Long-term confinement of nuclear waste is one of the main challenges faced by the nuclear industry. Fission products such as Sr-90 and Cs-137, both beta(-) emitters known to induce serious health hazards, represent the largest fraction of nuclear waste. Cement is a good candidate to store them, provided it can resist the effects of irradiation over time. Here, we have investigated the effects of beta(-) decay on cement by performing electron irradiation experiments on different samples. We show that H-2 production in cement, the main effect of water radiolysis, depends strongly on composition and relative humidity. First-principles calculations indicate that the water-rich interlayer regions with Ca2+ ions act as electron traps that promote the formation of H-2. They also show that holes localize in water-rich regions in low Ca content samples and are then able to participate in H-2 production. This work provides new understanding of radiolysis effects in cements.

VL - 100 UR - https://linkinghub.elsevier.com/retrieve/pii/S0008884617302065 JO - Cement and Concrete Research ER - TY - JOUR T1 - Cement As a Waste Form for Nuclear Fission Products: The Case of 90Sr and Its Daughters JF - Environmental Science & Technology Y1 - 2016 A1 - Dezerald, Lucile A1 - Kohanoff, Jorge J. A1 - Alfredo A. Correa A1 - Caro, Alfredo A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm A1 - Andres Saùl AB -One of the main challenges faced by the nuclear industry is the long-term confinement of nuclear waste. Because it is inexpensive and easy to manufacture, cement is the material of choice to store large volumes of radioactive materials, in particular the low-level medium-lived fission products. It is therefore of utmost importance to assess the chemical and structural stability of cement containing radioactive species. Here, we use ab initio calculations based on density functional theory (DFT) to study the effects of Sr-90 insertion and decay in C-S-H (calcium-silicate-hydrate) in order to test the ability of cement to trap and hold this radioactive fission product and to investigate the consequences of its beta-decay on the cement paste structure. We show that Sr-90 is stable when it substitutes the Ca2+ ions in C-S-H, and so is its daughter nucleus Y-90 after beta-decay. Interestingly, Zr-90, daughter of Y-90 and final product in the decay sequence, is found to be unstable compared to the bulk phase of the element at zero K but stable when compared to the solvated ion in water. Therefore, cement appears as a suitable waste form for Sr-90 storage.

VL - 49 UR - http://pubs.acs.org/doi/10.1021/acs.est.5b02609 IS - 22 JO - Environ. Sci. Technol. ER - TY - JOUR T1 - Cement As a Waste Form for Nuclear Fission Products: The Case of 90 Sr and Its Daughters JF - Environ Sci Technol Y1 - 2015 A1 - Dezerald, Lucile A1 - Kohanoff, Jorge J A1 - Alfredo A. Correa A1 - Caro, Alfredo A1 - Roland Jean-Marc Pellenq A1 - Franz-Josef Ulm A1 - Andres Saùl AB -One of the main challenges faced by the nuclear industry is the long-term confinement of nuclear waste. Because it is inexpensive and easy to manufacture, cement is the material of choice to store large volumes of radioactive materials, in particular the low-level medium-lived fission products. It is therefore of utmost importance to assess the chemical and structural stability of cement containing radioactive species. Here, we use ab initio calculations based on density functional theory (DFT) to study the effects of (90)Sr insertion and decay in C-S-H (calcium-silicate-hydrate) in order to test the ability of cement to trap and hold this radioactive fission product and to investigate the consequences of its β-decay on the cement paste structure. We show that (90)Sr is stable when it substitutes the Ca(2+) ions in C-S-H, and so is its daughter nucleus (90)Y after β-decay. Interestingly, (90)Zr, daughter of (90)Y and final product in the decay sequence, is found to be unstable compared to the bulk phase of the element at zero K but stable when compared to the solvated ion in water. Therefore, cement appears as a suitable waste form for (90)Sr storage.

VL - 49 IS - 22 ER - TY - JOUR T1 - Magnetic nanopantograph in the SrCu

Magnetic materials having competing, i.e., frustrated, interactions can display magnetism prolific in intricate structures, discrete jumps, plateaus, and exotic spin states with increasing applied magnetic fields. When the associated elastic energy cost is not too expensive, this high potential can be enhanced by the existence of an omnipresent magnetoelastic coupling. Here we report experimental and theoretical evidence of a nonnegligible magnetoelastic coupling in one of these fascinating materials, SrCu2(BO3)(2) (SCBO). First, using pulsed-field transversal and longitudinal magnetostriction measurements we show that its physical dimensions, indeed, mimic closely its unusually rich field-induced magnetism. Second, using density functional-based calculations we find that the driving force behind the magnetoelastic coupling is the CuOCu superexchange angle that, due to the orthogonal Cu2+ dimers acting as pantographs, can shrink significantly (0.44%) with minute (0.01%) variations in the lattice parameters. With this original approach we also find a reduction of similar to 10% in the intradimer exchange integral J, enough to make predictions for the highly magnetized states and the effects of applied pressure on SCBO.

VL - 112 UR - http://www.pnas.org/lookup/doi/10.1073/pnas.1421414112 IS - 7 JO - Proc Natl Acad Sci USA ER - TY - JOUR T1 - Magnetic nanopantograph in the SrCu2(BO3)2 Shastry–Sutherland lattice JF - Proceedings of the National Academy of Sciences Y1 - 2015 A1 - Guillaume Radtke A1 - Andres Saùl A1 - Dabkowska, Hanna A. A1 - Salamon, Myron B. A1 - Jaime, Marcelo AB -Magnetic materials having competing, i.e., frustrated, interactions can display magnetism prolific in intricate structures, discrete jumps, plateaus, and exotic spin states with increasing applied magnetic fields. When the associated elastic energy cost is not too expensive, this high potential can be enhanced by the existence of an omnipresent magnetoelastic coupling. Here we report experimental and theoretical evidence of a nonnegligible magnetoelastic coupling in one of these fascinating materials, SrCu2(BO3)2 (SCBO). First, using pulsed-field transversal and longitudinal magnetostriction measurements we show that its physical dimensions, indeed, mimic closely its unusually rich field-induced magnetism. Second, using density functional-based calculations we find that the driving force behind the magnetoelastic coupling is the CuOCu^ superexchange angle that, due to the orthogonal Cu2+ dimers acting as pantographs, can shrink significantly (0.44%) with minute (0.01%) variations in the lattice parameters. With this original approach we also find a reduction of ∼10% in the intradimer exchange integral J, enough to make predictions for the highly magnetized states and the effects of applied pressure on SCBO.

VL - 112 ER - TY - JOUR T1 - Density functional approach for the magnetism of JF - Physical Review B Y1 - 2014 A1 - Andres Saùl A1 - Guillaume Radtke AB -Density functional calculations have been carried out to investigate the microscopic origin of the magnetic properties of β-TeVO4. Two different approaches, based either on a perturbative treatment of the multiorbital Hubbard model in the strongly correlated limit or on the calculation of supercell total energy differences, have been employed to evaluate magnetic couplings in this compound. The picture provided by these two approaches is that of weakly coupled frustrated chains with ferromagnetic nearest-neighbor and antiferromagnetic second-nearest-neighbor couplings. These results, differing substantially from previous reports, should motivate further experimental investigations of the magnetic properties of this compound.

VL - 89 UR - https://link.aps.org/doi/10.1103/PhysRevB.89.104414 IS - 10 JO - Phys. Rev. B ER - TY - JOUR T1 - A density functional approach of the magnetism of b-TeVO4 JF - Phys. Rev. B. Y1 - 2014 A1 - Andres Saùl A1 - Guillaume Radtke AB -Density functional calculations have been carried out to investigate the microscopic origin of the magnetic properties of

β-TeVO4. Two different approaches, based either on a perturbative treatment of the multiorbital Hubbard model in the strongly correlated limit or on the calculation of supercell total energy differences, have been employed to evaluate magnetic couplings in this compound. The picture provided by these two approaches is that of weakly coupled frustrated chains with ferromagnetic nearest-neighbor and antiferromagnetic second-nearest-neighbor couplings. These results, differing substantially from previous reports, should motivate further experimental investigations of the magnetic properties of this compound.

In this work we present a systematic study of the magnetic interactions within 3d transition-metal chains adsorbed on Cu2N and Cu2O monolayers grown on Cu(001). We are interested in the particular geometric adsorption configuration which gives rise, after relaxation, to the development of diatomic TM-X (X = N, O) chains. By using density functional theory (DFT), we calculate the energy difference between the ferromagnetic and antiferromagnetic intrachain configurations for Ti, V, Cr, Mn, Fe, and Co. Both substrates give rise, with minor differences, to the same magnetic trends, the only chains which are ferromagnetic after adsorption are Cr chains. By performing similar calculations in unsupported chains and introducing a tight-binding-model Hamiltonian based on physically reasonable assumptions we reproduce the magnetic trends obtained from the DFT calculations.

VL - 90 ER - TY - JOUR T1 - Molecular dynamics simulations of the formation of 1D spin-valves from stretched Au-Co and Pt-Co nanowires JF - J. Phys.: Condens. Matter Y1 - 2014 A1 - Robinson Cortes-Huerto A1 - T. Sondon A1 - Andres Saùl AB -We have performed molecular dynamics (MD) simulations of stretched Aux-Co1 - x and Ptx-Co1 - x nanowires to investigate the formation of bimetallic monoatomic wires between two electrodes. We have considered nanowires with two concentrations x = 0.2 and 0.8, aspect ratio of 13, a cross section of 1 nm(2) and a wide range of temperatures (from 10 to 400 K). For the MD simulations we have used a semi-empirical interatomic potential based on the second moment approximation (SMA) of the density of states to the tight-binding Hamiltonian.For Au-Co alloys, Au atoms tends to migrate towards the narrowed region to form almost pure Au wires. In the PtCo case the formed chains usually consist of Pt enriched alternating structures. The most striking result is probably the Au(0.2)-Co(0.8) alloy where pure monoatomic Au chains form between two Co electrodes constituting a potential 1D spin valve. Despite the known ease with which the 5d metals (Pt, Ir, and Au) form monoatomic chains (MACS), our results show that in the presence of Co (x = 0.2), the percentage of chain formation is higher than in the Pt and Au rich cases (x = 0.8).

VL - 26 IS - Special issue: Break junctions ER - TY - JOUR T1 - Multiphase equation of state for carbon addressing high pressures and temperatures JF - Phys Rev. B Y1 - 2014 A1 - L. X. Benedict A1 - K. P. Driver A1 - S. Hamel A1 - B. Militzer A1 - T. Qi A1 - Alfredo A. Correa A1 - Andres Saùl A1 - E. Schwegler AB -We present a 5-phase equation of state for elemental carbon which addresses a wide range of density and temperature conditions: 3g/cc<ρ<20g/cc,0K<T<∞. The phases considered are diamond, BC8, simple cubic, simple hexagonal, and the liquid/plasma state. The solid phase free energies are constrained by density functional theory (DFT) calculations. Vibrational contributions to the free energy of each solid phase are treated within the quasiharmonic framework. The liquid free energy model is constrained by fitting to a combination of DFT molecular dynamics performed over the range 10000K<T<100000K, and path integral quantum Monte Carlo calculations for T>100000K (both for ρ between 3 and 12 g/cc, with select higher- ρ DFT calculations as well). The liquid free energy model includes an atom-in-jellium approach to account for the effects of ionization due to temperature and pressure in the plasma state, and an ion-thermal model which includes the approach to the ideal gas limit. The precise manner in which the ideal gas limit is reached is greatly constrained by both the highest-temperature DFT data and the path integral data, forcing us to discard an ion-thermal model we had used previously in favor of a new one. Predictions are made for the principal Hugoniot and the room-temperature isotherm, and comparisons are made to recent experimental results.

VL - 89 ER - TY - JOUR T1 - Role of temperature in the formation and growth of gold monoatomic chains: A molecular dynamics study JF - Physical Review B Y1 - 2013 A1 - Robinson Cortes-Huerto A1 - T. Sondon A1 - Andres Saùl AB -The effect of temperature on the formation and growth of monoatomic chains is investigated by extensive molecular dynamics simulations using a semiempirical potential based on the second-moment approximation to the tight-binding Hamiltonian. Gold nanowires, with an aspect ratio of

Some aspects of the thermodynamics and mechanics of solid surfaces, in particular with respect to surface stress and surface energy, are reviewed. The purpose is to enlighten the deep differences between these two physical quantities. We consider successively the case of atomic flat surfaces and the case of vicinal surfaces characterized by surface stress discontinuities. Finally, experimental examples, concerning Si surfaces, are described.

VL - 5 ER - TY - JOUR T1 - Structure and properties of nanoscale materials: theory and atomistic computer simulation JF - International Journal of Nanotechnology Y1 - 2012 A1 - Christophe Bichara A1 - Marsal, P A1 - Mottet, C A1 - Roland Jean-Marc Pellenq A1 - F. Ribeiro A1 - Andres Saùl A1 - G. Tréglia A1 - Weissker, H–Ch AB -We present a review of a few research topics developed within the "Theory and Atomistic Computer Simulation" Department at CINaM. The bottom line of the scientific activity is to use up–to–date theoretical and computer simulation techniques to address physics and materials science problems, often at the nanometric scale, in close contact with experimental groups. It ranges from the study of the structure and properties of molecular systems for organic electronics to metallic clusters and alloys, magnetic oxides, nuclear fuels and carbon–based nanostructures. These studies are motivated by fundamental research questions as well as more applied goals including environmental and energy issues, or information technologies. This broad spectrum of activities requires a large range of techniques, from theory and ab initio calculations to semi–empirical models incorporated in Monte Carlo or molecular dynamics simulations.

VL - 9 IS - 3-7 ER -