Na is studied by means of laser selective excitation. Five Pr centers are revealed. A three-parameter model is used to describe the Hamiltonian of rare earth ions in a cubic lattice with charge compensators. Two of the parameters are B 4 and B 6 of the original cubic crystal
the third is related to the distortion due to introducing the compensator. Fitting the splittings of
3
F
2
with such a model has shown that the observed centers are Pr
3+
with charge compensator situated at different distances.