High-spin and low-spin f-d transitions of Tb3+ in elpasolite hosts -: art. no. 085127

The electronic absorption spectra of Cs2NaTbCl6 and Cs2NaYCl6:Tb3+ have been recorded between 34000-43000 cm(-1) at temperatures down to 10 K. Well-resolved spin-forbidden and spin-allowed transitions are observed, comprising three and one electronic transitions, respectively, between the crystal field levels of 4f(8) and 4f(7)d (t(2g)) states. For transitions from the electronic ground state, symmetry selection rules restrict the upper crystal field levels to be Gamma(4u) (T-1u) symmetry. Three vibrational progression modes appear in the electronic transitions and these are assigned to breathing modes with the first, second, and third nearest neighbors of Tb3+. The experimental results are rationalized both from the use of a simple model and from the extended f-shell programs of Reid. The excitation process 4f(8)-> 4f(7)5d (t(2g)) involves a small (5.5 pm) bond length change which is considered to be a contraction rather than an expansion.