The inclusion of NaF or LIF into rare-earth (R3+) doped CaF2 and SrF2 incre
ases the concentration of isolated R3+ ions to the point where these cubic
symmetry centers dominate the defect site distribution. The optical spectra
of these centers are noteworthy in revealing sharp vibronic lines that hav
e intensities and line widths comparable to their parent zero-phonon transi
tion. The vibronic frequency, of approximately 140 cm(-1), is observed to d
ecrease along the rare-earth series. It is proposed that these vibronic tra
nsitions are associated with low-frequency resonance modes of the heavy tri
valent R3+ ions substituting for the divalent cations. The loose packing of
the smaller radius R3+ ions contributes to the weak coupling to the host g
iving rise to the sharp vibronic transitions. (C) 1999 Elsevier Science B.V
. All rights reserved.