Uranium is an element with a large radius and atomic mass, so its oxides are considered to be of very low δ~(18)O value at the bottom of the ~(18)O enrichment table for minerals. The oxygen isotope fractionation between uranium dioxide, uranium trioxide and water has been calculated by Hattori & Halas using force constants and low temperature heat capacity, which is a rare example for spectrum-heat capacity model (SHCM). But one could find some problems after detailed analysis of the calculated results. The force constants of uranium oxides are listed in table 1. Compared with the force constants of UO_2, K_2 of α-UO_3 is approximately equal to K_1 and F of α-UO_3 is obviously larger than that of UO_2. But α-UO_3 are depleted in ~(18)O comparedwith UO_2 in the calculation of Hattori & Halas. As we know, bonds with large force constants and small atomic mass will produce high vibrational frequencies and tend to be incorporated by heavy isotopes, which makes us feel hard to understand the calculation results of Hattori & Halas. For this reason, the oxygen isotope fractionation of α-UO_3 is recalculated using the spectroscopic data of Tsuboi and the modified increment method of Zheng.
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