Go-doping of rare-earth (R3+)-doped CaF2 and SrF2 with LiF, NaF or KF enhan
ces the concentration of single R3+ centres of cubic symmetry. C-2v centre
are also produced in which the R3+ ions are charge-compensated by Li+, Naor K+ ions located in the nearest-neighbour Ca2+ or Sr2+ sites in the [1 1
0] direction. Infrared-absorption studies have been carried out for many R3
+ co-doped crystals. The infrared-absorption lines of the cubic centres app
ear as magnetic-dipole allowed transitions and Zeeman measurements confirm
their cubic symmetry. Absorption lines of the Li+ and Na+ centres occur wit
hin 10 cm(-1) of the cubic-centre lines and shift in going from Li+ to Naindicating different placements of these ions in the (1 10) position. (C) 2
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