Work performed at Argonne National Laboratory was supported by the U.S. DOE Office of Basic Energy Science, Division of Chemical Sciences, and by DOE EMSP Programs, under contract N W-31-109-ENG-38. One of us (V.S.V.) was supported in Germany nad Russia by the German DAAD and RFBR Programs (N 00-02-16875) and by NATO (PST. CLG, 977348).
From uranium to americium, the lighter elements in the much lower in its excited states. The lowering of vibrational actinide series often bind with two oxygen anions to form energy in the excited uranyl states suggests strong structure actinyl ions. The uranyl ion, UO2+, is extraordinarily stable distortion induced by charge transfer-lattice interaction [7,8].
in solutions as well as in crystals and glasses. Under blue Strong charge trasfer-lattice interaction also results in large or shorter wavelength light illumination, it emits visible different equilibrium positions of the uranyl potential that fluorescence in the region of 500 to 700 nm. The fluores- prevent observation of zero-phonon transitions and change cence and spectral properties of uranyl in canary glass have the lifetime of the uranyl fluorescence.
been studied since the 19th century. After extensive and It is also well known that vibronic states originating from systematic investigation of actinyl properties that began in the charge transfer-lattice interaction are quite localized. As the middle of the last century, many current studies of the a result, the absorption and emission spectra of the UO2+ spectroscopic properties of actinyl species seek primarily ion in single crystals such as Cs2UO4Cl4 indeed exhibit to interpret their electronic structure and bonding, and extremely sharp lines . At low temperature, optical lines eventually to provide fundamental understanding of the are as narrow as 1cm-1, so that energy levels of various chemical and mechanical properties of nuclear waste-related vibrational modes and even the isotopic shift resulting phases. from substitution of oxygen-18 for exygen-16 is clearly After Jrgensen’s initial work in 1957  and subsequent observable . However, due to structural disorder in studies by others [2–4], it is now well-understood that amorphous environments, inhomogeneous line broadening optical absorption and emission in the uranyl ion are due is often significant and obscures the spectral structure of to oxygen-to-uranium charge transfer excitations. Significant vibronic transitions. The inhomogeneous line broadening, contribution to the uranium-related impurity center studies which often can be partially eliminated using selective laser was made by Feofilov and co-authors [5,6]. The optical excitation , provides fingerprint“ information about the ” transitions are due to electronic excitation primarily from uranyl local structure. Namely, one expects to observe sharp the 2p state of the ligands to the 5 f state of the uranium ion. lines in the absorption and emission spectra of uranyl ions in The intense vibronic features and absence of zero-phonon crystalline phases , where the charge transfer transitions lines in the optical transitions indicate that the charge can be so broad that the characteristic vibronic features transfer transition is strongly coupled to lattice vibrational become obscured for uranyl ions in amorphous phases. It modes. Therefore, the uranyl optical transitions indirectly has been realized that lanthanide and actinide ions in oxide probe the properties of local structure. Depending on glasses, such as borosilicate glass, have an ordered local the host materials, the lowest excited charge transfer state structure [11,12]. This means that an f -element ion has of UO2+ is on the order from 19 000 to 21 000 cm-1, and a definite coordination number and bond distance with its the vibrational energy of the O–U–O symmetric stretch first shell ligands, although there is no long-range order in mode is typically 750 to 900 cm-1 for its ground state and the glass matrix.
Structure and charge transfer dynamics of uranyl ions in boron oxide and borosilicate glasses Although the electronic properties of uranyl itself and its bonding in crystalline phases were extensively studied , there is still lack of knowledge about its bonding property in glasses. In the present paper, we report the results of experimental investigation and theoretical modeling of uranyl coordination in boron oxide and borosilicate glasses.
It is shown that the uranyl ions have a well-defined local structure that includes four additional oxygen ions in its equatorial plane to form a UO6 tetrahedral cluster in the glass matrix. A model structure of uranyl in boron oxide and borosilicate glasses is established using a molecular dynamics (MD) simulation method. The symmetry and stability of the uranyl cluster are interpreted based on the microscopic mechanisms of charge transferlattice interaction.
Figure 1. Ambient temperature excitation and fluorescence spectra of uranyl in B2O3 glass that contained 0.1% uranium.
1. Experimental results and discussion Samples of boron oxide (B2O3) and borosilicate (12% B2O3, 63% SiO2, 20% Na2O, 5% Al2O3, by weight) glass containing 0.1 and 1% (weight) uranium were prepared for this work. After the host glasses were made, a nitric acid solution of natural isotopic abundance uranium as uranyl was added to the ground glass, and the mixture was melted in a furnace by heating up to 1000C for the boron oxide glass and 1600C for the borosilicate glass before quenching to room temperature. Laser excitation and fluorescence spectra and fluorescence decay were recorded for the samples with 0.1 and 1% (by weight) uranium at room temperature and liquid helium temperature.
As showninFig. 1 for 0.1%UO2 in the boron oxide glass, the fluorescence spectrum consists of 5 sharp bands from 20 278 to 16 722 cm-1 that are due to vibronic transitions Figure 2. Fluorescence decay of uranyl in boron oxide glass.
from the excited state to the ground state of uranyl charge transfer configuration. Fig. 1 also includes a section of the excitation spectrum that shows the lowest two groups of excited states, one centered at 21 030 cm-1, and another exponential decay time of 2.7 ms at 4 K and 1.9 ms at at 21 570 cm-1. No zero-phonon line was observed in the room temperature. The single exponential decay suggests fluorescence spectrum. The spectrum is remarkably sharp that most uranyl ions have similar local environments in and little inhomogeneous line broadening due to structure the sample we studied. The uranyl fluorescence decay disorder was observed. No significant differences were in the boron oxide glass decay curves is insensitive to registered between the spectra of the 0.1 and 1% uranyl temperature from ambient temperature to 4 K. The initial samples. Uranyl ions in samples of different concentration decay rate at room temperature is slightly larger than that have the same line width and positions. The results shown at low temperatures. Without considering this faster decay in Fig. 1 suggest that UO2+ in the B2O3 glass matrix has at short times, the decay curves are not sensitive either to a well-defined local environment. This means that the temperature or uranium concentration. This result indicates uranyl ions have the same coordination number and local that the two most common relaxation mechanisms, namely, crystalline lattice, and disorder in the first shell of ligands is phonon relaxation and energy transfer between uranyle insignificant.
ions are not strong in the samples we studied. Without The lifetime of the uranyl ion in the lowest excited state much contribution from these two relaxation mechanisms, is longer than 1 ms, which is extremely long in comparison the ms-scale lifetime of uranyl in the lowest excited state with the µs scale of fluorescence decay dynamics for confirms that the charge transfer state is metastable, and many other uranyl compounds. As shown in Fig. 2, the the recombination process with electronic relaxation to the fluorescence decay curves of uranyl in the B2O3 glass ground states is extremely slow. This will be discussed are nonexponential immediately following the laser pulse later with regard to the mechanisms of charge transfer-lattice due to the energy transfer and approaches to a single interaction.
Физика твердого тела, 2002, том 44, вып. 1376 G.K. Liu, H.Z. Zhuang, J.V. Beitz, C.W. Williams, V.S. Vikhnin It should be pointed out that in contrast to our selective laser excitation spectra revealed four types of different uranyl sites plus significant inhomogeneous line broadening on each set of the site-resolved spectra, the EXAFS data [13,14] suggest only one type of uranyl cluster exist in the same borosilicate glass. It is presumably because that EXAFS is not capable of resulting in site selective spectrum and also has sensitivity much less than that of the laserinduced fluorescence detectrion, so that minor sits was not resolved in the EXAFS spectrum. Given that the three sets of laser-detect uranyl sits indeed have much weaker intensity in comparison with the major site shown in Fig. 3 (curves and 2), these results can be interpreted consistently with the assumption that in the borosilicate glass most of the uranyl ions coordinate with four SiO2 tetrahedra in their equatorial Figure 3. Fluorescence spectra of uranyl in borosilicate glass plane. This means that a uranyl ion tends to bond with that contained 0.1% uranium with laser excitation at 28 169 (1), SiO2 and excludes other metal oxides in formation of a 19 806 (2), and 19 550 cm-1 (3). All spectra were recorded at 4 K.
uranyl tetrahedral cluster. This hypothesis was verified in MD simulation.
As expected for uranyl ions in the borosilicate glass, 2. Molecular dynamics simulation the lines of the fluorescence spectrum are much broader than those in the boron oxide glass. Fig. 3 (curve 1) It is generally believed that there is no long-range order in shows the fluorescence spectrum recorded at 4 K and glasses even though short range structural order is common.
wiht laser excitation at 28 170 cm-1. It is apparent that This usually means that the structure order is limited to a uranyl ions have a common local structure, in which they metal ion and its nearest neighbor ligands. In B2O3 glass, have different electronic energy levels and vibronic energies for example, the BO3- cluster in a form of a plane triangle than in the B2O3 glass. It is also clearly shown in is the basic unit of the disordered glass matrix. The present Fig. 3 (curves 2 and 3) that selective excitation at lower results show that for uranium in the glasses that we studeied laser energies resulted in fluorescence line narrowing and not only is the linear structure of uranyl ion well defined and distinguished four different types of uranyl species that stable, but also structure ordering extends to the next nearest evidently are due to UO2 at different local environments.
neighbors to from a larger crystalline cluster. At present, These site-resolved spectra are shifted from each other by EXAFS and laser spectroscopic experiments provide the approximately 200 cm-1 and have similar vibronic peaks best experimental techniques for resolving in detail the that the separated by 750 to 760 cm-1. In comparison with structure of the uranyl clusters diluted in glasses. The the broad spectrum obtained with 28 170 cm-1 excitation, structure and vibrational spectrum of uranyl in vitreous glass the strongest set of peaks in Fig. 3 (curve 2) is attributed have been studied using a MD simulation method [15–17].
to uranyl ions surrounded by SiO2, the major constituent Our MD calculations utilize the Born–Mayer–Hugof the glass compositions. And the other three groups gins and Coulomb pair potentials and the Stillinof lines in weaker intensity likely arise from uranyl ions ger–Weber three-body potential . We simuthat are coordinated with the other three constituents of lated two systems, one is UO2–B2O3, and another the glass, namely Na2O, Ba2O3, and Al2O3. One also UO2–(70% mol)SiO2(30% mol)B2O3. The first system anticipates that in borosilicate glass a uranyl ion may be contained 2 124 atoms including four U6+ ions, and the coordinated with more than one type of these four metal second contained 3 000 atoms including nine U6+ ions.
oxides, more possibly, the presence of a different type of As a procedure of the MD simulation, the assignment metal oxides near the uranyl cluster that is coordinated of effective charge is -2 for each of the axial oxygen with one type of the metal oxides in its first shell. The in the uranyl ions and -1.5 for other four oxygen ions mixture of the surrounding metal oxides is expected to shift that located as the nearest ligands of the uranium. The the uranyl electronic energy levels. In the spectra obtained same structure configuration was always obtained in our from our sample, only four sets of spectra can be identified, MD simulations regardless of initial conditions as long although emission line shifting was observed when the laser as the simulated system undergoes a virtual quenching excitation energy was varied. We believe that the line from the initial temperature at 6000 to 1000C, then from shifting, indicative of inhomogeneous line broadening, is 1000Cto roomtemperature in 105 time steps (0.5 fs/step).
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