Electron paramagnetic resonance studies in photorefractive crystals I: hyperfine interaction and photoinduced charge transfer in 233U5+ and 238U5+ doped LiNbO3

Electron paramagnetic resonance studies were conducted on the photoinduced charge transfer and also hyperfine interaction of U5+ stabilized in photorefractive matrix LiNbO3. This work deals with: (i) first observation of hyperfine structure due to 233U (I=5/2) in its pentavalent state at octahedral sites and comparison with other possible site symmetries, (ii) photoinduced charge transfer as observable by EPR and its relevance to photorefractive behaviour of LiNbO3. The effect of chemical bonding on the hyperfine interaction of 5f1 configuration was also studied by converting the existing literature data on 235U5+ to that of 233U5+ by standard methods. This suggests that progressive substitution of oxygen by F-, in the series UO67-, (UO5F)6- and (UO4F2)5- drastically decreases the hyperfine coupling constant A, along the local distortion axis. This trend is explained as being due to the absence of ligand ion along the distortion axis at U5+ site in trigonal LiNbO3. The effects of illumination by copper vapor laser (CVL) on the intensity of the U5+ signal was studied in the 10-300 K region. The kinetics of decay and restoration of U5+ was also studied between 10-100 K range. The decay kinetics was found to obey double exponential. The reduction of concentration of U5+ with CVL-illumination and its restoration in the absence of light show that pentavalent uranium takes part in the photorefractive effects in LiNbO3. (author). 14 refs., 4 figs., 3 tabs