High temperature behavior of UO2 was examined by X-ray diffraction. The temperature factor of UO2 was evaluated at room temperature and at high temperatures from 1003 to 1573 K. Two types of factors were calculated: the overall temperature factor of U and O atoms, designated as B; and the temperature factor of U atom, BU. In both cases, the value of the factors increased with increasing temperature up to ca. 1400 K. However, an anomalous decrease in these factors was detected above 1400 K. X-ray Debye temperature, Θ, was derived from the temperature factors as a function of temperature. Corresponding to the anomaly of the temperature factor, Θ increased atypically at the temperature higher than ca. 1400 K.
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