The rare-earth ions can be stabilized in the divalent state in CaF₂ lattice, besides the trivalent state, with which can coexist. It is known that certain fraction of Yb³⁺ ions can be reduced to divalent state by various methods [1] or directly in the as-grown crystals using high deoxidization growth conditions [2]. The spectroscopic properties of the Yb²⁺ ions have been less investigated, mainly only for its intense and broad yellow-green luminescence [2].

The purpose of this work is to study the influence of Na⁺ and Pb²⁺ ions on the absorption and emission spectra of YbF₃-doped CaF₂ crystals with high divalent ytterbium content in the as-grown crystals.

NaF and PbF₂ codoped Ca_1-xYb_x F_2+x (x = 0.0007, 0.0017, 0.007, 0.0116) crystals have been grown using the conventional Bridgman technique [3]. In order to obtain high Yb³⁺ - Yb²⁺ conversion a special procedure has been developed [4]. Room temperature optical absorption spectra reveal the characteristic UV absorption bands of the Yb²⁺ ions. The presence of the Na⁺ ions drastically decrease the absorption intensity of the characteristic UV peaks of Yb²⁺ ions. The absorption peaks of the Pb²⁺ ions are in UV domain; the influence of these ions on the various Yb²⁺ ion peaks depends on the YbF₃ concentration in CaF₂. For low YbF₃ concentrations the peaks intensity increases, for high concentrations decreases. The luminescence studies reveal a strong YbF₃ concentration and codopant dependence of the near-UV emission bands. Besides the reported yellow-green luminescence, new emission bands have been observed in the near-UV spectral region. The Na⁺ ions drastically decrease the emission intensity of the characteristic UV peaks observed for the CaF₂:YbF₃ samples. The Na⁺ ions influence on the emission intensity depend on the λ_excitation  and on the NaF and YbF₃ concentration. For the crystals codoped with Pb²⁺ ions the emission intensity increases in comparison with the CaF₂:YbF₃ samples. This effect depends on the YbF₃ concentration and on the λ_excitation. The observed emissions in the visible domain are attributed to the anomalous luminescence [5].

 References

[1] Kaczmarek S M, Tsuboi T, Ito M, Boulon G and Leniec G   J. Phys.: Condens.  Mater. 17 (2005) 3771

[2] Feofilov P P  Opt. Spectrosc. 1 (1956) 992

[3] D. Nicoara and I. Nicoara, Mater. Sci. Eng. A 102 (1988) L1

[4] I. Nicoara, M. Stef, A. Pruna J. of CrystalGrowth 310 (2008) 1470

[5] P. Dorenbos, J. Phys.: Condens. Matter 15 (2003) 2645.

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