The role of Li+ and Na+ charge compensators in SM3+-doped CaF2 and SrF2

We have examined the role of co-doping of Sm3+-doped CaF2 and SrF2 with Li+ and Na+ using electron paramagnetic resonance (EPR). Three Sm3+ centers in CaF2:Sm3+:Li+ are identified as tetragonal (C-4v), trigonal (C-3v), and orthorhombic (C-2v)(_) symmetry centers, whereas two Sm3+ centers each in CaF2:Sm3+:Na+, SrF2:Sm3+:Li+ and SrF2:Sm3+:Na+ are identified as C-4v and C-2v symmetry centers with correspondingly different g values. The C-2v centers correspond to Li+/Na+-substitution at Ca2+/Sr2+ sites along the [110] direction. The absence of cubic symmetry centers, observed in optical measurements of the same samples studied here, can be explained in terms of fast spin-lattice relaxation between the split ground state ((4))Gamma(8) levels of the H-6(5/2) multiplet of Sm3+.