Iron phosphate glasses: Structure determination and radiation tolerance

AbstractIron phosphate glass (IPG) has gained recent interest for use in encapsulating radioactive waste for long term storage. In this work, we investigate 5 different compositions of iron phosphate glass. We consider amorphous structures of 3 known crystalline phases: Fe2+Fe23+(P2O7)2, Fe43+(P2O7)3 and Fe3+(PO3)3, and structures of IPG (40mol% Fe2O3 and 60mol% P2O5), with 4% and 17% Fe2+ ion concentrations.Using constant volume molecular dynamics (MD), we quench a set of structures for each glass composition, to find the optimal density structure. We found that the lowest energy structures of IPG with 4% and 17% concentration of Fe2+, have a density of 3.25 and 3.28g/cm3 respectively. This is slightly higher than the experimentally measured values of 2.9 and 2.95g/cm3 respectively.We also estimate an upper and lower bound on the melting temperatures of each glass, then for each glass, we simulate radiation damage cascades at 4keV. The cascade structures can be in the form of either a concentrated thermal spike or more diffuse with sub-cascade branching. We found that the glass compositions with a higher Fe/P atomic ratio, contained a greater number of displacements after the cascade. We also found that the IPG with 4% Fe2+, contained slightly fewer displacements than the IPG with 17% Fe2+. This is consistent with our previous work, which showed that the threshold displacement energies are lower for glasses with a lower Fe2+ content. In all the simulations, many PO4 polyhedra are destroyed during the early stages of irradiation, but recover strongly over a time scale of picoseconds, leaving very few over or under co-ordinated P atoms at the end of the ballistic phase. This is in contrast to recent work in apatite. The strong recovery indicates that phosphate glasses with a low Fe2+ content could be good materials for waste encapsulation

Stacking-Mediated Diffusion of Ruthenium Nanoclusters in Graphite

The diffusion, penetration and intercalation of metallic atomic dopants is an important question for various graphite applications in engineering and nanotechnology. We have performed systematic first-principles calculations of the behaviour of ruthenium nanoclusters on a graphene monolayer and intercalated into a bilayer. Our computational results show that at a sufficiently high density of single Ru atom interstitials, intercalated atoms can shear the surrounding lattice to an AA stacking configuration, an effect which weakens with increasing cluster size. Moreover, the interlayer stacking configuration, in turn, has a significant effect on cluster diffusion. We therefore find different trends in diffusivity as a function of cluster size and interlayer stacking. For monolayer graphene and an AA graphene bilayer, the formation of small clusters generally lowers diffusion barriers, while the opposite behaviour is found for the preferred AB stacking configuration. These results demonstrate that conditions of local impurity concentration and interlayer disregistry are able to regulate the diffusivity of metallic impurities in graphite.Comment: 11 pages, 10 figure

Ripplocations in Layered Materials: Sublinear Scaling and Basal Climb

The ripplocation is a crystallographic defect which is unique to layered materials, combining nanoscale delamination with the crystallographic slip of a basal dislocation. Here, we have studied basal dislocations and ripplocations, in single and multiple van der Waals layers, using analytical and computational techniques. Expressions for the energetic and structural scaling factors of surface ripplocations are derived, which are in close correspondence to the physics of a classical carpet ruck. Our simulations demonstrate that the lowest-energy structure of dislocation pile-ups in layered materials is the ripplocation, while large dislocation pile-ups in bulk graphite demonstrate multilayer delamination, curvature and voids. This can provide a concise explanation for the large volumetric expansion seen in irradiated graphite.Comment: 7 pages, 6 figure

Borosilicate glass potentials for radiation damage simulations

Three borosilicate glass (SiO2-B2O3) fixed charge potentials from the literature are compared (Delaye and Ghaleb, 1996; Kieu et al., 2011; Rushton, 2006) and their suitability for use in simulations of radiation damage is assessed.For a range of densities, we generate glass structures by quenching at 5×1012 K/s using constant volume Molecular Dynamics. In each case, the bond lengths, mean bond angles, bulk modulus, melting point and displacement energy thresholds are calculated, and where possible compared to experimental data. Whereas the bond lengths and mean bond angles are reasonably well predicted, we find that the potentials predict melting temperatures, bulk moduli and densities that are higher than experimental data.The displacement energy thresholds are generally lower than those for ionic crystalline materials, but show a wider spread of values. However, the barriers for atomic rearrangements, after atoms have been displaced in the equilibrium structures, are very high. This indicates, that the radiation damage produced in the ballistic phase of a collision cascade, is likely to persist for extended time scales. This is in contrast to crystals, where interstitials and vacancies can diffuse rapidly between successive radiation events

Inter-atomic potentials for radiation damage studies in CePO4 monazite

An original empirical potential used for modelling phosphate glasses is adapted to be suitable for use with monazite (CePO4) so as to have a consistent formulation for radiation damage studies of phosphates. This is done by adding a parameterisation for the Ce–O interaction to the existing potential set. The thermal and structural properties of the resulting computer model are compared to experimental results. The parameter set gives a stable monazite structure where the volume of the unit cell is almost identical to that measured experimentally, but with some shrinkage in the a and b lengths and a small expansion in the c direction compared to experiment. The thermal expansion, specific heat capacity and estimates of the melting point are also determined. The estimate of the melting temperature of 2500 K is comparable to the experimental value of 2318 ± 20 K, but the simulated thermal expansion of 49 106 K1 is larger than the usually reported value. The simulated specific heat capacity at constant pressure was found to be approximately constant at 657 J kg1 K1 in the range 300–1000 K, however, this is not observed experimentally or in more detailed ab initio calculations

Iron phosphate glasses: structure determination and displacement energy thresholds, using a fixed charge potential model

Iron phosphate glass is a versatile matrix for the immobilisation of various radioactive elements found in high-level nuclear waste (HLW). Quenched glass structures of iron phosphate glasses with Fe/P ratios of 0.33, 0.67 and 0.75 and with a composition of 40 mol% Fe2O3 and 60 mol% P2O5, with 4% and 17% Fe2 + ion concentrations were generated using molecular dynamics and the threshold displacement energies calculated. In the minimum energy structures, we found that in nearly all cases the P atoms were 4-fold coordinated. The potential energy per atom increased with increasing concentration of Fe2 + ions with similar Fe/P ratio, suggesting that decreasing the Fe2 + content is a stabilising factor. The average bond distances between Fe2 +-O, Fe3 +-O, P-O and O-O were calculated as 2.12, 1.88, 1.5 and 2.5 Å respectively. The threshold displacement energy (Ed) was found to be dependent upon the ion specie, less for Fe2 + ions compared to Fe3 + ions, and was overall slightly lower than that determined for borosilicate glass

Ab-initio calculations of fission product diffusion on graphene

A clear understanding of the diffusive behaviour of a wide variety of impurities is essential for the construction and safe operation of the class of nuclear reactors which employ graphite as a shielding material. As a means of gaining insight into this important problem, the bonding, activation energy and structural properties of a variety of the most common nuclear fission products on graphene have been examined using Density Functional Theory (DFT), illustrating the attendant mechanisms of bonding and ionic transport of the different species, as well as the tendency to form nanoscale clusters in bulk graphite. Simulations have been conducted using a variety of approximations to the exchange-correlation functional, and the relative importance of functional choice is discussed in the context of the adsorption and activation energies. Finally, our calculations are compared to the relevant experimental results, allowing us to draw some conclusions about the likely transport mechanisms at larger length and time scales

Near-surface structure and residual stress in as-machined synthetic graphite

We have used optical and electron microscopy and Raman spectroscopy to study the structural changes and residual stress induced by typical industrial machining and laboratory polishing of a synthetic graphite. An abrasion layer of up to 35 nm in thickness formed on both machined and polished surfaces, giving the same ID/IG ratios evidencing graphite crystal refinement from an La of ~110 nm down to an average of 21 nm, but with different residual compression levels. For the as-polished sample, structural change was limited to the near surface region. Underneath the as-machined surface, large pores were filled with crushed material; graphite crystals were split into multi-layered graphene units that were rearranged through kinking. Graphite crystal refinement in the sub-surface region, measured by La, showed an exponential relationship with depth (z) to a depth of 35–40 μm. The positive shift of the G band in the Raman spectrum indicates a residual compression accompanied by refinement with the highest average of ~2.5 GPa on top, followed by an exponential decay inside the refined region; beyond that depth, the compression decreased linearly down to a depth of ~200 μm. Mechanisms for the refinement and residual compression are discussed with the support of atomistic modelling

Sub-monolayer growth of Ag on flat and nanorippled SiO2 surfaces

In-situ Rutherford Backscattering Spectrometry (RBS) and Molecular Dynamics (MD) simulations have been used to investigate the growth dynamics of silver on a flat and the rippled silica surface. The calculated sticking coeficient of silver over a range of incidence angles shows a similar behaviour to the experimental results for an average surface binding energy of a silver adatom of 0.2 eV. This value was used to parameterise the MD model of the cumulative deposition of silver in order to understand the growth mechanisms. Both the model and the RBS results show marginal difference between the atomic concentration of silver on the at and the rippled silica surface, for the same growth conditions. For oblique incidence, cluster growth occurs mainly on the leading edge of the rippled structure

A new potential for radiation studies of borosilicate glass

Borosilicate glass containing 70 mol% SiO2 and 30 mol% B2O3 is investigated theoretically using fixed charge potentials. An existing potential parameterisation for borosilicate glass is found to give good agreement for the bond angle and bond length distributions compared to experimental values but the optimal density is 30% higher than experiment. Therefore the potential parameters are refitted to give an optimal density of 2.1 g=cm3, in line with experiment. To determine the optimal density, a series of random initial structures are quenched at a rate of 5 1012 K/s using constant volume molecular dynamics. An average of 10 such quenches is carried out for each fixed volume. For each quenched structure, the bond angles, bond lengths, mechanical properties and melting points are determined. The new parameterisation is found to give the density, bond angles, bond lengths and Young’s modulus comparable with experimental data, however, the melting points and Poisson’s ratio are higher than the reported experimental values. The displacement energy thresholds are computed to be similar to those determined with the earlier parameterisation, which is lower than those for ionic crystalline materials