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A: Spectroscopy, Molecular Structure, and Quantum Chemistry 2
Optical and Chemical Characterization of Uranium Dioxide (UO) and Uraninite Mineral: Calculation of the Fundamental Optical Constants Brent DeVetter, Tanya L Myers, Bret D. Cannon, Nicole K Scharko, Molly Rose K Kelly-Gorham, Jordan F. Corbey, Alan L Schemer-Kohrn, Charles Tom Resch, Dallas D. Reilly, and Timothy J Johnson J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b05943 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018
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The Journal of Physical Chemistry
Optical and Chemical Characterization of Uranium Dioxide (UO2) and Uraninite Mineral: Calculation of the Fundamental Optical Constants Brent M. DeVetter, Tanya L. Myers, Bret D. Cannon, Nicole K. Scharko, Molly Rose K. Kelly-Gorham, Jordan F. Corbey, Alan L. Schemer-Kohrn, C. Tom Resch, Dallas D. Reilly, Timothy J. Johnson* Pacific Northwest National Laboratory, 902 Battelle Blvd, Richland, WA 99352, USA *Corresponding author:
[email protected] Abstract Uranium dioxide (UO2) is a material with historical and emerging applications in numerous areas such as photonics, nuclear energy, and aerospace electronics. While often grown synthetically as single-crystal UO2, the mineralogical form of UO2 called uraninite is of interest as a precursor to various chemical processes involving uranium-bearing chemicals. Here, we investigate the optical and chemical properties of a series of three UO2 specimens: synthetic single-crystal UO2, uraninite ore of relatively high purity, and massive uraninite mineral containing numerous impurities. An optical technique called single-angle reflectance spectroscopy was used to derive the optical constants n and k of these uranium specimens by measuring the specular reflectance spectra of a polished surface across the mid- and far-infrared spectral domains (ca. 7000 – 50 cm1 ). X-ray diffractometry, scanning electron microscopy, and energy-dispersive x-ray spectroscopy were further used to analyze the surface composition of the mineralogical forms of UO2. Most notably, the massive uraninite mineral was observed to contain significant deposits of calcite and quartz in addition to UO2 (as well as other metal oxides and radioactive decay products). Knowledge of the infrared optical constants for this series of uranium chemicals facilitates nondestructive, noncontact detection of UO2 under a variety of conditions.
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1. Introduction High-quality experimental data reporting the optical properties of solid-state materials—in particular, the complex optical constants—are essential for developing predictive models for the scattering, reflection, and absorption of light across a wide variety of sample morphologies and configurations, such as particles, films, bulk, or samples embedded within other media.1-4 Computational methods offer an alternative approach for determining the optical constants and other intrinsic optical or mechanical properties, but even with today’s advanced supercomputers calculations of the properties for certain materials (particularly for actinides) remain challenging: Atomic basis sets and exchange correlation functionals for species such as uranium (containing 5f electrons) are difficult due to not only the lengthy computational times, but also due to the significant empirical adjustments required to successfully model the electronic and optical response.5-6 One such example is the work of Faulques et al.7 which extensively investigated the vibrational modes of five uranyl phosphate minerals using Raman, infrared (IR), and visible near-infrared (VNIR) spectroscopy along with ab initio calculations. This work emphasized the importance of the strength of certain bands found in the IR spectra. As such, it is necessary to understand, develop, and verify quantitative methods for performing the collection and tabulation of high-quality complex optical constants of actinide-bearing materials such as uranium dioxide (UO2) and other uranium-bearing species that are typically found in nature as minerals.8 These materials are of particular interest because oxidized uranium is a critical component of the nuclear fuel cycle,9 has historically been used in optically colored glasses,10 and more recently has shown promise as a novel semiconducting material.11 The complex refractive index is a key quantity for understanding the optical properties of solidstate materials that enables the calculation and prediction of optical behaviors over a great variety of applications in environmental sensing, online processing, etc.12-13 The optical properties of crystalline solids are well-approximated by classical dispersion models in which a summation of damped harmonic oscillators represents the lattice dynamics of the crystal.14 One may formulate optical properties in terms of a frequency-dependent complex permittivity ̂() which is related to the complex refractive index () by: () = () + i () = () + i ()
(1)
where is the frequency, () and () are the real and imaginary components of the permittivity, and () and () correspond to the dispersion and extinction of light in the material, respectively. The availability of the optical constants n and k allow one to model lightmatter interactions (scattering, absorption, reflection, etc.).1,3,15 In the case of solids, obtaining the optical constants can be significantly more complicated than e.g. simply measuring the vibrational modes of isolated gas-phase molecules in a transmission cell. For example, reflectance spectra have very strong dependencies on any of several morphological parameters including layer thickness, particulate size, shape, etc.15-16 Works by several researchers such as Christensen,17-18 Lane et al.,19 Salisbury et al.,20-21 and others22-23 have demonstrated there can be strong dependencies on the particle size that researchers must carefully consider especially for 2 ACS Paragon Plus Environment
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The Journal of Physical Chemistry
infrared reflectance spectra. More recent work by Myers et al.24 has characterized not only how the reflectance spectra vary as a function of particle size but demonstrated how this can be better quantified and understood if the particle size is itself quantified, typically via optical microscopy. There are two primary methods used to measure the optical constants of materials.25-27 The first method, called spectroscopic ellipsometry, has been heavily advanced by the semiconductor industry with a focus on thin films and layered materials. To obtain the optical constants of a material using ellipsometry, a physical model must be developed based on the measured phase shift and amplitude ratio of the reflected light. Our method of choice, however, is the more conceptually straightforward method for characterizing solids called single-angle reflectance spectroscopy. Reflectance spectroscopy measures the (specularly) reflected light, R(), intensity as a function of wavelength. In the infrared this is almost always carried out with a Fourier transform infrared (FTIR) spectrometer. The quantitative specular reflectance measures the amount of light reflected from the material of interest as a function of frequency, , over a wide wavelength range and at a near-normal angle of incidence. Reliable complex index of refraction values can be obtained from the single-angle reflectance data by use of the Kramers-Kronig transformation (KKT) as described in detail elsewhere.14,25 The KKT allows one to calculate (or n) if a complete set of n data (or k data) is known over an infinite number of frequencies.3 Of course it is not possible to measure over an infinite spectral domain, and, in practice, it is not possible to measure the real (or imaginary) index of refraction without determining the other. For our work, single-angle reflectance was performed at near-normal incidence (fixed at 11°). As we describe elsewhere,4 the polarization of light for near-normal incidence does not significantly impact the resulting reflectance spectra; therefore, we can neglect any instrumentally induced polarization, making data interpretation more straightforward. While obtaining the R() spectrum is in principle straightforward, the most common challenge for the single-angle reflectance method lies in the availability and preparation of the sample to successfully make quantitative specular reflectance measurements. In reality, the samples must have a large (~3 to 20 mm) optically polished planar face and be free of scratches, voids, and phase steps as well as be compositionally homogenous.2,4 Consequently, most n and k measurements have historically been made on materials with window-like properties. This includes materials such as polished semiconductor wafers, such as Ge or Si, or window-like materials4,28 such as KBr, BaF2, CaF2, or quartz. The KKT demonstrates the interdependence of n and k, but does not intuitively describe how to experimentally relate these two quantities to the quantitative reflectance spectra. To do so, the complex index of refraction is expanded into two terms: = + i = ln() e() where = / = ()/ and () is the phase rotation angle.29 From the quantitative reflectance spectra, the phase rotation angle () is calculated as: () =
" & !( )#
'
$ " %$
d) =
& ()
'
$
" %$
d)
(2)
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From Equation (2), it is seen that accurate calculations of the optical constants using the KKT require that the reflectance at 0 cm-1, R(0), is known. For many materials the static dielectric constant is well-characterized and may be used to calculate R(0) using Fresnel’s equations. Extrapolation from the lower limits of the measured experimental data to 0 cm-1 was performed using the following quartic4,23 relationship: () = (0) + + + ,-
(3)
where a and b are fitting parameters determined by linear least squares regression. After extrapolation and application of the KKT to the phase rotation angle, the optical constants were calculated in the OPUS v7.2 software using the following:29 () =
%!()
(4)
! //$ ()2 ()
(5)
.!()%! //$ ()012 ()
() = .!()%!//$ ()012 ()
For polar crystalline solids such as the ones studied here, the infrared reflectance spectra often exhibit a reststrahlen band.24,30 The reststrahlen band is typified by a high at lower frequency followed by a steep drop off to near 0% reflectance on the blue edge of the band – this is called the Christiansen minimum.18 As a first-order approximation, it is possible to extract the phonon modes—the longitudinal optical (LO) and transverse optical (TO) mode from the KramersKronig calculated permittivity by solving for the high frequency root, (34 ) = 0, and the frequency at the maximum value of the imaginary permittivity, max ( (84 )), respectively.31 The reststrahlen band width 9 is defined as the frequency difference between the TO and LO modes: 9 = :; -
