Eu^(3+) 이온이 첨가된 K₂YF_(5) 단결정의 사이트 선택 레이저 분광학 연구

Abstract

Luminescence and excited-state dynamics of Eu^(3+) in K₂YF_(5) crystals are investigated by site-selective laser-excitation spectroscopy at 15 K and room temperature. The K2YF5:Eu^(3+) crystals with different Eu^(3+) concentrations (0.1, 1.0, 6.0, 30 and 100 mol %) were grown by the hydrothermal method. The K₂YF_(5) crystal with 100 mol % Eu^(3+)-concentration corresponds to K₂EuF_(5) crystal. The excitation spectra of the ^(7)F0 → ^(5)D0 transition were obtained by monitoring the emission due to the ^(5)D0 → ^(7)FJ (J=1, 2, 3, …, 6) transitions. The emission spectra were measured by tuning the dye laser to resonance with each line in the excitation spectra. The excitation spectra of the K₂YF_(5):Eu^(3+) crystals with the Eu^(3+) concentrations of 0.1, 1.0, and 6.0 mol % exhibit two strong lines at 578.90 (line A) and 579.25 nm (line B) together with four weaker lines at 578.65 (line a1), 578.81 (line a2), 579.12 (line b1), and 579.52 nm (line b2). The weaker lines increase in intensity and broaden in spectral width for higher Eu^(3+) concentrations. However, the significantly different spectral features are observed in the excitation spectra for the Eu^(3+) concentrations of 30 and 100 mol % which indicate that these crystals maintain no more the structural properties of the K₂YF_(5) lattice. Two kinds of decay with long (~ 5.0 ms) decay times for line A, a1 and a2 and short (~ 0.5 ms) decay times for B, b1, and b2 are detected for the selectively excited emissions of K₂YF_(5):Eu^(3+) (0.1, 1.0 and 6.0 mol %) crystals. The results indicate that there exist at least six different sites for Eu^(3+) in K₂YF_(5) lattice. It is suggestible that the Eu^(3+) ions with longer decay times are attributed to the isolated Eu^(3+) ions at the Y^(3+) sites while, the Eu^(3+) ions with shorter decay times are due to Eu^(3+)-Eu^(3+) pairs occupying two-nearest-neighbor Y^(3+) sites. The exchange interaction in Eu^(3+)-Eu^(3+) pair is responsible for luminescence with short decay times because the distance between two Y^(3+) sites is only 3.69 Å. The strong exchange interaction occurs dominantly in the ion-pair with distance shorter than 5 Å. The decay time of the luminescence from paired ions is quite shorter than that from isolated ions because the former and later occur by exchange interaction and by electric dipole - electric dipole interaction, respectively. Therefore, the long decay times are ascribed to the isolated Eu^(3+) ions. The ratio of emission intensities from paired and isolated ions was estimated to be about 0.01 from the time-integrated excitation spectrum. This means that only 1.0 % of total luminescence comes from the paired Eu^(3+) ions. The substitution sites for Eu^(3+) with weaker excitation lines are ascribed to the Eu^(3+) ions perturbed by isolated and paired Eu^(3+) ions, i.e. the Eu^(3+) ions at next-nearest neighbors and next-next-nearest neighbors. At room temperature the decay times of paired Eu^(3+) ions maintain short decay at early time and become longer to those of isolated Eu^(3+) ions at late time. The indicates that the energy transfer takes place from paired Eu^(3+) ions to isolated Eu^(3+) ions by phonon assistance at room temperature.