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A three-dimensional illustration of a uranium ion ͑ represented by the larger sphere ͒ placed in the position of a void in the crystal structure of Fe 3 ͑ P 2 O 7 ͒ 2 . The Fe 2 ϩ ions in trigonal prism coordination are sandwiched between two Fe 3 ϩ ions in octahedral coordination which are connected via
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The atomic structure of iron phosphate glasses containing uranium has been studied by complementary neutron and x-ray scattering techniques. By combining x-ray and neutron structure factors, detailed information about different pair interactions has been obtained. Most of the basic structural features such as coordination numbers and O–O and P–O di...
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... crystalline counterpart of the base glass. 6 Three Fe–O distances are resolved from the Fe K -edge EXAFS data while only two Fe–O distances are resolved from the difference spectrum of the neutron and x-ray data. The Fe–P distances are similar, however, the Fe–P coordination numbers obtained in the present study are lower than those obtained from the EXAFS study. The structural parameters summarized in Tables III and IV indicate that the base glass and the uranium containing glasses have similar mean bond distances and coordination numbers. The approximate P–O, Fe–O, and U–O coordination numbers are four, five, and seven, respectively. The P–O and O–O mean bond distances do not change appreciably with the uranium content. However, both neutron and x-ray structural parameters indicate a small decrease ͑ 1.6% ͒ in the Fe–O distance when uranium is substituted for both iron and phosphorus in the base glass, i.e., in glasses B, C, D, and E. A similar trend was observed in a recent EXAFS study of glasses D and E. 9 In contrast, the Fe–O distance increases slightly when only iron is replaced by uranium, glasses F and G. However, glass H, 15 mol% UO 2 is substituted for Fe 2 O 3 , does not follow this trend. The U–O coordination numbers obtained from T N ( r ) and T X ( r ) are approximately four and seven, respectively ͑ see Tables III and IV ͒ . The reason for this disparity may be that U–O partial pair correlations in T N ( r ) are not well resolved. Uranium ions (U 4 ϩ ) in uranium phosphate com- pounds and in UO have eight-fold coordination with a mean U–O distance of ϳ 2.4 Å. The U–O coordination number of ϳ 7 measured from x-ray scattering in the present study implies that uranium valence in these glasses is close to four, which agree with the XANES results. Uranium ions found in ground water solutions generally have the hexava- lent oxidation state as the divalent UO 2 2 ϩ which is more mobile than the U 4 ϩ due to the low solubility of uranous phases. 23 If there were U 6 ϩ ions in the glass, one would expect a lower chemical durability would be expected to compared to the base glass. On the contrary, the dissolution rates, measured in distilled water at 90 °C for 16 days, of ϳ 10 Ϫ 9 g/cm 2 min for uranium containing iron phosphate glasses, are comparable to that measured for the base glass. 28 Combining this with the fact that the UL III -edge XANES spectra do not show any change in uranium oxidation state, it is highly probable that the peak observed at 1.77 Å in the difference spectrum, ⌬ T ( r ), given in Fig. 6 is due to an artifact such as Fourier termination ripple rather than to a U–O peak. A detailed XANES and EXAFS study on these waste-loaded iron phosphate glasses will be published elsewhere. The observation that the structural features of uranium containing glasses are similar to those of the base glass is consistent with recent XPS, Raman, and M ̈ ssbauer spectroscopic studies of these glasses. As shown in Fig. 8, M ̈ ssbauer hyperfine parameters, isomer shift, and quadrupole splitting, do not change appreciably with the uranium content. This indicates that iron environment in glass is not af- fected by the addition of uranium to a degree detectable by M ̈ ssbauer spectroscopy. The second uranium related peak in x-ray T ( r ) is observed at around 3.7 Å Fig. 3 which indicates that uranium ions are situated far from the Fe–P network in the glass. This may be the reason for the immunity of the M ̈ ssbauer hyperfine parameters to the addition of uranium. Both quadrupole splitting and isomer shift are sen- sitive to the changes in the iron environment. In addition, XPS and Raman spectra of glasses F through H have shown that P–O network features are equally insensitive to the addition of uranium. 29 This insensitivity of the Fe–P–O network to the addition of a nuclear waste component such as uranium may be the reason for the apparent immunity of the chemical durability to the additional waste components. 29 An in depth analysis of the crystalline structure of Fe 3 ͑ P 2 O 7 ͒ 2 helps in explaining some of the observed structural features in these iron phosphate glasses. The average oxygen packing, determined by the average volume per oxygen atom, in Fe 3 ͑ P 2 O 7 ͒ 2 is lower than that in most M 2 P 2 O 7 compounds, where M stands for a first raw transition metal. 22 The volume per oxygen atom for Fe 3 ͑ P 2 O 7 ͒ 2 and M 2 P 2 O 7 are 19.9 and 17 Å 3 , respectively. This difference in oxygen packing is due to the voids on each side of (Fe 3 O 12 ͒ 16 Ϫ clus- ters present in the Fe 3 ͑ P 2 O 7 ͒ 2 crystal structure. The positions of these voids were determined by looking at the polyhedra around each atom. The suitability of the voids for different waste components were then checked by placing the waste component in the position of a void and calculating near- neighbor distances and volumes of the resulting polyhedra. 30 These voids are found to be large enough to easily host a U 4 ϩ ion. Figure 9 schematically illustrates an uranium ion placed in one of these voids in the structure of crystalline Fe 3 P 2 O 7 2 . The density of the base glass and its crystalline counterpart are similar, ϳ 3.1 g/cm 3 , which suggest that similar voids may also exist in the glass structure which would explain why Fe–O–P network in glasses containing waste components is similar to that in the base glass. As the glass is quenched from the melt, the Fe–O–P network may form with voids that can be occupied by waste ions such as U 4 ϩ . The structure of iron phosphate glasses containing uranium has been studied by neutron and x-ray diffraction. More detailed structural information can be obtained from the real space correlation functions when the two techniques are used to complement each other. The structural parameters suggest that the basic structural features such as coordination numbers and O–O, P–O distances in uranium containing glasses are the same as those in the base glass. However, a slight change in the Fe–O distances occurs with the addition of uranium. The results obtained in the present study are consistent with the previously obtained XPS and Raman spectroscopic results. The observed structural parameters support a structural model in which the waste elements occupy voids already present in the Fe–O–P network in glass without appreciably altering the basic structure of the parent glass. This work was supported by the U.S. Department of Energy through the Environmental Management Science Pro- gram of the DOE ͑ Contract No. 96ER45618, University of Missouri-Rolla ͒ and the Division of Materials Science, Of- fice of Basic Energy Sciences ͑ Contract W-31-109-EGN-38, Argonne National Laboratory ͒ . The author acknowledges with thanks the facility staff of IPNS and APS for their as- sistance during his stay at ANL where the experiments and data analysis were ...
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... Such peak results from the influence of the m-tolyl groups of TCPs on adjacent P-O bonds of the growing film. Within the third stage of growth (see Fig. 8 [103,104]. The CN obtained from the MD study of Al-Hasni and Mountjoy [100] is 4.3 for the 40FeO system ( Fig. 13.a) and 3.7 for the 40Fe 2 O 3 glass ( Fig. 13.b). ...
The ability to understand and model the growth of amorphous thin films on solid surfaces is essential to a wide range of industrial applications, from the deposition of wear-resistant coatings to the production of solar cells. Here, a three-dimensional (3D) hybrid off-lattice kinetic Monte Carlo/molecular dynamics (kMC/MD) algorithm is developed to study the growth of thin amorphous films on solid substrates with atomistic resolution over timescales of tens of seconds. We use this method to study the growth of polyphosphate films from tricresyl phosphate (TCP) molecules on an iron substrate. Molecular adsorption/desorption, bond breaking/formation processes, and diffusion of iron ions through the film are simulated in the kMC stage and the film is relaxed during the MD stage. The kMC/MD method is approximately eleven orders of magnitude faster than equivalent reactive force field (ReaxFF) MD simulations. The simulated film growth rate and topology agree well with experimental results and the chemical structure of the film is consistent with previous molecular dynamics simulations of iron polyphosphates. The newly-developed hybrid kMC/MD methodology can be adapted to yield important insights into thin film growth for several other potential applications.
... Although the Fe-P interatomic distance acts as a background, it is expected to be at 3.2 Å; however, the coordination number cannot be accepted as reliable because of the overlapping of this peak with other peaks, such as Fe-Na distance, which is reported to be close to this distance as well. 18 Note that for the 40Fe composition, the XRD data outlined in Table 3 are obtained by Karabulut et al. 36 and ND data by Wright et al. 9 ...
The atomic structure of a series of sodium iron phosphate glasses is studied using different experimental techniques: X‐ray and neutron diffraction (ND), infrared spectroscopy, extended X‐ray absorption fine structure (EXAFS), and X‐ray absorption near‐edge structure (XANES). Detailed information about the atomic pair correlations is obtained. The high resolution of ND in real space resolves two P–O distances at 1.48 Ǻ and 1.59 Ǻ as expected. All the glasses are found to consist of a phosphate tetrahedral network with metaphosphate chains and pyrophosphate units, and every phosphate unit is found to have two or three nonbridging oxygen (NBO) links available to coordinate with Na and Fe cations. The Fe–O coordination number in these glasses is found to decrease from 5.7 to 4.8 with increasing the Fe content, whereas the Na coordination number of approximately 5 is detected for all the samples.
... Other researchers used unconstrained fitting with two doublets assigned to tetrahedral Fe 3+ and octahedral Fe 3+ , respectively [23,28]. Tetrahedral Fe 3+ sites have been identified in iron phosphate glass by x-ray absorption spectroscopy, high energy x-ray diffraction, and neutron scattering [29][30][31], and the Mössbauer spectra in Fig. 7 were analyzed assuming that both Fe 3+ (tet) and Fe 3+ (oct) sites were present in these glasses. Table 2 summarizes the corresponding parameters, including isomer shift (IS in mm/s), quadrupole splitting (QS in mm/s), FWHM (full width at half maximum), of the Lorentzian contribution to the peak (w in mm/s), along with the relative fractions of each site to the total iron content of the glass, and the fitting uncertainties (χ 2 ). ...
Chemically durable iron phosphate glasses are promising candidates for hosting nuclear wastes containing constituents like Cs 2 O and MoO 3 that have lower solubility in more conventional borosilicate glasses. In this work, two series of Cs-Mo-Fe-phosphate glasses were prepared and characterized. For glasses in Series A (xCs 2 O• (100-x)•(15MoO 3 •22.5Fe 2 O 3 •62.5P 2 O 5), x = 0, 7.5, 15.0, 22.5), increasing Cs 2 O contents increase the molar volume and decrease the glass transition temperature. Raman spectroscopy reveals that adding Cs 2 O breaks down linkages between MoO 6 octahedra, replacing MoeOeMo and Mo]O bonds with MoO − Cs + bonds. Phosphate anion chromatography reveals that adding Cs 2 O has little effect on the phosphate anion distributions. For glasses in Series B (15Cs 2 O• yMoO 3 •(28.75-y)Fe 2 O 3 •56.25P 2 O 5 , y = 0, 5.75, 11.50, 17.25, 23.00, 28.75), the glass transition temperature decreases and the aqueous dissolution rates increase when MoO 3 replaces Fe 2 O 3. The formation of isolated MoO 6 octahedra with MoO − Cs + bonds results in an increase in the average phosphate anion-length in the MoO 3-rich glasses. Mössbauer spectroscopy indicates that ferric ions dominate in both series, and that the average Fe-coordination number increases with increasing Cs 2 O content (series A) and with increasing MoO 3 content (series B).
... The model elaborated to fit the Fe K-edge EXAFS data comprised several distinct scattering paths, based on the crystal structure of Fe 3 (P 2 O 7 ) 2 [53] which has been shown to crystallise as the primary phase from these IPG glasses upon heat treatment [19,32,49]. Previous X-ray and neutron scattering studies of IPG have shown the average local Fe environment to be approximated by those in Fe 3 (P 2 O 7 ) 2 [54]. ...
The bulk properties such as glass transition temperature, density and thermal expansion of iron phosphate glass compositions, with replacement of Cs by Ba, are investigated as a surrogate for the transmutation of ¹³⁷Cs to ¹³⁷Ba, relevant to the immobilisation of Cs in glass. These studies are required to establish the appropriate incorporation rate of ¹³⁷Cs in iron phosphate glass. Density and glass transition temperature increases with the addition of BaO indicating the shrinkage and reticulation of iron phosphate glass network. Average thermal expansion coefficient reduces from 19.8 × 10⁻⁶ K⁻¹ to 13.4 × 10⁻⁶ K⁻¹, when 25 wt. % of Cs2O was replaced by 25 wt. % of BaO in caesium loaded iron phosphate glass. In addition to above bulk properties, the role of Ba as a network modifier in the structure of iron phosphate glass is examined using various spectroscopic techniques. The FeII content and average coordination number of iron in the glass network was estimated using Mössbauer spectroscopy. FeII content in the un-doped iron phosphate glass and barium doped iron phosphate glasses were 20, 21 and 22 ± 1% respectively and the average Fe coordination varied from 5.3 ± 0.2 to 5.7 ± 0.2 with increasing Ba content. The atomic scale structure was further probed by Fe K-edge X-ray absorption spectroscopy. The average coordination number provided by extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure was in good agreement with that given by Mössbauer data.
... One other type of HLW waiting for disposal is the actinide containing wastes resulting from the reprocessing of spent fuel from nuclear power stations which involves extraction of uranium and plutonium formed during the fuel burning process. Effect of uranium on the structure and properties of iron phosphate glasses have been investigated previously [12][13][14]. Actinide containing wastes are currently incorporated in borosilicate nuclear glass matrices. However, waste loading capacity is quite low and there is an increasing need for alternative glass hosts that can incorporate higher amounts of wastes, especially Pu containing HLWs. ...
... Melts were quenched in air by pouring into a 1 cm × 1 cm × 5 cm steel molds, annealed at 475°C for 2 h and slowly cooled down to room temperature. P 2 O 5 loss during melting is known to be negligible for iron phosphate/borophosphate glasses [5][6][7][8][9][10][12][13][14][19][20][21][22][23][24]. ...
... The spectrometer was calibrated using an α-iron spectrum whose linewidth (FWHM) of the outer lines was 0.27 mm/s. Mössbauer spectra were fitted with broadened Lorentzian doublets to match the absorption envelope and weighted averages of Mössbauer hyperfine parameters are reported which is an accepted method for obtaining iron valence states and their concentrations [6][7][8][9][10][12][13][14]24,25]. ...
Possible use of iron phosphate/borophosphate glasses for the vitrification of nuclear wastes containing Pu has
been investigated on the basis of cerium as it is considered one of the nonradioactive surrogates for Pu. A series
of glasses with nominal compositions of xCeO2–yB2O3−[40−(x+y)]Fe2O3–60P2O5 (x=2–8, y=0–4, mol%)
have been synthesized and their structure was studied by X-ray diffraction, scanning electron microscope, differential
thermal analysis, infrared and Mössbauer spectroscopy. All of the compositions studied formed glass. The
aqueous dissolution rates calculated from the bulk weight losses indicate that the chemical durability of these
glasses is comparable to base iron phosphate glass. Even though there are some variations in the low frequency
part of the IR spectra upon addition of CeO2, the basic structure of iron phosphate/borophosphate glasses does
not seem to be greatly affected by Ce addition. Analysis of Mössbauer spectra showed that iron exists as ferrous
and ferric ions in the glasses studied and both iron ions have octahedral coordination.
We propose here a method to generate random networked amorphous structure using only readily available short-range properties like bond lengths, bond angles and connectivity of the constituents. This method is a variant of Monte-Carlo (MC) method wherein the basic constituents of an amorphous network i.e. rigid polyhedral units are connected randomly obeying certain steric constraints. The algorithm is designed to reproduce the medium-range order universally observed in glasses. The method resembles the reverse MC (RMC) method where a random move of an atom inside a box is accepted or rejected depending upon whether it decreases or increases the deviation from the experimentally observed features. However unlike RMC, this method does not demand large experimental sets of scattering data which are difficult to obtain for glasses. It rather relies on the stochasticity of MC method to produce glassy structures. The method presented here examines the possibility of developing glassy structures without employing either the computationally demanding melt-quench simulation or the information demanding RMC method. The algorithm is first validated against SiO2 glass structure by comparing with the available structures from other methods and experimental data. The method is then extended for developing more complex Iron Phosphate Glass (IPG) structures and a comparative study of the generated models is done with existing models of IPG developed using melt-quench scheme in classical Molecular Dynamics (MD). Through this study we conclude that the IPG model developed using MC method and subsequently equilibrated using MD agrees in all structural aspects with the model of IPG generated using melt-quench simulation, projecting the current recipe as an interesting alternative to melt-quench simulation for developing glass models.
The chemical state of ruthenium in simulated iron phosphate radioactive waste glass was investigated by conventional X-ray absorption fine structure (XAFS) and imaging XAFS analyses. The EXAFS analysis suggested that ruthenium was contained as glass phase when content of the waste components was less than 10 wt.% in 30 mol%Fe2O3-P2O5 base glass. In other samples, crystalline RuO2 was observed and became prominent with the increase of waste content. According to the imaging XAFS analysis, RuO2 particles in all samples had length smaller than 50 μm. Large size aggregations of RuO2, which are found frequently in nuclear waste borosilicate glass, were not seen in any of the iron phosphate glass samples.
Structural features of hafnium or cerium doped iron borophosphate glasses have been studied by Raman and EPR spectroscopies. Hf and Ce are introduced into the composition as surrogates for Pu. Raman spectrum of Hf and Ce free base iron phosphate glass indicate that the glass network consisted mainly of Q ¹ units with contributions from Q ² units. The general features observed for base glass are maintained in the Raman spectra of Hf/Ce containing glasses with some modifications with increasing Ce/Hf content in the composition. The EPR spectra of Hf and Ce containing glasses show different features. Superparamagnetic domains were highlighted through EPR for all CeO 2 containing glasses.
