Structure Low Dimensionality and Lone-Pair Stereochemical Activity: the Key to Low Thermal Conductivity in the Pb-Sn-S System.

Recently, metal sulfides have begun to receive attention as potential cost-effective materials for thermoelectric applications beyond optoelectronic and photovoltaic devices. Herein, based on a comparative analysis of the structural and transport properties of 2D PbSnS2 and 1D PbSnS3, we demonstrate that the intrinsic effects that govern the low lattice thermal conductivity (κL) of these sulfides originate from the combination of the low dimensionality of their crystal structures with the stereochemical activity of the lone-pair electrons of cations. The presence of weak bonds in these materials, responsible for phonon scattering, results in inherently low κL of 1.0 W/m K in 1D PbSnS3 and 0.6 W/m K in 2D PbSnS2 at room temperature. However, the nature of the thermal transport is quite distinct. 1D PbSnS3 exhibits a higher thermal conductivity with a crystalline-like peak at low temperatures, while 2D PbSnS2 demonstrates glassy thermal conductivity in the entire temperature range investigated. First-principles density functional theory calculations reveal that the presence of antibonding states below the Fermi level, especially in PbSnS2, contributes to the very low κL. In addition, the calculated phonon dispersions exhibit very soft acoustic phonon branches that give rise to soft lattices and very low speeds of sounds.

From K6[Re6-xMoxS8(CN)5] Solid Solution to Individual Cluster Complexes: Separation and Investigation of [Re4Mo2S8(CN)6]n- and [Re3Mo3S8(CN)6]n- Heterometallic Clusters.

A series of new cluster compounds with {Re4Mo2S8} and {Re3Mo3S8} cores has been obtained and investigated. The clusters with different Re/Mo ratios were isolated as individual compounds, which made it possible to study their spectroscopic and electrochemical properties. The geometry of the new clusters was studied using a combination of X-ray diffraction analysis, XAS and quantum chemical DFT calculations. It was shown that the properties of the new clusters, such as the number and position of electrochemical transitions, electronic structure and change in geometry with a change in charge, are similar to the properties of clusters based on the {Re4Mo2Se8} and {Re3Mo3Se8} cores described earlier.

Exsolution of Co-Fe Alloy Nanoparticles on the PrBaFeCoO5+δ Layered Perovskite Monitored by Neutron Powder Diffraction and Catalytic Effect on Dry Reforming of Methane.

Reversible exsolution and dissolution of metal nanoparticles (NPs) in complex oxides have been investigated as an efficient strategy to improve the performance and durability of the catalysts for thermal and electrochemical energy conversion. Here, in situ exsolution of Co-Fe alloy NPs from the layered perovskite PrBaFeCoO5+δ (PBFC) and their dissolution back into the oxide host have been monitored for the first time by in situ neutron powder diffraction and confirmed by X-ray diffraction and electron microscopy. Catalytic tests for dry reforming of methane showed stable operation over ∼100 h at 800 °C with negligible carbon deposition (<0.3 mg/gcat h). The CO2 and CH4 conversions are among the highest achieved by layered double perovskites. The cyclability of the PBFC catalyst and the potential to improve the catalytic activity by adjusting the composition, size, and the NP distribution would pave the way for highly efficient energy conversion applications.

A Tunable Structural Family with Ultralow Thermal Conductivity: Copper-Deficient Cu1-x□xPb1-xBi1+xS3.

Understanding the mechanism that connects heat transport with crystal structures and order/disorder phenomena is crucial to develop materials with ultralow thermal conductivity (κ), for thermoelectric and thermal barrier applications, and requires the study of highly pure materials. We synthesized the n-type sulfide CuPbBi5S9 with an ultralow κ value of 0.6-0.4 W m-1 K-1 in the temperature range 300-700 K. In contrast to prior studies, we show that this synthetic sulfide does not exhibit the ordered gladite mineral structure but instead forms a copper-deficient disordered aikinite structure with partial Pb replacement by Bi, according to the chemical formula Cu1/3□2/3Pb1/3Bi5/3S3. By combining experiments and lattice dynamics calculations, we elucidated that the ultralow κ value of this compound is due to very low energy optical modes associated with Pb and Bi ions and, to a smaller extent, Cu. This vibrational complexity at low energy hints at substantial anharmonic effects that contribute to enhance phonon scattering. Importantly, we show that this aikinite-type sulfide, despite being a poor semiconductor, is a potential matrix for designing novel, efficient n-type thermoelectric compounds with ultralow κ values. A drastic improvement in the carrier concentration and thermoelectric figure of merit have been obtained upon Cl for S and Bi for Pb substitution. The Cu1-x□xPb1-xBi1+xS3 series provides a new, interesting structural prototype for engineering n-type thermoelectric sulfides by controlling disorder and optimizing doping.

Local-Disorder-Induced Low Thermal Conductivity in Degenerate Semiconductor Cu22Sn10S32.

S-based semiconductors are attracting attention as environmentally friendly materials for energy-conversion applications because of their structural complexity and chemical flexibility. Here, we show that the delicate interplay between the chemical composition and cationic order/disorder allows one to stabilize a new sphalerite derivative phase of cubic symmetry in the Cu-Sn-S diagram: Cu22Sn10S32. Interestingly, its crystal structure is characterized by a semiordered cationic distribution, with the Cu-Sn disorder being localized on one crystallographic site in a long-range-ordered matrix. The origin of the partial disorder and its influence on the electronic and thermal transport properties are addressed in detail using a combination of synchrotron X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy, theoretical modeling, and transport property measurements. These measurements evidence that this compound behaves as a pseudogap, degenerate p-type material with very low lattice thermal conductivity (0.5 W m-1 K-1 at 700 K). We show that localized disorder is very effective in lowering κL without compromising the integrity of the conductive framework. Substituting pentavalent Sb for tetravalent Sn is exploited to lower the hole concentration and doubles the thermoelectric figure of merit ZT to 0.55 at 700 K with respect to the pristine compound. The discovery of this semiordered cubic sphalerite derivative Cu22Sn10S32 furthers the understanding of the structure-property relationships in the Cu-Sn-S system and more generally in ternary and quaternary Cu-based systems.

Exsolution of Ni Nanoparticles from A-Site-Deficient Layered Double Perovskites for Dry Reforming of Methane and as an Anode Material for a Solid Oxide Fuel Cell.

Exsolution is a promising technique to design metal nanoparticles for electrocatalysis and renewable energy. In this work, Ni-doped perovskites, (Pr0.5Ba0.5)1-x/2Mn1-x/2Nix/2O3-δ with x = 0, 0.05, 0.1, and 0.2 (S-PBMNx), were prepared to design exsolution systems as solid oxide fuel cell anodes and for catalysis applications. X-ray diffraction and transmission electron microscopy (TEM) analyses demonstrated that correlating A-site deficiency with Ni content can effectively induce exsolution of all Ni under H2 atmosphere at T ∼ 875 °C, yielding the reduced (exsolved) R-PBMNx materials. On heating the exsolution systems in air, metal incorporation in the oxide lattice did not occur; instead, the Ni nanoparticles oxidized to NiO on the layered perovskite surface. The lowest area-specific resistance (ASR) under wet 5% H2/N2 in symmetrical cells was observed for R-PBMN0.2 anode (ASR ∼ 0.64 Ω cm2 at 850 °C) due to the highest Ni particle density in the R-PBMNx series. The best performance for dry reforming of methane (DRM) was also obtained for R-PBMN0.2, with CH4 and CO2 conversion rates at 11 and 32%, respectively, and the highest production of H2 (37%). The DRM activity of R-PBMN0.2 starts at 800 °C and is sustained for up to at least 5 h operation with little carbon deposition (0.017 g·gcat-1·h-1). These results clearly demonstrate that varying Ni-doping in layered double perovskite oxides is an effective strategy to manipulate the electrochemical performance and catalytic activity for energy conversion purposes.

Time and space resolved modelling of the heating induced by synchrotron X-ray nanobeams.

X-ray synchrotron sources, possessing high power density, nanometric spot size and short pulse duration, are extending their application frontiers up to the exploration of direct matter modification. In this field, the use of atomistic and continuum models is now becoming fundamental in the simulation of the photoinduced excitation states and eventually in the phase transition triggered by intense X-rays. In this work, the X-ray heating phenomenon is studied by coupling the Monte Carlo method (MC) with the Fourier heat equation, to first calculate the distribution of the energy absorbed by the systems and finally to predict the heating distribution and evolution. The results of the proposed model are also compared with those obtained removing the explicit definition of the energy distribution, as calculated by the MC. A good approximation of experimental thermal measurements produced irradiating a millimetric glass bead is found for both of the proposed models. A further step towards more complex systems is carried out, including in the models the different time patterns of the source, as determined by the filling modes of the synchrotron storage ring. The two models are applied in three prediction cases, in which the heating produced in Bi2Sr2CaCu2O8+δ microcrystals by means of nanopatterning experiments with intense hard X-ray nanobeams is calculated. It is demonstrated that the temperature evolution is strictly connected to the filling mode of the storage ring. By coupling the MC with the heat equation, X-ray pulses that are 48 ps long, possessing an instantaneous photon flux of ∼44 × 1013 photons s-1, were found to be able to induce a maximum temperature increase of 42 K, after a time of 350 ps. Inversely, by ignoring the energy redistribution calculated with the MC, peaks temperatures up to hundreds of degrees higher were found. These results highlight the importance of the energy redistribution operated by primary and secondary electrons in the theoretical simulation of the X-ray heating effects.

Tailoring Heterometallic Cluster Functional Building Blocks: Synthesis, Separation, Structural and DFT Studies of [Re6-x Mox Se8 (CN)6 ]n.

Influence of the metal core composition and geometry on the structure, spectroscopic properties and redox potentials was investigated for the first time for heterometallic (Re/Mo)6 octahedral clusters. The discrete anionic clusters [Re6-x Mox Se8 (CN)6 ]n- (x=2, 3; n=4, 5) were obtained as individual salts. Their isomeric composition and bond-length distribution were inspected using a combination of single-crystal X-ray structure analysis, NMR, EXAFS, and DFT calculations.

In Situ Synchrotron Powder Diffraction Study of Cd Intercalation into Chevrel Phases: Crystal Structure and Kinetic Effect.

Chevrel phases are molybdenum chalcogenides of formula M xMo6X8 (where M is a cation and X is a chalcogen) that present a complex and captivating intercalation chemistry that has drawn the interest of the solid-state chemistry community since their discovery. This property has a huge potential for applied science and device development for energy storage and pollutant removal and detection, but a deeper knowledge of the intercalation processes and chemistry is still necessary. In the present work, the intercalation of Cd2+ in aqueous solution has been studied, taking advantage of the complementarity of electrochemical characterization and synchrotron powder diffraction acquired during an in situ combined experiment. During the experiment, industrially adequate electrochemical conditions (room temperature and reduced process time) were applied, allowing a better understanding of the intercalation processes. The intercalated phases obtained by electrochemistry have been characterized ex situ, and for the first time the structures of Cd2Mo6X8 (X = S, Se) have been determined. Unexpectedly, Cd2Mo6Se8 presents a trigonal crystal structure with only cavity 2 occupied, which has not been encountered before for Chevrel phases.

Maskless X-Ray Writing of Electrical Devices on a Superconducting Oxide with Nanometer Resolution and Online Process Monitoring.

X-ray nanofabrication has so far been usually limited to mask methods involving photoresist impression and subsequent etching. Herein we show that an innovative maskless X-ray nanopatterning approach allows writing electrical devices with nanometer feature size. In particular we fabricated a Josephson device on a Bi2Sr2CaCu2O8+δ (Bi-2212) superconducting oxide micro-crystal by drawing two single lines of only 50 nm in width using a 17.4 keV synchrotron nano-beam. A precise control of the fabrication process was achieved by monitoring in situ the variations of the device electrical resistance during X-ray irradiation, thus finely tuning the irradiation time to drive the material into a non-superconducting state only in the irradiated regions, without significantly perturbing the crystal structure. Time-dependent finite element model simulations show that a possible microscopic origin of this effect can be related to the instantaneous temperature increase induced by the intense synchrotron picosecond X-ray pulses. These results prove that a conceptually new patterning method for oxide electrical devices, based on the local change of electrical properties, is actually possible with potential advantages in terms of heat dissipation, chemical contamination, miniaturization and high aspect ratio of the devices.

Chemical Short-Range Order in Selenide and Telluride Glasses.

The structure of Ge20SbxSe80-x (x = 5, 15, 20) glasses was investigated by neutron diffraction, X-ray diffraction, and extended X-ray fine structure measurements at the Ge, Sb, and Se K-edges. For each composition, large-scale structural models were obtained by fitting simultaneously the experimental data sets in the framework of the reverse Monte Carlo simulation technique. It was found that the structures of these glasses can be described mostly by the chemically ordered network model. Ge-Se and Sb-Se bonds are preferred; Se-Se bonds in the Se-poor composition (x = 20) and M-M (M = Ge, Sb) bonds in strongly Se-rich glass (x = 5) are not needed. The quality of the fits was significantly improved by introducing Ge-Ge bonding in the nearly stoichiometric composition (x = 15), showing a violation of chemical ordering. The structure of Ge20SbxSe80-x was compared to that of several glasses from the three analogous systems (Ge-As-Se, Ge-As-Te, Ge-Sb-Te), and it was found that chemical short-range order becomes more pronounced upon substituting As with Sb and Se with Te. Ge-As-Se glasses behave as random covalent networks over a very broad composition range. Chemical short-range order and disorder coexist in both Te-rich and Te-poor Ge-As-Te glasses, whereas amorphous Ge14Sb29Te57 and Ge22Sb22Te56 are governed by strict chemical preferences.

Structure and reactivity with oxygen of Pr2NiO(4+δ): an in situ synchrotron X-ray powder diffraction study.

The promising SOFC cathode material Pr2NiO(4.22) has been studied in situ under a pure oxygen atmosphere from 25 to 950 °C by high resolution synchrotron X-ray powder diffraction. At room temperature (RT) δ = 0.22(1), the average crystal structure turns out to be monoclinic. The subtle monoclinic distortion (γ = 90.066(1)° at RT), retained up to 460 °C, is interpreted in terms of specific tilt schemes of the NiO6 octahedra. It is also shown that Pr2NiO(4.22) is incommensurately structurally modulated already at room temperature, in the same manner as the homologous cobaltate La2CoO(4.14). The phase transition to the High Temperature Tetragonal (HTT) phase was completed at 480 °C without any evidence for the Low Temperature Orthorhombic (LTO) phase allowing clarifying the phase diagram of this K2NiF4-type ternary oxide. Moreover, it turns out that above 800 °C, the HTT phase transforms reversibly into two coexisting isomorphous tetragonal phases. The incommensurate modulation subsists up to 950 °C, although modified concomitantly with the two abovementioned phase transformations. In addition, the role of kinetics on the decomposition process is highlighted through thermo-gravimetric analyses.

Time-resolved, in situ DRIFTS/EDE/MS studies on alumina-supported rhodium catalysts: effects of ceriation and zirconiation on rhodium-CO interactions.

The effects of ceria and zirconia on the structure-function properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ-Al2O3) during CO exposure are described. Ceria and zirconia are introduced through two preparation methods: 1) ceria is deposited on γ-Al2O3 from [Ce(acac)3] and rhodium metal is subsequently added, and 2) through the controlled surface modification (CSM) technique, which involves the decomposition of [M(acac)x] (M=Ce, x=3; M=Zr, x=4) on Rh/γ-Al2O3. The structure-function correlations of ceria and/or zirconia-doped rhodium catalysts are investigated by diffuse reflectance infrared Fourier-transform spectroscopy/energy-dispersive extended X-ray absorption spectroscopy/mass spectrometry (DRIFTS/EDE/MS) under time-resolved, in situ conditions. CeOx and ZrO2 facilitate the protection of Rh particles against extensive oxidation in air and CO. Larger Rh core particles of ceriated and zirconiated Rh catalysts prepared by CSM are observed and compared with Rh/γ-Al2O3 samples, whereas supported Rh particles are easily disrupted by CO forming mononuclear Rh geminal dicarbonyl species. DRIFTS results indicate that, through the interaction of CO with ceriated Rh particles, a significantly larger amount of linear CO species form; this suggests the predominance of a metallic Rh phase.

EXAFS and XANES investigation of (Li, Ni) codoped ZnO thin films grown by pulsed laser deposition.

Ni doped, Li doped and (Li, Ni) codoped ZnO thin films were successfully grown using a pulsed laser deposition technique. Undoped and doped ZnO thin films were investigated using extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge spectroscopy (XANES). Preliminary investigations on the Zn K-edge of the undoped and doped ZnO thin films revealed that doping has not influenced the average Zn-Zn bond length and Debye-Waller factor. This shows that both Ni and Li doping do not appreciably affect the average local environment of Zn. All the doped ZnO thin films exhibited more than 50% of substitutional Ni, with a maximum of 77% for 2% Ni and 2% Li doped ZnO thin film. The contribution of Ni metal to the EXAFS signal clearly reveals the presence of Ni clusters. The Ni-Ni distance in the Ni(0) nanoclusters, which are formed in the film, is shorter with respect to the reference Ni metal foil and the Debye-Waller factor is higher. Both facts perfectly reflect what is expected for metal nanoparticles. At the highest doping concentration (5%), the presence of Li favors the growth of a secondary NiO phase. Indeed, 2% Ni and 5% Li doped ZnO thin film shows %Nisub = 75 ± 11, %Nimet = 10 ± 8, %NiO = 15 ± 8. XANES studies further confirm that the substitutional Ni is more than 50% in all the samples. These results explain the observed magnetic properties.

Unprecedented electron-poor octahedral Ta(6) clusters in a solid-state compound: synthesis, characterisations and theoretical investigations of Cs(2)BaTa(6)Br(15)O(3).

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 [(Ta6Bri9Oi3)Bra6]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 [(Ta6Bri9Oi3)Bra6]5− motif. The poorer electron-count cluster presents longer metal­metal 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. Oxygen­barium 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.

Structural modulation and phase transitions in La2CoO(4.14) investigated by synchrotron X-ray and neutron single-crystal diffraction.

We report a combined synchrotron X-ray and neutron diffraction study on as-grown La(2)CoO(4.14) single-crystal from 10 to 470 K. Unprecedented structural features in terms of a (3 + 2)D incommensurate modulation have been detected and characterized in the Low Temperature Orthorhombic (LTO) phase already at room temperature despite the complex twinning that was unravelled. A new intermediate phase between the LTO and High Temperature Tetragonal (HTT) phases has been observed for the first time (in the range of 413-433 K). The transformation from LTO to this so-called HTLO (High Temperature Less Orthorhombic) phase is associated to a lowering of orthorhombicity and a loss of one modulation vector, yielding a (3 + 1)D incommensurate modulation. Conversely, above 433 K the HTT phase appears as nonmodulated but exhibits a strong dynamic disorder of CoO(6) octahedra, which has been characterized in detail by reconstruction of nuclear densities via the Maximum Entropy Method (MEM).

Quick-EXAFS implementation on the general purpose EXAFS beamline at ESRF.

The new implementation of QEXAFS acquisition on the general purpose EXAFS beamline BM29 at the European Synchrotron Radiation Facility is presented. By adopting a continuous-scan mode, a high signal-to-noise ratio can be maintained, together with a fixed exit beam and full compatibility with step-by-step operation, while reducing acquisition times to a few seconds. The new equipment implemented on the beamline is described and the potential of this application is demonstrated with a few examples.

Tracking the formation of cobalt substituted ALPO-5 using simultaneous in situ X-ray diffraction and X-ray absorption spectroscopy techniques.

We followed the formation of metal ion substituted aluminophosphate, a redox molecular sieve, using a newly developed in situ simultaneous X-ray diffraction and X-ray absorption spectroscopic technique. The study showed that a cobalt-phosphorous network forms prior to the crystallisation.

EXAFS and XRD characterization of palladium sorbents for high temperature mercury capture from fuel gas.

Removal of pollutants such as mercury at elevated temperatures provides improvements in the overall thermal efficiency during the process of coal gasification. The two high temperature sorbents studied were 5 wt% Pd/Al(2)O(3) and 5 wt% Pd/SiO(2): materials shown to have significantly different Hg adsorption capacities. A combination of XRD and EXAFS has been used to characterize the Pd-Hg alloy formed when these Pd-based sorbents were exposed to fuel gas (CO, CO(2), H(2)) containing Hg vapour at 204 degrees C. Significant differences were found in the nature of the alloy formed on the two sorbents following Hg exposure. The Pd/Al(2)O(3) sorbent produced a single homogeneous solid solution of Pd-Hg whilst the silica-supported Pd produced an alloy of varying composition.

Novel high-temperature reactors for in situ studies of three-way catalysts using turbo-XAS.

Two novel high-temperature reactors for in situ X-ray absorption spectroscopy (XAS) measurements in fluorescence are presented, each of them being optimized for a particular purpose. The powerful combination of these reactors with the turbo-XAS technique used in a dispersive-XAS beamline permits the study of commercial three-way catalysts under realistic gas composition and temporal conditions.