Crystal field energy level scheme of Er³⁺ in GdOCl Parametric analysis

Abstract

The optical absorption spectra of the trivalent erbium ion (Er³⁺) in polycrystalline GdOCl (x_Er = 0.09) samples were measured from 200 to 1650 nm at selected temperatures between 9 and 300 K. The luminescence from the ⁴S_3/2 and ⁴F_9/2 levels in GdOCl:Er³⁺ (x_Er = 0.02) was also observed at 77 K under argon-ion laser excitation in the region between 540 and 870 nm. The interpretation of the absorption and emission spectra yielded an energy level scheme consisting of 17 ^2S+1L_J levels; i.e. 61 Stark components out of the 182 Kramers doublets for the 4f¹¹ configuration. This scheme was fit to a Hamiltonian of 19 adjustable parameters including the free ion [Racah (E_0–3), Trees (α, β, γ) and Judd (T^k; k = 2–4, 6–8) parameters as well as the spin–orbit coupling constant (ζ_4f)] together with the crystal field B_q^k (B₀², B₀⁴, B₄⁴, B₀⁶ and B₄⁶) parameters according to the C_4v point group symmetry of the Er³⁺ site in the tetragonal lanthanide (Ln) oxychloride. The experimental energy level scheme was reproduced with a satisfactory root-mean-square deviation equal to 19 cm⁻¹. The present results were found consistent with the energy level parametrisation of Pr³⁺ (4f²) in PrOCl, Nd³⁺ (4f³) in NdOCl, Tb³⁺ (4f⁸) in TbOCl as well as of Eu³⁺ (4f⁶) and Tm³⁺ (4f¹²) in GdOCl. The influence of the nuclear charge, the energy of the 5s (and 5d) orbitals and the two-electron crystal field terms are discussed as possible interactions responsible for the evolution of the B_q^k parameters along the Ln series.