Edge Extended X-ray Absorption Fine Structure Research Articles

Atomic-level investigation of structure and formation process of Y3Al5O12 with super-high content of Ce emitting anomalous orange-red emissions

Ce-doped yttrium aluminum garnet (YAG:Ce, Y3Al5O12:Ce) is the most commonly used yellow phosphor in white light-emitting diodes. Recently, we found that (Y1–xCex)3Al5O12:Ce with a super-high content of Ce (x = 0.11–0.21) can be prepared by a polymerized complex method and exhibits anomalous orange–red emission. To understand the nature of this anomalous behavior from the atomic level, we applied high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and scanning transmission electron microscopy–energy dispersive X-ray spectroscopy (STEM-EDS), a state-of-the-art technique that enables analysis of the atomic-level elemental distributions of Al, Y, and Ce. STEM-EDS images on the atomic level were successfully obtained for YAG:Ce. It was found that Ce in Y site has spatially uneven for few nanometers. Ce–K edge extended X-ray absorption fine structure (EXAFS) analysis was also conducted, which revealed a high level of asymmetry around Ce. By combining the STEM-EDS and EXAFS results, a dynamic model for the red emission was proposed. STEM-EDS also revealed that CeO2 nanocrystals were firstly formed with low-temperature annealing and that the reaction of Y with the CeO2 nanocrystals subsequently yielded YAG:Ce, unlike in the conventional one-step high-temperature synthesis.

Local structure of solid and liquid Au as a function of temperature by x-ray absorption spectroscopy

Accurate Au $L$-edge EXAFS (extended x-ray absorption fine structure) spectra of elemental Au have been collected and analyzed in a wide range of temperatures from 80 to 1400 K in the solid and liquid phases. Suitable samples for high-temperature measurements in the liquid phase were obtained by in situ reduction of an organic precursor. The data analysis was carried out using reliable multiple-scattering calculations including relativistic corrections (GnXAS), especially important for this heavy element. Simultaneous ${L}_{1}$-, ${L}_{2}$-, and ${L}_{3}$-edge refinements were performed and local structural results were compared with those obtained in previous studies based mainly on x-ray diffraction experiments. In the solid phase, an increased asymmetry of the first-neighbor distribution is observed for increasing temperatures while the average distance is found to be compatible with present thermal expansion data. The structure refinement of liquid Au was carried out considering the short-range contribution to the pair distribution function $g(r)$, using available x-ray diffraction data as a constraint for long-range order. We have shown that EXAFS is extremely sensitive to the local structure and that the $g(r)$ of liquid Au is characterized by a clear shift of the foot of the $g(r)$ and a slight shortening of the position of the maximum as compared with the previous data. The present data put to a test advanced computer simulations based on embedded atom models.

Efficient removal of antimony by a facile liquid-controlled strategy reinforced hematite-spinel (Fe2O3-MnFe2O4) composite: construction, simulation and practical evaluation

Currently, aqueous antimony contamination has become a non-negligible issue due to its scale industrious production and potential health risk. Herein, we prepare a novel hematite-spinel (Fe2O3-MnFe2O4) adsorbent by a one-step hydrothermal method for aqueous antimony deep purification, in which interfacial Fe-Mn bidentate complex sites are determined as the reinforced one by X-ray absorption spectroscopy (XAS) analysis and density functional theory (DFT) simulation. Through facile alkaline control by NH3·H2O and Fe/Mn proportion adjustment, the intrinsic synthesis kinetics of materials are precisely regulated that MnFe2O4 nano-particles successfully in-situ form on the Fe2O3 surface, in which sufficient adsorption sites are guaranteed by the small size effect. The adsorption experiment further demonstrates that the composite material is effective in Sb(III) and Sb(V) elimination, in which the adsorption capacity reaches up to 42.7 (Sb(III)) and 32.4 mg·g−1 (Sb(V)) (initial concentration 5.0 mg·L−1). The Sb K edge Extended X-ray absorption fine structure (EXAFS) analysis shows that antimony molecules occur as two typical complexation forms on the MnFe2O4 interface, in which an edge-sharing with Fe-O octahedron (Fe-Fe-ES) and two corner-sharing with Fe-O octahedron and Mn-O tetrahedron (Fe-Mn-CS) are exhibited. Further DFT calculations indicate that the Fe-Mn-CS has the highest adsorption energy for Sb(III) (-3.31 eV) and Sb(V) (-5.12 eV), which is rationally interpreted by the compatible overlapping of Fe 3d and O 2p as well as interfacial electron redistribution in Mn atom. This work may represent a new strategy in structure design and provide an alternative perspective in adsorption mechanism illustration.

Structural and spectroscopic investigation of the charge-ordered, short-range ordered, and disordered phases of the Co3O2BO3 ludwigite

In this work, we investigate the representative case of the homometallic Co ludwigite Co$^{2+}_2$Co$^{3+}$O$_2$BO$_3$ ($Pbam$ space group) with four distinct Co crystallographic sites [$M1$-$M4$] surrounded by oxygen octahedra. The mixed-valent character of the Co ions up to at least $T=873$ K is verified through x-ray absorption near-edge structure (XANES) experiments. Single crystal x-ray diffraction (XRD) and neutron powder diffraction (NPD) confirm that the Co ions at the $M4$ site are much smaller than the others at low temperatures, consistent with a Co$^{3+}$ oxidation state at $M4$ and Co$^{2+}$ at the remaining sites. The size difference between the Co ions in the $M4$ and $M2$ sites is continuously reduced upon warming above $\approx 370$ K, indicating a gradual charge redistribution within the $M4$-$M2$-$M4$ (424) ladder in the average structure. An increasing structural disorder, is noted above $\approx 370$ K, The local Co-O distance distribution, revealed by Co $K$-edge Extended X-Ray Absorption Fine Structure (EXAFS) data and analyzed with an evolutionary algorithm method, is similar to that inferred from the XRD crystal structure below $\approx 370$ K. At higher temperatures, the local Co-O distance distribution remains similar to that found at low temperatures, at variance with the average crystal structure obtained with XRD. We conclude that the oxidation states Co$^{2+}$ and Co$^{3+}$ are instantaneously well defined in a local atomic level at all temperatures, however the thermal energy promotes local defects in the charge-ordered configuration of the 424 ladders upon warming. These defects coalesce into a phase-segregated state within a narrow temperature interval ($475< T < 495$ K). Finally, a transition at $\approx 500$ K revealed by differential scanning calorimetry (DSC) in the iron ludwigite Fe$_3$O$_2$BO$_3$ is discussed.

Local electronic and atomic structures of the mixed B-site ions in SrFe1−xMnxO3−δ studied with X-ray absorption spectroscopy

Local electronic and atomic structures of the B-site ions in perovskite SrFe[Formula: see text]Mn[Formula: see text]O[Formula: see text] were evaluated with X-ray absorption spectroscopy (XAS) at Mn, Fe [Formula: see text]- and [Formula: see text]-edges and O [Formula: see text]-edge. The energy of [Formula: see text]- and [Formula: see text]-edge peaks for Mn and Fe ions stayed unchanged against the change in Mn content [Formula: see text]. The analysis of Mn [Formula: see text]-edge extended X-ray absorption fine structure (EXAFS) revealed that Mn−O polyhedra was kept as an octahedron with Mn[Formula: see text] irrespective of the [Formula: see text] value, agreeing with the results of [Formula: see text]-edge spectra. The Fe [Formula: see text]-edge EXAFS analysis showed that the Fe–O distance decreased with decrease of the Mn contents, suggesting that the ratio of FeO5 polyhedra to FeO6 octahedra is increased due to smaller oxygen amount. By analyzing both the [Formula: see text]-edge and [Formula: see text]-edge spectra, we clarified the local electronic and atomic structural changes particularly occurred in the B-site mixed perovskite oxides.

Anomalous Behavior in the Atomic Structure of Nb3Sn under High Pressure

In the present study, the local atomic structure of a Nb3Sn superconductor sample has been probed by X-ray absorption fine structure (XAFS) as a function of hydrostatic pressure (from ambient up to 26 GPa) using a diamond anvil cell set-up. The analysis of the Nb-K edge extended X-ray absorption fine structure (EXAFS) data was carried out combining standard multi shell structural refinement and reverse Monte Carlo method to provide detailed in situ characterization of the pressure-induced evolution of the Nb local structure in Nb3Sn. The results highlight a complex evolution of Nb chains at the local atomic scale, with a peculiar correlated displacement of Nb–Nb and Nb–Nb–Nb configurations. Such a local effect appears related to anomalies evidenced by X-ray diffraction in other superconductors belonging to the same A15 crystallographic structure.

Magnetism and structure in nanocomposite Fe nanoparticle/Al matrix films

We describe magnetometry measurements performed on nanocomposite films formed of Fe nanoparticles embedded in Al matrix. The samples were prepared using a flexible co-deposition technique under Ultra-High Vacuum (UHV) environments. Fe nanoparticles, made by using a gas aggregation source, were co-deposited with an atomic Al beam, made by an MBE source. The Volume Filling Fraction (VFF) of nanoparticles was varied controllably between 4% and 45%, while the mean diameter of Fe nanoparticles produced by the source was ∼ 2 nm. Fe K edge extended x-ray absorption fine structure (EXAFS) experiments show that there is a high degree of alloying between Fe nanoparticles and Al atoms. Magnetism in the embedded Fe nanoparticle samples was investigated using a SQUID magnetometer. Atomic moment of Fe in the Fe/Al nanocomposite films increases slightly when the proportion of Fe nanoparticles is increased, but is still significantly lower than the bulk Fe value. In a more detailed analysis, the magnetisation curves were fitted using the Random Anisotropy Model (RAM) which allowed the exchange field and random anisotropy field to be evaluated. The findings and issues raised by this approach are discussed.

The peculiar redox mechanism of copper nitroprusside disclosed by a multi-technique approach

We report the structural analysis on copper nitroprusside (CuNP) used as cathode material in a Li-ion battery during the first lithiation process. The investigation involved a multi-technique approach, where multiple edge Extended X-ray Absorption Fine Structure (EXAFS) refinement has been coupled with Density Functional Theory (DFT) calculations. The reduction occurring at the Cu site involves structural distortion, which is believed to be the main reason for the capacity fading in the first cycles.

The PmoB subunit of particulate methane monooxygenase (pMMO) in Methylococcus capsulatus (Bath): The CuI sponge and its function

In this study, we describe efforts to clarify the role of the copper cofactors associated with subunit B (PmoB) of the particulate methane monooxygenase (pMMO) from Methylococcus capsulatus (Bath) (M. capsulatus). This subunit exhibits strong affinity toward CuI ions. To elucidate the high copper affinity of the subunit, the full-length PmoB, and the N-terminal truncated mutants PmoB33–414 and PmoB55–414, each fused to the maltose-binding protein (MBP), are cloned and over-expressed into Escherichia coli (E. coli) K12 TB1 cells. The Y374F, Y374S and M300L mutants of these protein constructs are also studied. When this E. coli is grown with the pmoB gene in 1.0 mM CuII, it behaves like M. capsulatus (Bath) cultured under high copper stress with abundant membrane accumulation and high CuI content. The recombinant PmoB proteins are verified by Western blotting of antibodies directed against the MBP sub-domain in each of the copper-enriched PmoB proteins. Cu K-edge X-ray absorption near edge spectroscopy (XANES) of the copper ions confirms that all the PmoB recombinants are CuI proteins. All the PmoB proteins show evidence of a “dicopper site” according to analysis of the Cu extended X-ray absorption edge fine structure (EXAFS) of the membranes. No specific activities toward methane and propene oxidation are observed with the recombinant membrane-bound PmoB proteins. However, significant production of hydrogen peroxide is observed in the case of the PmoB33–414 mutant. Reaction of the dicopper site with dioxygen produces hydrogen peroxide and leads to oxidation of the CuI ions residing in the C-terminal sub-domain of the PmoB subunit.

Identification of Surface Structures in Pt3Cr Intermetallic Nanocatalysts

A precise understanding of the catalytic surface of nanoparticles is critical for relating their structure to activity. For silica-supported Pt–Cr bimetallic catalysts containing nominal Cr/Pt molar ratios of 0, 1.9, and 5.6, a fundamental difference in selectivity was observed as a function of composition for propane dehydrogenation, suggesting different surface structures. The formation of bimetallic catalysts and the phases present were confirmed by synchrotron in situ X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) of the nanoparticle as a function of reduction temperature. With the increasing reduction temperature, there is a systematic increase in the Pt LIII edge X-ray absorption near edge structure (XANES) energy, which is consistent with the incorporation of more metallic Cr into the nanoparticles. Pt LIII edge extended X-ray absorption fine structure (EXAFS) shows that the nanoparticles are Pt rich regardless of the reduction temperature, and XRD shows the presence of both Pt and Pt3Cr phases at temperatures below about 700 °C. For the latter, a full Pt3Cr intermetallic alloy forms after reduction at 800 °C. This work also presents a method for the characterization of the catalytic surface by the analysis of XAS difference spectra and XRD difference patterns of the (reduced and oxidized) catalysts. The surface analysis suggests that Pt3Cr formation begins at the surface, and at low reduction temperatures, a core–shell morphology is formed containing a Pt core with a Pt3Cr surface. By combining the XAS and XRD analyses with transmission electron microscopy (TEM) particle sizes, the thickness of the shell can be approximated. All evidence indicates that the shell thickness increases with the increasing reduction temperature until a full alloy is formed after reduction at about 800 °C but only if there is enough Cr2O3 available near Pt nanoparticles to form Pt3Cr. Catalysts containing a full monolayer coverage of Pt3Cr have higher olefin selectivity (>97%) compared with partially covered Pt surfaces (88%).

Vapor-phase hydrodeoxygenation of guaiacol over carbon-supported Pd, Re and PdRe catalysts

Vapor-phase hydrodeoxygenation (HDO) of guaiacol was investigated over a commercial Pd/C (A) catalyst (Evonik) and Pd/C (B), Re/C and PdRe/C catalysts prepared by incipient wetness impregnation of Norit SX-1 G activated carbon. The Pd/C catalysts had equivalent dispersions after reduction at 300 °C; however, Pd/C (B) had very low dispersion after reduction at 400 °C. CO chemisorption, Re LIII edge extended x-ray absorption fine structure (EXAFS) spectroscopy, and high-angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM) of the Re/C catalyst after reduction at 400 °C evidenced the formation of supported Re clusters. EXAFS spectroscopy of the PdRe/C catalyst after in situ reduction at 300 °C indicated the presence of Pd nanoparticles and Re clusters; a 2.70 Å Pd-Re contribution was required to adequately fit the Re LIII EXAFS spectrum. HAADF-STEM with energy-dispersive x-ray (EDX) analysis of the PdRe/C catalyst after reduction at 400 °C revealed Re clusters and Pd nanoparticles, some in intimate contact. In guaiacol HDO at 300 °C and 1 atm, Pd/C (A) was selective to phenol and cyclohexan-one/-ol and did not produce significant yields of benzene and cyclohexane, despite its high activity. Turnover frequencies for phenol (and cyclohexan-one/-ol) formation over the Pd/C catalysts were equivalent. Phenol, benzene and anisole were major products over Re/C after in situ reduction at 400 °C. The highest yield (52%) of fully deoxygenated products was obtained over PdRe/C after in situ reduction at 400 °C. We infer that the bimetallic catalyst combines synergistically the demethoxylation and hydrogenation functions of Pd/C with the capability of Re/C to deoxygenate phenol [Ghampson, et al., Catal. Sci. Technol. 2016].

On the hydrophilicity of Ni-doped TiO2 thin films. A study by X-ray absorption spectroscopy

Undoped and Ni-doped TiO2 thin films, with Ni concentration in the range 3–9 at.%, were obtained by reactive magnetron co-sputtering. A combined analysis by X-ray photoelectron spectroscopy and extended X-ray absorption-edge fine structure shows that Ni2+ substitutes for Ti4+ in the TiO2 lattice and promotes, at higher Ni amounts, a structural transition of the host from anatase to rutile. Several difficulties of the extended X-ray absorption-edge fine structure analysis, in the case of Ni-doped TiO2, are also discussed. X-ray diffraction data revealed that a large amount of the films material is in amorphous state. The films are very smooth, with root mean square roughness values (below 1.5 nm) decreasing with the increase of the Ni content. The hydrophilic properties of TiO2, modified by Ni doping, are also investigated.

Pb sorption on montmorillonite-bacteria composites: A combination study by XAFS, ITC and SCM

Though abundant studies have targeted the characterization of heavy metal adsorption by either clay minerals or bacteria, to date, minimal literature exists which specifically assesses bacteria-clay mineral interactions in the context of metal immobilization. The adsorption of Pb onto montmorillonite, Pseudomonas putida, and their 1:1, 2:1, 6:1 and 12:1 mass ratio composites were investigated by using a combination of atomic force microscope (AFM), X-ray diffraction (XRD), surface complexation modeling (SCM), Pb-LIII edge extended X-ray absorption fine structure (EXAFS) spectroscopy and isothermal titration calorimetry (ITC). The SCM and EXAFS demonstrated that Pb ions coordinate with phosphoryl and carboxyl functional groups on bacteria at low and high concentrations, respectively. The ITC analysis found adverse enthalpy values for Pb adsorption to permanent (−2.91 kJ/mol) and variable charge sites (6.93 kJ/mol) on montmorillonite. The ternary bridging model, EXAFS and ITC provide molecular and thermodynamic evidences for the formation of enthalpy driven (−4.74 kJ/mol) ternary complex (>AlO-Pb-PO4) in the composites. The proportion for the bridging structures increased at pH > 5 and high bacterial mass ratios. The formation of ternary complex did not result in the enhanced adsorption of Pb on the composites, but promoted the allocation of Pb on the mineral fraction. The results obtained from SCM, EXAFS and ITC may provide an essential assumption for predicting the speciation and fate of Pb in soils and associated environments.

Local thermal expansions and lattice strains in Elinvar and stainless steel alloys

Local thermal expansions and lattice strains in the Elinvar alloy ${\mathrm{Fe}}_{49.66}{\mathrm{Ni}}_{42.38}{\mathrm{Cr}}_{5.49}{\mathrm{Ti}}_{2.47}$ (Ni Span C) and the stainless steel $\text{SUS}304{\mathrm{Fe}}_{71.98}{\mathrm{Ni}}_{9.07}{\mathrm{Cr}}_{18.09}{\mathrm{Mn}}_{0.86}$ (AISI304) were investigated by the temperature-dependent Cr, Fe, and Ni $K$-edge extended x-ray absorption fine-structure (EXAFS) measurements, combined with the path-integral effective classical potential Monte Carlo (PIECP MC) theoretical simulations. From the EXAFS analysis of the Elinvar alloy, the local thermal expansion around Fe is found to be considerably smaller than the ones around Ni and Cr. This observation can be understood simply because Fe in the Elinvar alloy exhibit an incomplete Invar-like effect. Moreover, in both the Elinvar and SUS304 alloys, the local thermal expansions and the lattice strains around Cr are found to be larger than those around Fe and Ni. From the PIECP MC simulations of both the alloys, the first-nearest neighbor Cr-Fe pair shows extraordinarily large thermal expansion, while the Cr-Cr pair exhibits quite small or even negative thermal expansion. These findings consequently indicate that the lattice strains in both the Elinvar and SUS304 alloys are concentrated predominantly on the Cr atoms. Although the role of Cr in stainless steel has been known to inhibit corrosion by the formation of surface chromium oxide, the present investigation may interestingly suggest that the Cr atoms in the bulk play a hidden new role of absorbing inevitable lattice strains in the alloys.

Reactions of Antitumor Active Dirhodium(II) Tetraacetate Rh2(CH3COO)4 with Cysteine and Its Derivatives.

We have combined results from several spectroscopic techniques to investigate the aerobic reactions of Rh2(AcO)4 (AcO– = CH3COO–) with l-cysteine (H2Cys) and its derivatives d-penicillamine (3,3′-dimethylcysteine, H2Pen), with steric hindrance at the thiol group, and N-acetyl-l-cysteine (H2NAC), with its amino group blocked. Previous investigations have shown that antitumor active dirhodium(II) carboxylates may irreversibly inhibit enzymes containing a thiol group at or near their active sites. Also, cysteine, the only thiol-containing proteinogenic amino acid, interacts in vivo with this class of antitumor compounds, but structural information on the products of such reactions is lacking. In the present study, the reactions of Rh2(AcO)4 and H2L were carried out in aqueous solutions at the pH of mixing (acidic) and at physiological pH, using the different mole ratios 1:2, 1:4, and 1:6, which resulted in the same products in increasing yields. Electrospray ionization mass spectrometry (ESI-MS) indicates formation of dimeric [RhIII2Pen4]2– or oligomeric {RhIII2L4}n (L = Cys, NAC) complexes with bridging thiolate groups. Analyses of Rh K edge extended X-ray absorption fine structure (EXAFS) data reveal 3–4 Rh–S and 2–3 Rh–(N/O) bonds around six-coordinated Rh(III) ions at mean distances of 2.33 ± 0.02 and 2.09 ± 0.02 Å, respectively. In the N-acetyl-l-cysteine compound, the RhIII···RhIII distance 3.10 ± 0.02 Å obtained from the EXAFS spectrum supports trithiolate bridges between the Rh(III) ions, as was also found when using glutathione as ligand. In the cysteine and penicillamine complexes, double thiolate bridges join the Rh(III) ions, with the nonbridging Cys2– and Pen2– ligands in tridentate chelating (S,N,O) mode, which is consistent with the ΔδC = 7.3–8.4 ppm shift of the COO– signal in their carbon-13 cross polarization magic angle spinning (CPMAS) NMR spectra. For the penicillamine complex, the 2475.6 eV peak in its S K edge X-ray absorption near edge structure (XANES) spectrum shows partial oxidation, probably caused by peroxide generated from reduction of dissolved O2, of thiolato to sulfenato (S=O) groups, which were also identified by ESI-MS for all three {RhIII2L4}n compounds.

Ti1-Sn O2 nanofilms: Layer-by-layer deposition with extended Sn solubility and characterization

Abstract High quality rutile Ti 1- x Sn x O 2 nanofilms were successfully grown in a layer-by-layer mode at a moderately low temperature of 400 °C using pulsed laser deposition (PLD). High solid solubility of up to x = 0.216 has been achieved in the Ti 1- x Sn x O 2 films despite theoretical prediction by Density functional theory (DFT) of large formation energy (∼5.64 eV) required for the substitutional alloy to exist at such high Sn concentration. The resultant films have smooth interfaces and step-terraced surfaces with well controlled stoichiometry and are optically transparent. Sn L 3 -edge Extended X-ray absorption fine structure (EXAFS) reveals the substitution of Sn 4+ in the Ti 4+ lattice sites of TiO 2 . The lattice spacing along [110] increases linearly with increment in x due to substitution of Sn 4+ ions in the Ti lattice sites of the Ti 1- x Sn x O 2 films. X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering (RBS) show that Sn is uniformly distributed on the surface and in the bulk of the films. These results are crucial when considering Ti 1- x Sn x O 2 with suitable composition for making TiO 2 based quantum structures in advanced optoelectronic devices and solar energy materials, where high-quality crystalline thin film-substrates are important.

Structure and Magnetic Properties of FeCo Clusters: Carbon Environment and Annealing Effects

In this article, we study the thermally activated chemical order transition in FeCo nanoalloys prepared by low energy clusters beam deposition (LECBD). Due to the low Z-contrast (ΔZ = 1) between Fe and Co atoms, a particular strategy based on extended X-ray absorption edge fine structure (EXAFS) experiments and simulations at both Fe and Co K-edges has been developed as well as accurate magnetic characterizations on as-prepared and annealed FeCo clusters embedded in a carbon matrix. We show that upon annealing crystallographic order and magnetic properties are improved, but we are not able to detect without ambiguity the B2 (CsCl-type) chemical order as expected in bulk-like phase, due to competition between stable iron carbide formation and cobalt–carbon demixing in bimetallic nanoparticles.

Defect compensation by Cr vacancies and oxygen interstitials inTi4+-dopedCr2O3epitaxial thin films

Epitaxial thin films of $\mathrm{C}{\mathrm{r}}_{2\ensuremath{-}x}\mathrm{T}{\mathrm{i}}_{x}{\mathrm{O}}_{3}$ were deposited by oxygen-plasma-assisted molecular beam epitaxy for $0.04\ensuremath{\le}x\ensuremath{\le}0.26$. Ti valence is verified by both x-ray photoelectron spectroscopy (XPS) and Ti $K$-edge x-ray absorption near-edge spectroscopy (XANES) to be $\mathrm{T}{\mathrm{i}}^{4+}$. Substitution of Ti for Cr in the corundum lattice is verified by fitting the Ti $K$-edge extended x-ray absorption fine structure (EXAFS) data. Room temperature electrical transport measurements confirm the highly resistive nature of Ti-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$, despite the presence of aliovalent $\mathrm{T}{\mathrm{i}}^{4+}$. For comparison, the resistivity of very pure, undoped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ was measured to be four orders of magnitude higher than for Ti-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$. Analysis of the XPS and EXAFS data reveal the presence of Cr vacancies at intermediate and high Ti concentrations. This conclusion is corroborated by the results of density functional modeling. At low Ti concentrations, a strong increase of the XPS $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}2p$ core level peak width is observed as the Ti concentration decreases. In this limit, the formation of Cr vacancies becomes less favorable due to the increased distance between Ti dopants, and compensation by O interstitials contributes to broadening of the $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}2p$ XPS peak. The differences in electronic structure which render $\mathrm{T}{\mathrm{i}}^{4+}$-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ resistive due to the formation of compensating defects, but $\mathrm{T}{\mathrm{i}}^{4+}$-doped $\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ conductive, are discussed. The defect compensation model developed here provides insight into previous, conflicting reports of $n$-type versus $p$-type conductivity in Ti-doped $\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{3}$ at high temperature, and will inform future studies to exploit the wide variety of electronic and magnetic properties of corundum structure oxides.

Order, disorder and mixing: The atomic structure of amorphous mixtures of titania and tantala

The atomic structure of mixtures of titania (TiO2) and tantala (Ta2O5) ion-beam sputtered amorphous thin film coatings at various post-deposition annealing temperatures have been studied using Ta LIII and Ti K edge Extended X-ray Absorption Fine Structure (EXAFS). The results indicate that post-deposition annealing produces subtle changes in the short range order (<1nm) for samples which remain amorphous. We also show that titania–tantala mixtures maintain a structure similar to that of pure tantala, with the titanium atoms preferring to sit at the second shell distance, which is similar to the Ta–Ta distance seen in the pure tantala structure. A discussion is also included on interpretation of the general trends identified in the EXAFS data and how this relates to previous and ongoing studies of the structure and mechanical loss measurements of titania–tantala coatings.

Structure and magnetism in Cr-embedded Co nanoparticles

We present the results of an investigation into the atomic structure and magnetism of 2 nm diameter Co nanoparticles embedded in an antiferromagnetic Cr matrix. The nanocomposite films used in this study were prepared by co-deposition directly from the gas phase, using a gas aggregation source for the Co nanoparticles and a molecular beam epitaxy (MBE) source for the Cr matrix material. Co K and Cr K edge extended x-ray absorption fine structure (EXAFS) experiments were performed in order to investigate atomic structure in the embedded nanoparticles and matrix respectively, while magnetism was investigated by means of a vibrating sample magnetometer. The atomic structure type of the Co nanoparticles is the same as that of the Cr matrix (bcc) although with a degree of disorder. The net Co moment per atom in the Co/Cr nanocomposite films is significantly reduced from the value for bulk Co, and decreases as the proportion of Co nanoparticles in the film is decreased; for the sample with the most dilute concentration of Co nanoparticles (4.9% by volume), the net Co moment was 0.25 μB/atom. After field cooling to below 30 K all samples showed an exchange bias, which was largest for the most dilute sample. Both the structural and magnetic results point towards a degree of alloying at the nanoparticle/matrix interface, leading to a core/shell structure in the embedded nanoparticles consisting of an antiferromagnetic CoCr alloy shell surrounding a reduced ferromagnetic Co core.