Electron paramagnetic resonance and optical spectroscopy of Yb³ ions in SrF₂and BaF₂; an analysis of distortions of the crystal lattice near Yb³ 

Abstract

SrF2 and BaF2 crystals, doped with the Yb3+ ions, have been investigated by electron paramagnetic resonance and optical spectroscopy. As-grown crystals of SrF2 and BaF2 show the two paramagnetic centres for the cubic (Tc) and trigonal (T4) symmetries of the Yb3+ ions. Empirical diagrams of the energy levels were established and the potentials of the crystal field were determined. Information was obtained on the SrF2 and BaF2 phonon spectra from the electron-vibrational structure of the optical spectra. The crystal field parameters were used to analyse the crystal lattice distortions in the vicinity of the impurity ion and the F− ion compensating for the excess positive charge in T4. Within the frames of a superposition model, it is shown that three F− ions from the nearest surrounding cube, located symmetrically with respect to the C3 axis from the side of the ion-compensator, approach the impurity ion and cling to the axis of the centre when forming T4. The F− ion located on the axis of the centre between the Yb3+ ion and ion-compensator, also approaches close to the impurity ion.

EPR of rare-earth ion clusters in mixed crystals Ba1−x La x F2+x doped with Yb3+ Ion

“…The g-factor in the ground state 1"7 equals gc = 24/7, which ls very close to the experimental value gcub = 3.422 [3,8]. The crystal field potential for the Yb 3+ ion in a trigonal center is G= v~.b + av, where the first term corresponds to the single cubic center and gives rise to the above-mentioned splitting of the 2F7/2 octet, and AV is the perturbation which has trigonal symmetry and is due mainly to local compensators F-.…”

“…Intensive lines O~ and 02 were observed already for "pure" BaF a (x = 0), so they were attributed to the well-known single trigonal center Yb-F (O-center with gll = 2.77, g• = 3.75) [3,7,8]). These lines are accompanied by the hyperfine structure due to Yb ~7~ isotopes (natural abundance of 14.4%, nuclear spin I = 1/2, hyperfŸ interaction parameter Aj : 887.2 MHz [9]) and Yb 173 (natural abundance of 16.2%, I = 5/2, A: =-243.3 MHz).…”

“…Experimental values are B ~ = 101 cm -~ and Agdt = -0.66 [3], so the estimates testify that the main part (about 75%) of the AV ~ is due to the charge of the compensator F-and the minor part (however notable, 25 cm -~, or nearly 25%) is given by the deformation of the F8 cube forming the nearest surrounding of the rare-earth ion.…”

“…In the cubic field, the ground multiplet ZFv2 of Yb 3+ ion is split into two doublets 1-'6 and F v and quartet Fs, the doublet F 7 being the ground state for the cubic center Yb 3+ in BaF z [3]: E(F7) = 0, E(Fs) = 535 cm -1, E(F6) = 542 cm -l. The corresponding wave functions in cubic axes (z li C4) are given, e.g., in ref. 9.…”

EPR and optical spectroscopy of Yb3+ ions in CaF2 and SrF2

“…In contrast to fluorite, other MeF 2 crystals do not exhibit many types of PCs, which considerably simplifies identification of the optical lines. Earlier we have studied Yb 3+ PCs of the cubic symmetry (T c ) in PbF 2 [5], SrF 2 , BaF 2 [6] and Yb 3+ PCs of the trigonal symmetry (T 4 ) in SrF 2 , BaF 2 [6]. Asgrown crystals of SrF 2 and BaF 2 show two Yb 3+ PCs: a cubic one (nonlocal compensation of the excess positive charge) and a trigonal one [1].…”