Eva Beurer, Judith Grimm, Pascal Gerner and Hans U. Güdel
Absorption, Light Emission, and Upconversion Properties of Tm²⁺-Doped CsCaI₃ and RbCaI₃
Inorg. Chem. 45, 9901-9906 (2006)                 

Abstract: Absorption, light emission, and upconversion properties of Tm²⁺-doped CsCaI₃ and RbCaI₃ single crystals are presented and compared. Both compounds show multiple emissions after excitation at 21834 cm^–1 between 10 and 300 K. Besides sharp 4f-4f transitions around 8800 cm^–1, five and three broad 4f-5d emission bands are observed at higher energies in CsCaI₃:Tm²⁺ and RbCaI₃:Tm²⁺, respectively. The 4f-5d transitions are very sensitive to the crystalline environment: the onset of the 4f-5d excitations is red-shifted by about 1000 cm^–1 in RbCaI₃:Tm²⁺ compared to CsCaI₃:Tm²⁺. In addition, a broadening of bands is observed in the former compound. These differences are attributed to the structural changes that occur when the alkali metal is changed from Cs to Rb in these crystal lattices. An increased energy splitting of the multiplets and a red shift of the barycenter of the (4f)¹²(5d)¹ electron configuration in RbCaI₃:Tm²⁺ is the result. This affects not only the color of the visible emission, which turns from green in CsCaI₃:Tm²⁺ to yellow in RbCaI₃:Tm²⁺, but also the excited state dynamics. As a consequence, the dominant upconversion processes are different in the two compounds. Thus, the two title compounds nicely illustrate the influence of the structural environment on the optical spectroscopic properties of Tm²⁺.