Neon diffusion in goethite, α-FeO(OH): a theoretical multi-scale study

International audienceThe neon diffusion behavior in goethite has been investigated for the purpose of geological (U-Th)/Ne dating, as Ne produced in goethite by nucleogenic reactions related to natural U and Th alpha decay can diffuse out of the crystal. According to previous works, a multi-scale computational approach combining Density Functional Theory studies at the atomic scale and Kinetic Monte Carlo simulations at the macroscopic scale has been used to determine Ne diffusion behavior in goethite. Periodic-DFT calculations have been performed to study the structural, electronic, and magnetic properties of goethite, and therefore to identify the Ne insertion sites in pure defect-free goethite as well as in goethite containing iron-aluminum substitution and goethite containing crystallographic defects, to obtain a crystal structure closest to a natural goethite crystal. The Nudged Elastic Band method was used to define the minimum energy pathway for Ne migration, between neighboring interstitial sites. The Climbing Image Nudged Elastic Band method was adopted to obtain more accuracy on the transition state. The 3-dimensional random walk of Ne jumps between interstitial sites was simulated using the Kinetic Monte Carlo method. We found that a Ne atom diffuses in pure defect-free goethite following a zig-zag pathway along the unoccupied channel of goethite, with an effective activation energy of E-a = 0.50 eV and a pre-exponential factor of D-0 = 6.38 x 10(-4) cm(2) s(-1). Moreover, the iron-aluminum substitution induces a small volume contraction of the unoccupied channel, which increases the energy barrier of Ne diffusion to 0.66 eV. Nevertheless, this energy barrier remains insufficient to retain Ne atom in the goethite structure at surface temperature. However, crystallographic defects impact strongly Ne diffusivity in goethite. In the case of a Schottky defect, i.e. a large vacancy, the Ne atom is retained in the new stable site generated by the vacancy. In the case of a hydrated Fe vacancy, steric constraints remain a barrier that inhibit the Ne jumping between two adjacent unoccupied channels

Exploring the excited-states of squaraine dyes with TD-DFT, SOS-CIS(D) and ADC(2)

International audienc

QTAIM Analysis in the Context of Quasirelativistic Quantum Calculations

International audienceComputational chemistry currently lacks ad hoc tools for probing the nature of chemical bonds in heavy and superheavy-atom systems where the consideration of spinorbit coupling (SOC) effects is mandatory. We report an implementation of the Quantum Theory of Atoms-In-Molecules in the framework of two-component relativistic calculations. Used in conjunction with the topological analysis of the Electron Localization Function, we show for astatine (At) species that SOC significantly lowers At electronegativity and boosts its propensity to make charge-shift bonds. Relativistic spin-dependent effects are furthermore able to change some bonds from mainly covalent to charge-shift type. The implication of the disclosed features regarding the rationalization of the labeling protocols used in nuclear medicine for At-211 radioisotope nicely illustrates the potential of the introduced methodology for investigating the chemistry of (super)heavy elements

Revealing the radiation damage and Al-content impacts on He diffusion in goethite

International audienceOver the past few decades, the (U-Th)/He geochronological method on goethite has been more and more applied to date laterite formation and evolution or ore-deposit formation. However, questions remain on possible He loss by diffusion due to the polycrystalline structure of goethite and associated underestimation of the goethite crystallization date given by the (U-Th)/He age. Prior studies estimated helium loss in goethite to range from 2 to 30%, but no relation or models have been produced to explain such values. To clarify the situation, we firstly performed a complete review of experimental He diffusion data in natural goethite, that reveals the link between activation energy and He loss with the damage dose. To understand He diffusion behavior in goethite and model the He loss, natural defect and alpha damage as well as the chemical composition and growth structure effect on He diffusion have been investigated thanks to a multi-scale study. We used numerical simulations combining the Density Functional Theory (DFT) at the atomic scale and Kinetic Monte Carlo (KMC) simulations at the macroscopic scale. We found that He diffusion is purely anisotropic along the preferential elongated axis (i.e., b-axis) and He leaks out easily in defect-free goethite and Al-goethite. The consequence of this anisotropy is that crystallographic defects and alpha damage strongly lower the He diffusivity in goethite and Al-goethite by obstructing the diffusion channel or trapping He along the b-axis. Defect and damage impact on He diffusion is even larger for Al-goethite. The obtained He diffusion parameters for goethite containing defect and damage are similar to the activation energy and He diffusional loss obtained in natural goethite from the literature. We demonstrate the systematic dependence of the diffusion coefficient with damage dose and the impact of Al on He retention. He atoms are retained only at the favor of obstructions blocking the diffusion and vacancies trapping them in the goethite structure. The consequence of the diffusive behavior is that a part of He diffuses out of the crystal until sufficient damage accumulated along the b-axis. The diffusion domain size is the channel length along the b-axis rather than the whole crystallite size. To correct the He loss this study proposes estimation of the He retention and needed corrections for different types of goethite

Etude de la chimie de l'astate à l'aide de la modélisation moléculaire quantique

National audienc

Role of Defects and Radiation Damage on He Diffusion in Magnetite: Implication for (U-Th)/He Thermochronology

International audienceThe discovery of He retentivity in magnetite has opened up the use of the magnetite (U-Th)/He method as a thermochronometer to date the exhumation of mafic and ultramafic rocks, and also as a chronometer to date magnetite crystallization during serpentinization. However, published He diffusion data reveal more complex behavior than expected. To resolve this issue and generalize the understanding of He retention in magnetite, we conducted a multiscale theoretical study. We investigated the impact of natural point-defects (i.e., vacancies unrelated to radiation damage) and defects associated with radiation damage (i.e., vacancies and recoil damage that form amorphous zones) on He diffusion in magnetite. The theoretical results show that He diffusion is purely isotropic, and that defect-free magnetite is more He diffusive than indicated by experimental data on natural specimen. Interestingly, the obtained theoretical trapping energy of vacancies and recoil damage are very similar to those obtained from experimental diffusion data. These results suggest that He diffusion in magnetite is strongly controlled by the presence of vacancies and radiation damage, even at very low damage dose. We propose that, when using magnetite (U-Th)/He thermochronometry, the impact of vacancies and radiation damage on He retention behavior should be integrated.</jats:p

Exploration of the metallic character of astatine

International audienc

QTAIM Analysis in the Context of Quasirelativistic Quantum Calculations

Computational chemistry currently lacks ad hoc tools for probing the nature of chemical bonds in heavy and superheavy-atom systems where the consideration of spin–orbit coupling (SOC) effects is mandatory. We report an implementation of the Quantum Theory of Atoms-In-Molecules in the framework of two-component relativistic calculations. Used in conjunction with the topological analysis of the Electron Localization Function, we show for astatine (At) species that SOC significantly lowers At electronegativity and boosts its propensity to make charge-shift bonds. Relativistic spin-dependent effects are furthermore able to change some bonds from mainly covalent to charge-shift type. The implication of the disclosed features regarding the rationalization of the labeling protocols used in nuclear medicine for ²¹¹At radioisotope nicely illustrates the potential of the introduced methodology for investigating the chemistry of (super)­heavy elements

211 At-labeled agents for alpha-immunotherapy: On the in vivo stability of astatine-agent bonds

International audienceThe application of 211 At to targeted cancer therapy is currently hindered by the rapid deastatination that occurs in vivo. As the deastatination mechanism is unknown, we tackled this issue from the viewpoint of the intrinsic properties of At-involving chemical bonds. An apparent correlation has been evidenced between in vivo stability of 211 At-labeled compounds and the AtÀR (R ¼ C, B) bond enthalpies obtained from relativistic quantum mechanical calculations. Furthermore, we highlight important differences in the nature of the AtÀC and AtÀB bonds of interest, e.g. the opposite signs of the effective astatine charges, which implies different stabilities with respect to the biological medium. Beyond their practical use for rationalizing the labeling protocols used for 211 At, the proposed computational approach can readily be used to investigate bioactive molecules labeled with other heavy radionuclides

Questioning the Affinity of Electrophilic Astatine for Sulfur-containing Compounds: Unexpected Bindings Revealed

International audienc