Electron paramagnetic resonance studies in photorefractive crystals I: Hyperfine interaction and photoinduced charge transfer in233U5+ and238U5+ 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 to²³³U (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 5f ¹ configuration was also studied by converting the existing literature data on²³⁵U⁵⁺ to that of²³³U⁵⁺ by standard methods. This suggests that progressive substitution of oxygen by F⁻, in the series UO ₆ ⁷⁻ , (UO₅F)⁶⁻ and (UO₄F₂)⁵⁻ drastically decreases the hyperfine coupling constantA _∥, 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 10–300K region. The kinetics of decay and restoration of U⁵⁺ was also studied between 10–100K 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.