Yizhak Yacoby

The UWXAFS analysis package: philosophy and details

Abstract The requirements for a reliable analysis program for XAFS spectra to obtain structural information are listed. Besides the more generally recognized items, it is emphasized that criteria to assess whether the model found is reasonable, and a fast and reliable error analysis which assures that the information content of the data is not exceeded, should be included. The University of Washington analysis program UWXAFS is described which satisfies the listed requirements and includes multiple scattering (MS) contributions in an efficient manner utilizing the FEFF program, and fits in R-space to allow limiting the MS paths.

Analysis of multiple-scattering XAFS data using theoretical standards

Abstract Theoretical standards for scattering amplitudes and phase shifts are often necessary for XAFS analysis, as for cases in which multiple scattering paths are important over the R-range of interest. Even when not necessary, they are often more convenient and reliable than experimental standards. We discuss several important considerations that must be taken into account to successfully compare ab initio theoretical calculations from FEFF to experimental XAFS spectra, and present a computer program, FEFFIT, to assist in using FEFF to get reliable information from experimental XAFS data.

Ferroelectric phase transitions in Perovskites with off-center ion displacements

Abstract We present a model of ferroelectrics which takes into account the softening of a transverse optical mode, the existence of dynamical spontaneous off-center displacements and their interaction with each other. In this paper we confine ourselves to temperatures above T c . The model is quantitatively compared with experimental results in two very different materials, PbTiO 3 and KNbO 3 . The results show that the existence of off-center displacements and their interaction with the soft mode play a crucial role in explaining the properties of ferroelectric phase transitions in perovskite crystals and possibly other crystal families.

Anomalous Interface and Surface Strontium Segregation in (La1–ySry)2CoO4±δ/La1–xSrxCoO3−δ Heterostructured Thin Films

Heterostructured oxides have shown unusual electrochemical properties including enhanced catalytic activity, ion transport, and stability. In particular, it has been shown recently that the activity of oxygen electrocatalysis on the Ruddlesden-Popper/perovskite (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ heterostructure is remarkably enhanced relative to the Ruddlesden-Popper and perovskite constituents. Here we report the first atomic-scale structure and composition of (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ grown on SrTiO3. We observe anomalous strontium segregation from the perovskite to the interface and the Ruddlesden-Popper phase using direct X-ray methods as well as with ab initio calculations. Such Sr segregation occurred during the film growth, and no significant changes were found upon subsequent annealing in O2. Our findings provide insights into the design of highly active catalysts for oxygen electrocatalysis.

Ferroelectric phase transitions and off-centre displacements in systems with strong electron-phonon interaction

We present a microscopic theory of ferroelectric phase transitions and off-centre displacements in perovskites. We show that the inclusion of strong intraband electron-phonon interaction (i.e. the existence of small polarons) into the framework of the interband theory of displacive-like ferroelectrics leads to local spin-like structural distortions in the paraelectric phase. Their interaction with the soft mode induces spin-spin coupling through the soft phonon leading to a spin-ordering phase transition. The resulting theory is shown to quantitatively explain both displacive-like and order-disorder-like features of two representative perovskites: KNbO3 and PbTiO3.

Structural disorder in perovskite ferroelectric crystals as revealed by XAFS

Abstract The question whether ferroelectrics of the perovskite structure are driven by displacive or order-disorder mechanisms has been decided currently in favor of displacive based primarily on their dynamic behavior, particularly the existence of a soft mode both below and above Tc. However, various recent experiments have suggested that perovskite ferroelectrics and other ferroelectric crystals, are, in fact, disordered above Tc, contrary to the displacive mechanism. Recent developments in X-ray Absorption Fine Structure (XAFS) analysis have facilitated detailed investigation of the temperature dependence of the local structure providing, for the first time, a direct quantitative way to investigate the form of the local potential energy and the extent of disorder in both pure and mixed ferroelectrics as well as pure and mixed antiferrodistortive crystals. The surprising result is that in all the crystals studied so far, the low temperature local distortions remain large well above Tc. The macroscopic distortions vary by essentially disordering these local distortions and not by their decrease, indicating that the phase transition driving mechanism has a significant order-disorder component.

Anomalous expansion of the copper-apical-oxygen distance in superconducting cuprate bilayers

We have introduced an improved x-ray phase-retrieval method with unprecedented speed of convergence and precision, and used it to determine with sub-Ångstrom resolution the complete atomic structure of epitaxial La2-xSrxCuO4 ultrathin films. We focus on superconducting heterostructures built from constituent materials that are not superconducting in bulk samples. Single-phase metallic or superconducting films are also studied for comparison. The results show that this phase-retrieval diffraction method enables accurate measurement of structural modifications in near-surface layers, which may be critically important for elucidation of surface-sensitive experiments. Specifically we find that, while the copper-apical-oxygen distance remains approximately constant in single-phase films, it shows a dramatic increase from the metallic-insulating interface of the bilayer towards the surface by as much as 0.45 Å. The apical-oxygen displacement is known to have a profound effect on the superconducting transition temperature.

Atomic-scale mapping of quantum dots formed by droplet epitaxy.

Quantum dots (QDs) have applications in optoelectronic devices, quantum information processing and energy harvesting. Although the droplet epitaxy fabrication method allows for a wide range of material combinations to be used, little is known about the growth mechanisms involved. Here we apply direct X-ray methods to derive sub-ångström resolution maps of QDs crystallized from indium droplets exposed to antimony, as well as their interface with a GaAs (100) substrate. We find that the QDs form coherently and extend a few unit cells below the substrate surface. This facilitates a droplet-substrate exchange of atoms, resulting in core-shell structures that contain a surprisingly small amount of In. The work provides the first atomic-scale mapping of the interface between epitaxial QDs and a substrate, and establishes the usefulness of X-ray phasing techniques for this and similar systems.

Direct structure determination of systems with two dimensional periodicity

A method is disclosed for determining the three-dimensional atomic structure of systems that are periodic in two dimensions, a-periodic in the third dimension, and are commensurate with an underlying three-dimensional crystal. The system is considered as composed of two components: an unknown system periodic in two dimensions, a-periodic in the third with an unknown structure and a known system also periodic in two dimensions with a known structure. The two systems are commensurate with each other. The method provides the structure of the unknown and therefore of the entire system.

Catalytic Activity and Stability of Oxides: The Role of Near-Surface Atomic Structures and Compositions

Electrocatalysts play an important role in catalyzing the kinetics for oxygen reduction and oxygen evolution reactions for many air-based energy storage and conversion devices, such as metal-air batteries and fuel cells. Although noble metals have been extensively used as electrocatalysts, their limited natural abundance and high costs have motivated the search for more cost-effective catalysts. Oxides are suitable candidates since they are relatively inexpensive and have shown reasonably high activity for various electrochemical reactions. However, a lack of fundamental understanding of the reaction mechanisms has been a major hurdle toward improving electrocatalytic activity. Detailed studies of the oxide surface atomic structure and chemistry (e.g., cation migration) can provide much needed insights for the design of highly efficient and stable oxide electrocatalysts. In this Account, we focus on recent advances in characterizing strontium (Sr) cation segregation and enrichment near the surface of Sr-substituted perovskite oxides under different operating conditions (e.g., high temperature, applied potential), as well as their influence on the surface oxygen exchange kinetics at elevated temperatures. We contrast Sr segregation, which is associated with Sr redistribution in the crystal lattice near the surface, with Sr enrichment, which involves Sr redistribution via the formation of secondary phases. The newly developed coherent Bragg rod analysis (COBRA) and energy-modulated differential COBRA are uniquely powerful ways of providing information about surface and interfacial cation segregation at the atomic scale for these thin film electrocatalysts. In situ ambient pressure X-ray photoelectron spectroscopy (APXPS) studies under electrochemical operating conditions give additional insights into cation migration. Direct COBRA and APXPS evidence for surface Sr segregation was found for La1-xSrxCoO3-δ and (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ oxide thin films, and the physical origin of segregation is discussed in comparison with (La1-ySry)2CoO4±δ/La1-xSrxCo0.2Fe0.8O3-δ. Sr enrichment in many electrocatalysts, such as La1-xSrxMO3-δ (M = Cr, Co, Mn, or Co and Fe) and Sm1-xSrxCoO3, has been probed using alternative techniques, including low energy ion scattering, secondary ion mass spectrometry, and X-ray fluorescence-based methods for depth-dependent, element-specific analysis. We highlight a strong connection between cation segregation and electrocatalytic properties, because cation segregation enhances oxygen transport and surface oxygen exchange kinetics. On the other hand, the formation of cation-enriched secondary phases can lead to the blocking of active sites, inhibiting oxygen exchange. With help from density functional theory, the links between cation migration, catalyst stability, and catalytic activity are provided, and the oxygen p-band center relative to the Fermi level can be identified as an activity descriptor. Based on these findings, we discuss strategies to increase a catalysts activity while maintaining stability to design efficient, cost-effective electrocatalysts.