Michel Sassi

Capturing ultrafast photoinduced local structural distortions of BiFeO3.

The interaction of light with materials is an intensively studied research forefront, in which the coupling of radiation energy to selective degrees of freedom offers contact-free tuning of functionalities on ultrafast time scales. Capturing the fundamental processes and understanding the mechanism of photoinduced structural rearrangement are essential to applications such as photo-active actuators and efficient photovoltaic devices. Using ultrafast x-ray absorption spectroscopy aided by density functional theory calculations, we reveal the local structural arrangement around the transition metal atom in a unit cell of the photoferroelectric archetype BiFeO3 film. The out-of-plane elongation of the unit cell is accompanied by the in-plane shrinkage with minimal change of interaxial lattice angles upon photoexcitation. This anisotropic elastic deformation of the unit cell is driven by localized electric field as a result of photoinduced charge separation, in contrast to a global lattice constant increase and lattice angle variations as a result of heating. The finding of a photoinduced elastic unit cell deformation elucidates a microscopic picture of photocarrier-mediated non-equilibrium processes in polar materials.

RADIATION-DAMAGE RESISTANCE IN PHYLLOSILICATE MINERALS FROM FIRST PRINCIPLES AND IMPLICATIONS FOR RADIOCESIUM AND STRONTIUM RETENTION IN SOILS

Accidental discharges of the hazardous nuclear fission products 137Cs+ and 90Sr2+ into the environment, such as during the Fukushima Dai-ichi nuclear accident, have occurred repeatedly throughout the ‘nuclear age.’ Numerous studies of the fate and transport of 137Cs+ and 90Sr2+ in soils and sediments have demonstrated their strong and selective binding to phyllosilicate clay minerals, primarily by means of cation exchange into interlayer sites. The locally concentrated amounts of these radioactive beta-emitters that can be found in these host minerals raise important questions regarding the long-term interplay and durability of radioisotope—clay associations, which is not well known. The present study goes beyond the usual short-term focus to address the permanence of radioisotope retention in clay minerals, by developing a general theoretical understanding of their resistance to the creation of defects. The present study reports ab initio molecular dynamics (AIMD) calculations of the threshold displacement energy (TDE) of each symmetry-unique atomic species comprising the unit cell of model vermiculite. The TDE values determined are material specific, radiation independent, and can be used to estimate the probability of Frenkel-pair creation by direct electron—ion collision, as could be induced by the passage of a high-energy electron emitted during the beta-decay of 137Cs, 90Sr, and daughter 90Y. For 137Cs and 90Sr, the calculated probability is ~36%, while for 90Y the probability is much greater at ~89%. The long-term retention picture that emerges is that decay will progressively alter the clay interlayer structure and charge, probably leading to delamination of the clay, and re-release of residual parent isotopes. Further work examining the effect of Frenkel defect accumulation on the binding energy of parent and daughter radionuclides in the interlayer is thus justified and potentially important for accurate long-term forecasting of radionuclide transport in the environment.

Origin of 6-fold coordinated aluminum at (010)-type pyrophyllite edges

To better understand the aqueous chemical reactivity of clay mineral edges we explored the relationships between hydration and the structure of (010)-type edges of pyrophyllite. In particular, we used density functional theory and the quantum theory of atoms in molecules to evaluate the stability of 6-fold coordinated hydrated aluminum at the edge in terms of the electron density distribution. Geometrical optimization revealed an intra-edge hydrogen bond network between aluminol hydroxyls and water ligands completing the aluminum coordination shell. From the electron density isosurfaces one water ligand is not covalently bonded to aluminum. Bader charge analysis revealed that OH2 ligands have small negative charge. In addition, it is also found that the charge of the 6-fold coordinated aluminum is larger than one of the 5-fold aluminum. From these results, the charging of the OH2 ligands is interpreted as charge transfer originated from the formation of the hydrogen bond network and not from Al-OH2 intera...

Consequences of realistic embedding for the L2,3 edge XAS of α-Fe2O3

Cluster models of condensed systems are often used to simulate the core-level spectra obtained with X-ray Photoelectron Spectroscopy, XPS, or with X-ray Absorption Spectroscopy, XAS, especially for near edge features. The main objective of this paper is to examine the dependence of the predicted L2,3 edge XAS of α-Fe2O3, an example of a high spin ionic crystal, on increasingly realistic models of the condensed system. It is shown that an FeO6 cluster model possessing the appropriate local site symmetry describes most features of the XAS and is a major improvement over the isolated Fe3+ cation. In contrast, replacing next nearest neighbor positive point charges with Sc3+, a closed shell cation of similar spatial extent to Fe3+, only marginally improves the match to experiment. This work suggests that second nearest neighbor effects are negligible. Rather, major improvements to the predicted L2,3 edge XAS likely requires additional many body effects that go beyond the present study in which the multiplets are restricted to arise from angular momentum coupling within a single open shell configuration.

First-Principles Fe L2,3-Edge and O K-Edge XANES and XMCD Spectra for Iron Oxides

X-ray absorption near-edge structure (XANES) and X-ray magnetic circular dichroism (XMCD) spectroscopies are tools in widespread use for providing detailed local atomic structure, oxidation state, and magnetic structure information for materials and organometallic complexes. The analysis of these spectra for transition-metal L-edges is routinely performed on the basis of ligand-field multiplet theory because one- and two-particle mean-field ab initio methods typically cannot describe the multiplet structure. Here we show that multireference configuration interaction (MRCI) calculations can satisfactorily reproduce measured XANES spectra for a range of complex iron oxide materials including hematite and magnetite. MRCI Fe L2,3-edge XANES and XMCD spectra of Fe(II)O6, Fe(III)O6, and Fe(III)O4 in magnetite are found to be in very good qualitative agreement with experiment and multiplet calculations. Point-charge embedding and small distortions of the first-shell oxygen ligands have only small effects. Oxygen K-edge XANES/XMCD spectra for magnetite investigated by a real-space Greens function approach complete the very good qualitative agreement with experiment. Material-specific differences in local coordination and site symmetry are well reproduced, making the approach useful for assigning spectral features to specific oxidation states and coordination environments.

Analysis of X-ray adsorption edges: L2,3 edge of FeCl4−

We describe a detailed analysis of the features of the X-ray adsorption spectra at the Fe L2,3 edge of FeCl4-. The objective of this analysis is to explain the origin of the complex features in relation to properties of the wavefunctions, especially for the excited states. These properties include spin-orbit and ligand field splittings where a novel aspect of the dipole selection rules is applied to understand the influence of these splittings on the spectra. We also explicitly take account of the intermediate coupling of the open core and valence shell electrons. Our analysis also includes comparison of theory and experiment for the Fe L2,3 edge and comparison of theoretical predictions for the Fe3+ cation and FeCl4-. The electronic structure is obtained from theoretical wavefunctions for the ground and excited states.

X-ray Linear Dichroism in Apatite

The recent observation in parrotfish teeth of X-ray linear dichroism motivated an in-depth investigation into this spectroscopic effect in various apatite crystals, including geologic hydroxyapatite (Ca5(PO4)3OH), fluorapatite (Ca5(PO4)3F), and their biogenic counterparts in human bone, mouse enamel, and in parrotfish bone, dentin, and enameloid, the equivalent of dental enamel in certain fish. These data are important because they now enable visualization of the nano- to microscale structure of apatite crystals in teeth and bone. Polarization-dependent imaging contrast (PIC) maps of lamellar bone, obtained with a new method that minimizes space-charge and charging effects, show the expected rotating apatite crystal orientations. PIC maps of mouse enamel reveal a complex arrangement of hydroxyapatite crystals perpendicular to the dentin-enamel junction, with rods arranged in a decussation pattern in inner enamel and nearly parallel to one another in outer enamel. In both inner and outer enamel crystal c-axes are not always aligned with the rod elongation direction.

Radiocesium interaction with clay minerals: Theory and simulation advances Post–Fukushima

Insights at the microscopic level of the process of radiocesium adsorption and interaction with clay mineral particles have improved substantially over the past several years, triggered by pressing social issues such as management of huge amounts of waste soil accumulated after the Fukushima Dai-ichi nuclear power plant accident. In particular, computer-based molecular modeling supported by advanced hardware and algorithms has proven to be a powerful approach. Its application can now generally encompass the full complexity of clay particle adsorption sites from basal surfaces to interlayers with inserted water molecules, to edges including fresh and weathered frayed ones. On the other hand, its methodological schemes are now varied from traditional force-field molecular dynamics on large-scale realizations composed of many thousands of atoms including water molecules to first-principles methods on smaller models in rather exacting fashion. In this article, we overview new understanding enabled by simulations across methodological variations, focusing on recent insights that connect with experimental observations, namely: 1) the energy scale for cesium adsorption on the basal surface, 2) progress in understanding the structure of clay edges, which is difficult to probe experimentally, 3) cesium adsorption properties at hydrated interlayer sites, 4) the importance of the size relationship between the ionic radius of cesium and the interlayer distance at frayed edge sites, 5) the migration of cesium into deep interlayer sites, and 6) the effects of nuclear decay of radiocesium. Key experimental observations that motivate these simulation advances are also summarized. Furthermore, some directions toward future solutions of waste soil management are discussed based on the obtained microscopic insights.

The Role of Defects in Fe(II)–Goethite Electron Transfer

Despite substantial experimental evidence for Fe(II)-Fe(III) oxide electron transfer, computational chemistry calculations suggest that oxidation of sorbed Fe(II) by goethite is kinetically inhibited on structurally perfect surfaces. We used a combination of 57Fe Mössbauer spectroscopy, synchrotron X-ray absorption and magnetic circular dichroism (XAS/XMCD) spectroscopies to investigate whether Fe(II)-goethite electron transfer is influenced by defects. Specifically, Fe L-edge and O K-edge XAS indicates that the outermost few Angstroms of goethite synthesized by low temperature Fe(III) hydrolysis is iron deficient relative to oxygen, suggesting the presence of defects from Fe vacancies. This nonstoichiometric goethite undergoes facile Fe(II)-Fe(III) oxide electron transfer, depositing additional goethite consistent with experimental precedent. Hydrothermal treatment of this goethite, however, appears to remove defects, decrease the amount of Fe(II) oxidation, and change the composition of the oxidation product. When hydrothermally treated goethite was ground, surface defect characteristics as well as the extent of electron transfer were largely restored. Our findings suggest that surface defects play a commanding role in Fe(II)-goethite redox interaction, as predicted by computational chemistry. Moreover, it suggests that, in the environment, the extent of this interaction will vary depending on diagenetic history, local redox conditions, as well as being subject to regeneration via seasonal fluctuations.