It was noted that the addition of NaF or LiF to rare-earth (R3+)-doped samples increased the concentration of isolated R3+ ions to an extent such that the cubic-symmetry centers dominated the defect-site distribution. The optical spectra of the centers exhibited sharp vibronic lines that had intensities and line-widths which were comparable to their parent zero-phonon transition. The vibronic frequency, of about 140/cm, decreased along the rare-earth series. It was suggested that these vibronic transitions were associated with low-frequency resonance modes of the heavy trivalent R3+ ions which replaced divalent cations. The loose packing of the smaller-radius R3+ ions then contributed to the weak coupling to the host and gave rise to the sharp vibronic transitions.

Sharp Resonance Modes of Cubic Symmetry Rare-Earth Ions in CaF2-Type Crystals. S.P.Jamison, R.J.Reeves, P.P.Pavlichuk, G.D.Jones: Journal of Luminescence, 1999, 83-84, 429-34