Time-Resolved Infrared Reflectance Studies of the Dehydration-Induced Transformation of Uranyl Nitrate Hexahydrate to the Trihydrate Form

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• 作者：Timothy J. Johnson ; Lucas E. Sweet ; David E. Meier ; Edward J. Mausolf ; Eunja Kim ; Philippe F. Weck ; Edgar C. Buck ; Bruce K. McNamara
• 刊名：Journal of Physical Chemistry A
• 出版年：2015
• 出版时间：October 1, 2015
• 年：2015
• 卷：119
• 期：39
• 页码：9996-10006
• 全文大小：472K
• ISSN：1520-5215

文摘

Uranyl nitrate is a key species in the nuclear fuel cycle. However, this species is known to exist in different states of hydration, including the hexahydrate ([UO₂(NO₃)₂(H₂O)₆] often called UNH), the trihydrate [UO₂(NO₃)₂(H₂O)₃ or UNT], and in very dry environments the dihydrate form [UO₂(NO₃)₂(H₂O)₂]. Their relative stabilities depend on both water vapor pressure and temperature. In the 1950s and 1960s, the different phases were studied by infrared transmission spectroscopy but were limited both by instrumental resolution and by the ability to prepare the samples for transmission. We have revisited this problem using time-resolved reflectance spectroscopy, which requires no sample preparation and allows dynamic analysis while the sample is exposed to a flow of N₂ gas. Samples of known hydration state were prepared and confirmed via X-ray diffraction patterns of known species. In reflectance mode the hexahydrate UO₂(NO₃)₂(H₂O)₆ has a distinct uranyl asymmetric stretch band at 949.0 cm^鈥? that shifts to shorter wavelengths and broadens as the sample desiccates and recrystallizes to the trihydrate, first as a shoulder growing in on the blue edge but ultimately results in a doublet band with reflectance peaks at 966 and 957 cm^鈥?. The data are consistent with transformation from UNH to UNT as UNT has two inequivalent UO₂²⁺ sites. The dehydration of UO₂(NO₃)₂(H₂O)₆ to UO₂(NO₃)₂(H₂O)₃ is both a structural and morphological change that has the lustrous lime green UO₂(NO₃)₂(H₂O)₆ crystals changing to the matte greenish yellow of the trihydrate solid. The phase transformation and crystal structures were confirmed by density functional theory calculations and optical microscopy methods, both of which showed a transformation with two distinct sites for the uranyl cation in the trihydrate, with only one in the hexahydrate.