Correlation between Luminescence and EPR Spectroscopy as Evidence of Ytterbium Pair Formation in Li(6) Ln(BO(3) )(3) :Yb(3+) (Ln=Gd, Y) Borate Single Crystals.

Synthesized powders and grown single crystals of nominal compositions Li(6) Ln(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 Å, R(cryst) =3.849 Å