Distribution and spectrum properties of uranium in U:CaF2 crystal

Transparent and integral U:CaF2 single crystals with diameter of 75 mm were successfully grown by temperature gradient technique. The distribution coefficient of uranium in CaF2 is equal to 0.53, which was calculated using the formula, K0=Cs/C1. The calculated concentrations of uranium in CaF2 according to general distribution equation, Cs =K0C0(1-g)K0-1, are consistent with the measured values, indicating that the process of crystal growth was nearly in equilibrium. Unit cell parameters and optical absorption spectra of U:CaF2 crystals grown under different conditions have been analyzed. Results indicate that the growth atmospheres determine the valences of uranium ions and the mechanism of charge compensation: the uranium ion is U4+ with green color in the absence of PbF2; uranium is apt to form U3+ with radius most close to that of Ca2+ in the presence of PbF2 as a scavenger. From the beginning part to the end of U3+:CaF2 crystal, only optical densities of all absorption bands gradually increase with the concentration of uranium. With increasing concentration of doped uranium, the color of the as-grown U:CaF2 crystals change from pink to brownish. The 5-mm-thickness periphery of the U3+:CaF2 crystal with a yellow color was confirmed to contain mixed valences of U2+ and U3+, as a result of the reduction effect of graphite crucible.