Effect of ultraviolet irradiation on CaF2 containing trivalent rare earth-interstitial hydride ion pairs

CaF2 crystals containing RE3+-H1- pairs of C4v symmetry have been irradiated by UV light and x rays at 77 K. These treatments destroy the C4v centres and produce new centres which are believed to have trigonal symmetry. The new centres are unstable on warming the samples to about 130 K when the original tetragonal centres are reformed.