F and photochromic centers in LiYF4:Nd crystals

Samples of Nd doped LiYF4 single crystals (scheelite structure) were X-irradiated at room temperature and valence changes plus defect production by radiation damage were studied by optical spectroscopy techniques. The band centered at 3.70 eV showed a temperature dependent gaussian shape indicating a strong electron phonon coupling suggestive of an F type center defect. Reversible valence changes of Nd3+ -> Nd2+ after irradiation were observed throughout luminescence studies when (Nd3+)* -> Nd3+ transitions were identified. A new band centered at 4.57 eV was found. This band showed an optical switching mechanism and a correlation to the 4Dsub(3/2) transition of the Nd3+ ion. This band was assigned as being due to a photochromic (F-Nd3+) center. (author)