolinium gallium garnet crystals doped with Ce are presented. In the region of intraconfigurational 4f-4f transitions the spectra exhibit existence of at least two different, major Ce³⁺ related centers in the GGG crystals and also some other centers at lower concentration. The spectrum of 4f-4f intrashell transitions of Ce³⁺ ions extends up to about 3700 cm^-1 due to the large splitting of the ²{F}7/2$ excited state. In the visible region the absorption spectrum shows influence of symmetry-related selection rules. The absorption coefficient changes in the region of 4${f}¹$-5${d}¹$ transitions due to thermal population of the second level, belonging to the ${}²F5/2 ground state. This suggests that the symmetry of the site occupied by Ce³⁺ ions, which substitute Gd³⁺, is higher than D₂ expected for garnet hosts.