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.

}, issn = {00088846}, doi = {10.1016/j.cemconres.2017.05.022}, url = {https://linkinghub.elsevier.com/retrieve/pii/S0008884617302065}, author = {Le Caer, Sophie and Dezerald, Lucile and Boukari, Khaoula and Laine, Maxime and Taupin, s{\'e}bastien and Kavanagh, Ryan M. and Johnston, Conrad S.N. and Foy, Eddy and Charpentier, Thibault and Konrad J. Krakowiak and Roland Jean-Marc Pellenq and Franz-Josef Ulm and Tribello, Gareth A. and Kohanoff, Jorge J. and Andres Sa{\`u}l} } @article {643, title = {Cement As a Waste Form for Nuclear Fission Products: The Case of 90Sr and Its Daughters}, journal = {Environmental Science \& Technology}, volume = {49}, year = {2016}, month = {Nov-17-2016}, pages = {13676-13683}, abstract = {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.

}, issn = {0013-936X}, doi = {10.1021/acs.est.5b02609}, url = {http://pubs.acs.org/doi/10.1021/acs.est.5b02609}, author = {Dezerald, Lucile and Kohanoff, Jorge J. and Alfredo A. Correa and Caro, Alfredo and Roland Jean-Marc Pellenq and Franz-Josef Ulm and Andres Sa{\`u}l} } @article {133, title = {Cement As a Waste Form for Nuclear Fission Products: The Case of 90 Sr and Its Daughters}, journal = {Environ Sci Technol}, volume = {49}, year = {2015}, month = {Oct-2015}, pages = {13676-83}, abstract = {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.

\

}, issn = {1520-5851}, doi = {10.1021/acs.est.5b02609}, author = {Dezerald, Lucile and Kohanoff, Jorge J and Alfredo A. Correa and Caro, Alfredo and Roland Jean-Marc Pellenq and Franz-Josef Ulm and Andres Sa{\`u}l} } @article {645, title = {Magnetic nanopantograph in the SrCuMagnetic 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.

}, issn = {0027-8424}, doi = {10.1073/pnas.1421414112}, url = {http://www.pnas.org/lookup/doi/10.1073/pnas.1421414112}, author = {Guillaume Radtke and Andres Sa{\`u}l and Dabkowska, Hanna A. and Salamon, Myron B. and Jaime, Marcelo} } @article {117, title = {Magnetic nanopantograph in the SrCu2(BO3)2 Shastry{\textendash}Sutherland lattice}, journal = {Proceedings of the National Academy of Sciences}, volume = {112}, year = {2015}, month = {Feb-02-2015}, pages = {1971-1976}, abstract = {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.

}, doi = {10.1073/pnas.1421414112}, author = {Guillaume Radtke and Andres Sa{\`u}l and Dabkowska, Hanna A. and Salamon, Myron B. and Jaime, Marcelo} } @article {651, title = {Density functional approach for the magnetism of}, journal = {Physical Review B}, volume = {89}, year = {2014}, month = {Mar-01-2014}, abstract = {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.

}, issn = {1098-0121}, doi = {10.1103/PhysRevB.89.104414}, url = {https://link.aps.org/doi/10.1103/PhysRevB.89.104414}, author = {Andres Sa{\`u}l and Guillaume Radtke} } @article {62, title = {A density functional approach of the magnetism of b-TeVO4}, journal = {Phys. Rev. B.}, volume = {89}, year = {2014}, month = {Mar-2014}, chapter = {104414}, abstract = {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.

}, issn = {1098-0121}, doi = {10.1103/PhysRevB.90.195423}, author = {Urdaniz, M. C. and Barral, M. A. and Llois, A. M. and Andres Sa{\`u}l} } @article {69, title = {Molecular dynamics simulations of the formation of 1D spin-valves from stretched Au-Co and Pt-Co nanowires}, journal = {J. Phys.: Condens. Matter}, volume = {26}, year = {2014}, month = {Oct-2014}, chapter = {474206}, abstract = {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).

}, doi = {10.1088/0953-8984/26/47/474206}, author = {Robinson Cortes-Huerto and T. Sondon and Andres Sa{\`u}l} } @article {70, title = {Multiphase equation of state for carbon addressing high pressures and temperatures}, journal = {Phys Rev. B}, volume = {89}, year = {2014}, month = {Jun-2014}, chapter = {224109}, abstract = {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.

}, doi = {10.1103/PhysRevB.89.224109}, author = {L. X. Benedict and K. P. Driver and S. Hamel and B. Militzer and T. Qi and Alfredo A. Correa and Andres Sa{\`u}l and E. Schwegler} } @article {61, title = {Role of temperature in the formation and growth of gold monoatomic chains: A molecular dynamics study}, journal = {Physical Review B}, volume = {88}, year = {2013}, month = {Dec-31-2013}, pages = {235438}, abstract = {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.

}, doi = {10.1088/2043-6262/5/1/013002}, author = {M{\"u}ller, Pierre and Andres Sa{\`u}l and Leroy, Fr{\'e}d{\'e}ric} } @article {58, title = {Structure and properties of nanoscale materials: theory and atomistic computer simulation}, journal = {International Journal of Nanotechnology}, volume = {9}, year = {2012}, month = {2012}, pages = {576{\textendash}604}, abstract = {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.

}, doi = {10.1504/IJNT.2012.045335 }, author = {Christophe Bichara and Marsal, P and Mottet, C and Roland Jean-Marc Pellenq and F. Ribeiro and Andres Sa{\`u}l and G. Tr{\'e}glia and Weissker, H{\textendash}Ch} }