Kirkegaard M C, Langford J, Steill J, Anderson B, Miskowiec A
University of Tennessee-Knoxville, Knoxville, Tennessee 37996, USA.
Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
J Chem Phys. 2017 Jan 14;146(2):024502. doi: 10.1063/1.4973430.
Uranyl fluoride (UOF) is a hygroscopic powder with two main structural phases: an anhydrous crystal and a partially hydrated crystal of the same R3¯m symmetry. The formally closed-shell electron structure of anhydrous UOF is amenable to density functional theory calculations. We use density functional perturbation theory (DFPT) to calculate the vibrational frequencies of the anhydrous crystal structure and employ complementary inelastic neutron scattering and temperature-dependent Raman scattering to validate those frequencies. As a model closed-shell actinide, we investigated the effect of LDA, GGA, and non-local vdW functionals as well as the spherically averaged Hubbard +U correction on vibrational frequencies, electronic structure, and geometry of anhydrous UOF. A particular choice of U=5.5 eV yields the correct U-O bond distance and vibrational frequencies for the characteristic E and A modes that are within the resolution of experiment. Inelastic neutron scattering and Raman scattering suggest a degree of water coupling to the lattice vibrations in the more experimentally accessible partially hydrated UOF system, with the symmetric stretching vibration shifted approximately 47 cm lower in energy compared to the anhydrous structure. Evidence of water interaction with the uranyl ion is present from a two-peak decomposition of the uranyl stretching vibration in the Raman spectra and anion-hydrogen stretching vibrations in the inelastic neutron scattering spectra. A first-order dehydration phase transition temperature is definitively identified to be 125 °C using temperature-dependent Raman scattering.
氟化铀酰（UOF）是一种吸湿性粉末，有两种主要结构相：一种无水晶体和一种具有相同R3¯m对称性的部分水合晶体。无水UOF形式上的闭壳层电子结构适合进行密度泛函理论计算。我们使用密度泛函微扰理论（DFPT）来计算无水晶体结构的振动频率，并采用互补的非弹性中子散射和温度相关拉曼散射来验证这些频率。作为一个闭壳层锕系元素模型，我们研究了局域密度近似（LDA）、广义梯度近似（GGA）和非局域范德华泛函以及球平均哈伯德+U校正对无水UOF的振动频率、电子结构和几何结构的影响。U = 5.5 eV的特定选择给出了在实验分辨率范围内特征E和A模式的正确U - O键长和振动频率。非弹性中子散射和拉曼散射表明，在实验上更容易获得的部分水合UOF系统中，水与晶格振动存在一定程度的耦合，对称伸缩振动的能量比无水结构低约47 cm。拉曼光谱中铀酰伸缩振动的双峰分解以及非弹性中子散射光谱中的阴离子 - 氢伸缩振动证明了水与铀酰离子的相互作用。利用温度相关拉曼散射明确确定一阶脱水相变温度为125 °C。
