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Large-offset detachment faults are common at slow-spreading mid-ocean ridges (MORs). They are typically thought to form in ridge portions that receive a moderate supply of magma. However, they are also found along certain sections of ultraslow-spreading MORs that are largely amagmatic, and feature unusually cold and thick (>15 km) brittle lithosphere. Here we combine geological observations and numerical simulations to assess how these unusual conditions enable and modulate the growth of detachments.We simulate amagmatic seafloor spreading using 2-D thermo-mechanical models with self-consistent thermal evolution. The brittle lithosphere is modeled as a Mohr-Coulomb elasto-plastic material whose friction decreases with accumulated plastic strain. Ductile deformation is parameterized through experimentally-derived olivine flow laws.We first investigate how the strength contrast between the fault zone and surrounding lithosphere affects tectonic styles. Geological observations suggest fault zones have lower effective friction coefficients due to serpentinization and fluid circulation. Evidence for grain size reduction in ultramafic rocks also suggests additional ductile weakening. In our simulations, varying the strength contrast between faults and lithosphere leads to 3 regimes: (1) a stable detachment that migrates toward its hanging wall; (2) the sequential growth of horsts bound by two active antithetic faults; and (3) “flip-flopping” detachments that cross-cut each other, comparable to those documented in the natural case. A greater contrast in friction and/or cohesion favors the stable detachment mode, which is consistent with previous studies.We next focus on the specific effect of a strong, viscous lower lithosphere on brittle deformation in the upper lithosphere. We do so by comparing simulations that use dry olivine flow laws for rocks hotter than ~700ºC with models in which the brittle lithosphere sharply transitions into a low-viscosity asthenosphere. We find that a strong lower lithosphere favors more distributed faulting and shifts the transition to the stable detachment regime to greater strength contrasts.We also investigate the impact of pervasive fluid circulation in the shallow axial lithosphere, which manifests as active hydrothermal sites. We parameterize its mechanical and thermal effect, i.e., reducing the effective normal stress through a hydrostatic fluid pressure and efficiently cooling young lithosphere. While the latter strongly modulates the depth to the brittle-ductile transition, we find that the former has small effect on tectonic styles, akin to a slight weakening of unfaulted lithosphere.Finally, extensive mass wasting is also documented at mid-ocean ridge detachments, but its potential effect on tectonics remains poorly known. We implement diffusive erosion of the model's free surface, which promotes a transition from the stable to flip-flopping detachment regime. This is possibly due to a modulation of topographic stresses.Overall, because of the delocalizing effect of a strong ductile lithosphere, the growth of detachments at cold, amagmatic MOR sections requires some degree of rheological weakening, both in the brittle and ductile domains. We find, however, that even moderate frictional weakening (e.g., a friction coefficient of 0.4) which can be attributed to serpentinization of the fault zone, can be sufficient to promote large-offset faulting, a process that may be aided by mass redistribution at the seafloor.

The reaction of the K4[{Re6Si8}(OH)a6]·8H2O rhenium cluster salt with pyrazine (Pz) in aqueous solutions of alkaline or alkaline earth salts at 4 °C or at room temperature leads to apical ligand exchange and to the formation of five new compounds: [trans-{Re6Si8}(Pz)a2(OH)a2(H2O)a2] (1), [cis-{Re6Si8}(Pz)a2(OH)a2(H2O)a2] (2), (NO3)[cis-{Re6Si8}(Pz)a2(OH)a(H2O)a3](Pz)·3H2O (3), [Mg(H2O)6]0.5[cis-{Re6Si8}(Pz)a2(OH)a3(H2O)a]·8.5H2O (4), and K[cis-{Re6Si8}(Pz)a2(OH)a3(H2O)a]·8H2O (5). Their crystal structures are built up from trans- or cis-[{Re6Si8}(Pz)a2(OH)a4−x(H2O)ax]x−2 cluster units. The cohesions of the 3D supramolecular frameworks are based on stacking and H bonding, as well as on H3O2−bridges in the cases of (1), (2), (4), and (5) compounds, while (3) is built from stacking and H bonding only. This evidences that the nature of the synthons governing the cluster unit assembly is dependent on the hydration rate of the unit.

Tectonic plate boundaries are shaped by localized and distributed brittle-plastic deformation such as slip on major faults and folding of fault-bounded blocks. However, the microstructural to outcrop-scale mechanisms that enable such deformation and the factors that control the onset of localization remain a matter of debate. Here we combine field-based strain measurements and numerical modeling of a half-graben to investigate patterns of distributed inelastic strain induced by footwall-flexure of the upper crust. We focus on the Sandia Mountains (New Mexico, USA), which have marked the eastern edge of the Rio Grande rift's middle section for the last ~10–25 Myrs. This half-graben is uniquely suited for our study: it consists of a layer of Pennsylvanian limestone which experienced little deformation prior to Cenozoic rifting and lies uncomformably above Proterozoic granite. Furthermore, most of the present-day topography and up-warping of the limestone can be attributed to slip on the Sandia fault system and is well modeled as the deflection of an anomalously weak elastic upper crust. The Sandia limestone thus constitutes a unique record of distributed brittle strain related to inelastic shoulder flexure.In the field, deformation within the up-warped footwall-block primarily manifests as small faults (To test this idea, we designed 2-D numerical simulations of half-graben growth using a visco-elasto-plastic rheology coupled with plastic strain softening to enable spontaneous fault localization. Our models predict spatial patterns of distributed inelastic strain within the footwall block that are consistent with our field-based fracture intensity profiles. We find that the strain softening rate is a key control on (1) the distribution of footwall inelastic strain and (2) whether distributed strain can localize onto a new crustal fault. This enables us to constrain values of weakening rate (~100 MPa/strain) that reproduce the observed pattern of distributed cracking while allowing prolonged slip on a single master fault. Our results demonstrate that numerical geodynamic simulations can be benchmarked against microstructural observations to quantify the strain localization properties of the lithosphere. They also suggest that the low effective rigidity of warped crust stems from the growth and interaction of tensile defects on a range of spatial scales, as is commonly observed in rock deformation experiments.

The reaction of a K4[\Re6Si8\(OH)a6]·8H2O rhenium cluster salt with 4,4'-bipyridine (bpy) or 4-aminopyridine (apy) in an aq. soln. at mild temp. and pressure conditions lead to cluster functionalization and to the formation of two new compds. [\Re6Si8\(OH)a2(bpy)a4]·bpy·4H2O (1) and [\Re6Si8\(OH)a2(apy)a4]·2apy·2H2O (2). Their crystal structures are built up from trans-[\Re6Si8\(OH)a2(L)a4] neutral units and the cohesions of both supramol. frameworks are based on π-π stacking as well as hydrogen bonds, involving the cluster units trans-[\Re6Si8\(OH)a2(L)a4], interstitial L ligands and water mols. 1 and 2 present 2D and 3D cluster frameworks, resp. Both compds. show red-NIR luminescence in the solid states. DFT calcns. demonstrate that assocn. of face capped Re6 clusters with bpy and apy ligands enabled to form supramol. frameworks that retain luminescent properties of the cluster units.

Solar-driven two-step thermochemical splitting of carbon dioxide represents one of the most promising candidate processes for carbon-neutral fuel production and solar energy storage. This redox process yields CO, one of the main precursors for liquid hydrocarbon fuels, and inherently results in the recycling and valorization of the CO2 greenhouse gas. In this context, non-stoichiometric perovskites such as La1−xSrxMnO3−δ have emerged as attractive candidate redox materials because of their large oxygen storage capacities and exchange capabilities that are required for applications in high temperature thermochemical redox cycles for splitting CO2. This study addresses the investigation of reactive Mn-containing substituted perovskites derived from this La1−xSrxMnO3−δ series, where Ba2+, Ca2+ or Y3+ have been incorporated on the A-site. Conversely, introduction of Al3+ and Mg2+ on the B-site was also examined. The materials were synthesized and their structures were validated by powder X-ray diffraction. Their thermochemical redox performances were subsequently quantified by thermogravimetric analysis, while the microstructure of the polycrystalline materials was examined by scanning electron microscopy. High redox activity for CO2 splitting was observed for the majority of the substituted perovskites examined here, in which Mn was the single reducible cation and Mn4+/Mn3+ redox pair was activated in the oxygen-exchange process. The structural evolutions and thermochemical behavior are discussed with respect to the distinct chemistries of the additional cations incorporated in these Mn-based perovskites.

The electrochemical performance of the layered perovskite YSr 2 Cu 3− x Co x O 7+ δ , a potential solid oxide fuel cell (SOFC) cathode, is improved by increasing the Co content from x = 1.00 to a maximum of x = 1.30. Single phase samples with x > 1.00 are obtained by tuning the Y/Sr ratio, yielding the composition Y 1− y Sr 2+ y Cu 3− x Co x O 7+ δ (where y ≤ 0.05). The high temperature structure of Y 0.95 Sr 2.05 Cu 1.7 Co 1.3 O 7+ δ at 740 °C is characterised by powder neutron diffraction and the potential of this Co-enriched material as a SOFC cathode is investigated by combining AC impedance spectroscopy, four-probe DC conductivity and powder XRD measurements to determine its electrochemical properties along with its thermal stability and compatibility with a range of commercially available electrolytes. The material is shown to be compatible with doped ceria electrolytes at 900 °C.

The La1−xSrxMnO3−δ series of non-stoichiometric perovskites (x = 0.35, 0.50, 0.65, 0.80) was examined in the context of solar-driven two-step thermochemical dissociation of CO2. Powder X-ray diffraction and thermochemical performance characterization were performed in order to assess the redox activity of these materials toward thermal reduction under inert atmosphere followed by re-oxidation for CO generation from CO2. To a certain extent, controlled introduction of Sr2+ into LaMnO3 allowed tuning the redox thermodynamics within the series, thus resulting in high activity toward both thermal reduction and CO2 dissociation. La0.50Sr0.50MnO3−δ composition appeared to be the most suitable trade-off for thermochemical CO2 splitting. Maximum CO production of about 270 μmol g−1 was reached during the CO2 splitting step with an optimal re-oxidation temperature of 1050 °C (after thermal reduction under Ar at 1400 °C), although the re-oxidation yield was limited to around 50%. Decreasing the amount of substituted Sr enhanced the re-oxidation yield at the expense of a lower final reduction extent, thus lowering the global amount of produced CO. The evolution of the Mn oxidation state implied partial re-oxidation of Mn3+ into Mn4+, thereby confirming the activation of Mn4+/Mn3+ redox pair in the perovskites. An elevated electronic transfer occurred within the Mn4+/Mn3+ redox pair (superior to that involved in the case of ceria within the Ce4+/Ce3+ redox pair), showing that mixed valence perovskites have clear potential for displaying redox properties suitable for efficient solar-driven thermochemical CO2 dissociation.

Modules of Desire Using computational methods to design materials with specific properties has found some limited success. Dyer et al. (p. 847 , published online 11 April) have devised a method, based on extended module materials assembly, that combines chemical intuition and ab initio calculations starting from fragments or modules of structure types that show the desired functionality. The method was tested by identifying materials suitable for a solid oxide fuel cell cathode.

A series of hydration experiments of the Ruddlesden-Popper phase PrSr(3)Co(1.5)Fe(1.5)O(10-δ) with varying levels of oxygen nonstoichiometry were performed with the goal to clarify phase formation and underlying mechanisms and driving forces. The hydration reaction is most intense for partly reduced samples with a vacancy concentration corresponding to δ ≈ 1. Fully oxidized samples show little or no tendency toward hydration. Presence of oxygen vacancies acts as a prerequisite for hydration. Probably, the basicity of the materials owing to A-site cations is another contributing factor to the hydration ability. Under CO(2) free conditions pure hydrates and oxide hydroxides are formed. In CO(2)-containing atmosphere, additional carbonate anions are easily incorporated into the hydrate, probably at the expense of hydroxyl groups. The I-centered PrSr(3)Co(1.5)Fe(1.5)O(8)(OH)(2)·1H(2)O achieves a highly expanded c-axis upon the topochemical insertion reactions. In situ powder synchrotron X-ray diffraction (SXRD) shows that the hydrate converts to an oxide hydroxide, PrSr(3)Co(1.5)Fe(1.5)O(8)(OH)(2), at 70 °C with a primitive orthorhombic unit cell. Upon heating above 170 °C, an I-centered product is formed for which further dehydroxylation occurs at around 400-500 °C. Rietveld refinement of SXRD data shows that the absorbed water molecules fill the tetrahedral voids of the [AO](RS) rock salt layer of the monoclinic hydrate.

Ba1.6Ca2.3Y1.1Fe5O13 is an Fe3+ oxide adopting a complex perovskite superstructure, which is an ordered intergrowth between the Ca2Fe2O5 and YBa2Fe3O8 structures featuring octahedral, square pyramidal, and tetrahedral B sites and three distinct A site environments. The distribution of A site cations was evaluated by combined neutron and X-ray powder diffraction. Consistent with the Fe3+ charge state, the material is an antiferromagnetic insulator with a Neel temperature of 480−485 °C and has a relatively low d.c. conductivity of 2.06 S cm−1 at 700 °C. The observed area specific resistance in symmetrical cell cathodes with the samarium-doped ceria electrolyte is 0.87 Ω cm2 at 700 °C, consistent with the square pyramidal Fe3+ layer favoring oxide ion formation and mobility in the oxygen reduction reaction. Density functional theory calculations reveal factors favoring the observed cation ordering and its influence on the electronic structure, in particular the frontier occupied and unoccupied electronic states.

International audience; This study addresses the synthesis, characterization, and thermochemical redox performance evaluation of perovskites and parent structures (Ruddlesden–Popper phases) as a class of oxygen-exchange materials for hydrogen generation via solar two-step water splitting. The investigated materials are LaxSr1–xMO3 (M = Mn, Co, Fe), BaxSr1–x(Co,Fe)O3, LaSrCoO4, and LaSrFeO4, also used as mixed ionic-electronic conductors in fuel cells. Temperature-programmed reduction, powder X-ray diffraction, and thermogravimetric analysis were used to obtain a preliminary assessment of these materials performances. Most of the perovskites studied here stand out by larger thermal reduction capabilities and oxygen vacancies formation at modest temperatures in the range 1000–1400 °C when compared with reference nonstoichiometric compounds such as spinel ferrites or fluorite-structured ceria-based materials. In addition, these materials offer noticeable access to metallic valence transitions during reoxidation in steam atmosphere that are not available in stoichiometric oxides. The promising behaviors characterized here are discussed in regard to the crystal chemistry of the perovskite and parent phases.

In this work we use calculations to obtain reaction enthalpies for the formation of YBa2Fe3−xMxO8 (where M = Co, Ni and Mn and x = 1, 2 and 3) from binary oxides and oxygen gas using Density Functional Theory (DFT). Based upon these calculations we are able to make predictions on favourable levels of doping and B-site ordering for YBa2Fe3−xMxO8, followed by experimental investigation in the same study. The composition where we predict doping to be favourable was experimentally investigated and a triple perovskite is found to be the major phase, confirming the prediction. Optimisation of the synthesis produced a phase-pure triple perovskite, Y1.175Ba1.825Fe2MnO8±δ, formed in a narrow compositional window. The crystal structure of this phase was analysed using Powder X-ray Diffraction (PXRD), iodometric titrations, Mossbauer spectroscopy and Neutron Powder Diffraction (NPD). This is the first reported example of ordered or disordered Fe and Mn coexistence in this structure type. We compare the observed structure against the initial DFT predictions and find them to be in good agreement and conclude that the computational methods presented within this work can be used as a predictive guide to the synthesis of oxide materials.

Cs2BaTa6Br15O3 and Cs2LaTa6Br15O3 are obtained from stoichiometric mixtures of Ta, Ta2O5, TaBr5, CsCl, and BaBr2 or La2O3 (sealed silica container, 650—690 °C, 3 d).

International audience; The crystal structure of Cs2BaTa6Br15O3 has been elucidated by using synchrotron X-ray powder diffraction and absorption experiments. It is built from edge-bridged octahedral [(Ta6equation imageequation image)equation image]4− cluster units with a singular poor metallic electron (ME) count equal to thirteen. This leads to a paramagnetic behaviour related to one unpaired electron. The arrangement of the Ta6 clusters is similar to that of Cs2LaTa6Br15O3 exhibiting 14-MEs per [(Ta6equation imageequation image)equation image]5− motif. The poorer electron-count cluster presents longer metal-metal distances as foreseen according to the electronic structure of edge-bridged hexanuclear cluster. Density functional theory (DFT) calculations on molecular models were used to rationalise the structural properties of 13- and 14-ME clusters. Periodic DFT calculations demonstrate that the electronic structure of these solid-state compounds is related to those of the discrete octahedral units. Oxygen-barium interactions seem to prevent the geometry of the octahedral cluster to strongly distort, allowing stabilisation of this unprecedented electron-poor Ta6 cluster in the solid state.

The hydration behavior of the title phase (space group I4/mmm) is investigated to clarify phase formation and underlying mechanisms and driving forces.

Ba1.6Ca2.3Y1.1Fe5O13 is an FeIII oxide with a complex perovskite superstructure, which is an intergrowth between the Ca2Fe2O5 and YBa2Fe3O8 structures featuring octahedral, square pyramidal, and tetrahedral B sites and three distinct A site environments.