Analysis of Water Coupling in Inelastic Neutron Spectra of Uranyl Fluoride

Andrew Miskowiec,J L Niedziela,Ashley E Shields,Marie C Kirkegaard

doi:10.1038/s41598-019-46675-x

Abstract

Inelastic neutron scattering (INS) is uniquely sensitive to hydrogen due to its comparatively large thermal neutron scattering cross-section (82 b). Consequently, the inclusion of water in real samples presents significant challenges to INS data analysis due directly to the scattering strength of hydrogen. Here, we investigate uranyl fluoride (UO2F2) with inelastic neutron scattering. UO2F2 is the hydrolysis product of uranium hexafluoride (UF6), and is a hygroscopic, uranyl-ion containing particulate. Raman spectral signatures are commonly used for inferential understanding of the chemical environment for the uranyl ion in UO2F2, but no direct measurement of the influence of absorbed water molecules on the overall lattice dynamics has been performed until now. To deconvolute the influence of waters on the observed INS spectra, we use density functional theory with full spectral modeling to separate lattice motion from water coupling. In particular, we present a careful and novel analysis of the Q-dependent Debye–Waller factor, allowing us to separate spectral contributions by mass, which reveals preferential water coupling to the uranyl stretching vibrations. Coupled with the detailed partial phonon densities of states calculated via DFT, we infer the probable adsorption locations of interlayer waters. We explain that a common spectral feature in Raman spectra of uranyl fluoride originates from the interaction of water molecules with the uranyl ion based on this analysis. The Debye–Waller analysis is applicable to all INS spectra and could be used to identify light element contributions in other systems.

Highlights

UO2F2 is comprised of layers of uranyl ions hexagonally coordinated by fluoride ligands
H atoms coupled to the lattice dynamics complicate analysis, because there is no intrinsic mechanism for separating contributions in inelastic neutron scattering (INS) by mass
It applies a Gaussian broadening to the points and bins them in 0.08, 0.1, and 0.2 meV bins, which were chosen to match the energy binning in the data sets

Summary

Introduction

UO2F2 is comprised of layers of uranyl ions hexagonally coordinated by fluoride ligands. Need to improve analytical spectroscopic methods for the identification and characterization of non-ideal, and otherwise degraded, fuel cycle solids The goal of this investigation is to identify the existence and understand the influence of nonremovable water on the lattice dynamics in uranyl fluoride. H atoms coupled to the lattice dynamics complicate analysis, because there is no intrinsic mechanism for separating contributions in INS by mass To tackle this problem, we offer a novel mechanism to analyze neutron scattering spectra based on application of the equipartition theorem to directly identify motion of adsorbed water in the UO2F2 structure. We further employ a self-consistent procedure for extracting the phonon density of states from the measured data enabling direct comparison to the phonon density of states from DFT12–14 These combined approaches allow us for the first time, to directly ascertain the dynamic affects of the adhered water in the anhydrous uranyl fluoride. The new analysis technique presented here is directly applicable to understanding the water dynamics in a wide variety of materials where residual water is retained in the crystalline structure

Methods

Results

Conclusion