Correlation between luminescence and EPR spectroscopy as evidence of ytterbium Pair formation in Li_6Ln(BO_3)_3:Yb ~(3+) (Ln=Gd, Y) borate single crystals

摘要

Synthesized powders and grown single crystals of nominal compositions Li_6Ln(BO_3)_3:Yb~(3+) (Ln=Y, Gd) were investigated by means of powder and single-crystal X-ray diffraction (XRD), as well as optical near-IR spectroscopy in conjunction with electron paramagnetic resonance (EPR) spectroscopy. The appearance of two distinct zero-phonon lines suggests the existence of two kinds of Yb~(3+) ions in the single crystals. The XRD results exclude the possibility of a phase transition occurring between room and low temperatures. EPR spectra of single crystals show the presence of both isolated ions and pairs of ytterbium ions substituted for Y~(3+). A strong temperature dependence of the intensity of Yb-Yb pairs resonance lines coincides with temperature dependence of emission peak at 978 nm, confirming a common origin of the defect giving rise to these spectra. Calculated from EPR spectra, the distance between pairs of Yb~(3+) is in good agreement with crystallographic ones: R=3.856 ?, R cryst=3.849 ?. Emission spectra of an Yb-doped borate single crystal (see picture) show an unexpected multiplication of the 2F5/2-2F7/2 transitions around 975 nm. The crystals are investigated by means of X-ray diffraction and EPR spectroscopy. The results suggest the existence of two kinds of Yb~(3+) ions in the single crystals.