U:CaF_2晶体的分凝特性与光谱性质

应用TGT法生长了直径为 75mm的U :CaF2 晶体 ,宏观上透明完整 .应用公式K0 =Cs Cl 计算了U在CaF2 晶体中的分凝系数等于 0 5 3.应用溶质分布一般公式Cs=K0 C0 (1-g) K0 - 1 ,计算U的浓度分布与测量值 ,数值符合说明晶体生长过程接近平衡状态 .分析不同条件下生长的U :CaF2 晶体的晶胞参数和吸收光谱 ,结果表明生长气氛决定U的价态及电荷补偿机理 :无PbF2 存在的条件下 ,U为 4价 ,晶体呈绿色 ;PbF2 的加入起到氟化去氧作用 ,U倾向于以离子半径最接近于Ca2 的U3 存在 ,晶体呈红色 .从晶体生长开始到结束的部位 ,U3 :CaF2 晶体吸收光谱的峰位不变 ,峰强呈现与U浓度相同的增加趋势 .U3 :CaF2 晶体外层厚约 5mm处呈黄色 ,含有U3 和U2 的混合价态离子 ,其原理是石墨坩埚的还原作用通过单质铅 ,使部分的U3 进一步还原成了U2 .

Distribution and spectrum properties of uranium in U:CaF_2 crystal

Transparent and integral U:CaF 2 single crystals with diameter of 75mm were successfully grown by temperature gradient technique. The distribution coefficient of uranium in CaF 2 is equal to 0.53, which was calculated using the formula, K 0=C s/C l.The calculated concentrations of uranium in CaF 2 according to general distribution equation, C s=K 0C 0(1-g) K 0-1, are consistent with the measured values, indicating that the process of crystal growth was nearly in equilibrium. Unit cell paramenters and optical absorption spectra of U:CaF 2 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 U 4 with green color in the absence of PbF 2; uranium is apt to form U 3 with radius most close to that of Ca 2 in the presence of PbF 2 as a scavenger. From the beginning part to the end of U 3 :CaF 2 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:CaF 2 crystals change from pink to brownish. The 5-mm-thickness periphery of the U 3 :CaF 2 crystal with a yellow color was confirmed to contain mixed valences of U 2 and U 3 , as a result of the reduction effect of graphite crucible.