The authors show that CaF₂:Y crystals that have been X-irradiated at low temperatures contain Y²⁺ ions associated with a trigonally located interstitial F^- ion. Experimental results are presented that support this identification and that show that a trigonal Jahn-Teller effect (with warping) is involved. An estimate for the Jahn-Teller energy of E_JT approximately=490 cm^-1 is obtained.

An EPR study has been carried out on a Cu²⁺ center in NaF single crystals (with 2 % mole of copper) from 1.6 k – 150 K. The symmetry of the EPR spectra is triclinic at 1.6 K, monoclinic at 4.2 K, tetragonal at T ≥ 40 K. No hyperfine structure has been observed. Models which include both the Jahn-Teller effect and a low symmetry crystal field are proposed.