Abstract: In the present work, the fluorescence spectra of single crystal of KTbP₄O₁₂ have been measured at 77K. Most of the observed transitions were identified and the energy level structure of the ⁷F multiplet was also defined. The coordination of Tb³⁺ ion by oxygen atoms is eight-fold with the geometry of a slightly distorted square antiprism. In other words, the local symmetry of Tb³⁺ ion in KTbP₄O₁₂ belongs to C_4v. There is the same symmetry of Tb³⁺ ion in TbP₅O₁₄. So the symbols of Stark energy levels are of the same as in TbP₅O₁₄. The ligand field calculation was performed for such symmetry and gave a respectab-le to fit the observed Stark splitting of ⁷F_J(J = 0~6) multiplet.The comparison between the experimental and the calculated energy level shows that the RMSdeviation and the maximum deviation are 10cm^-1 and 24cm^-1, respectively.

Key words: KTbP₄O₁₂, Stark energy level, Calculation by fitting