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

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–1 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–1. 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.