Melanie Nentwich

Picometer polar atomic displacements in strontium titanate determined by resonant X-ray diffraction

Physical properties of crystalline materials often manifest themselves as atomic displacements either away from symmetry positions or driven by external fields. Especially the origin of multiferroic or magnetoelectric effects may be hard to ascertain as the related displacements can reach the detection limit. Here we present a resonant X-ray crystal structure analysis technique that shows enhanced sensitivity to minute atomic displacements. It is applied to a recently found crystalline modification of strontium titanate that forms in single crystals under electric field due to oxygen vacancy migration. The phase has demonstrated unexpected properties, including piezoelectricity and pyroelectricity, which can only exist in non-centrosymmetric crystals. Apart from that, the atomic structure has remained elusive and could not be obtained by standard methods. Using resonant X-ray diffraction, we determine atomic displacements with sub-picometer precision and show that the modified structure of strontium titanate corresponds to that of well-known ferroelectrics such as lead titanate.It is a challenge to measure changes in the crystal structures in picometer scale and the associated phase. Here the authors demonstrate the lattice expansion and polar distortions of oxygen deficient SrTiO3 using a resonance X-ray scattering technique.

Fundamental principles of battery design

Abstract With an increasing diversity of electrical energy sources, in particular with respect to the pool of renewable energies, and a growing complexity of electrical energy usage, the need for storage solutions to counterbalance the discrepancy of demand and offer is inevitable. In principle, a battery seems to be a simple device since it just requires three basic components – two electrodes and an electrolyte – in contact with each other. However, only the control of the interplay of these components as well as their dynamics, in particular the chemical reactions, can yield a high-performance system. Moreover, specific aspects such as production costs, weight, material composition and morphology, material criticality, and production conditions, among many others, need to be fulfilled at the same time. They present some of the countless challenges, which make battery design a long-lasting, effortful task. This chapter gives an introduction to the fundamental concepts of batteries. The principles are exemplified for the basic Daniell cell followed by a review of Nernst equation, electrified interface reactions, and ionic transport. The focus is addressed to crystalline materials. A comprehensive discussion of crystal chemical and crystal physical peculiarities reflects favourable and unfavourable local structural aspects from a crystallographic view as well as considerations with respect to electronic structure and bonding. A brief classification of battery types concludes the chapter.

Battery concepts: The past, the present, and research highlights

Abstract The concept of a battery is not a modern invention, as first proofs go back to 200 bc. The development of electrochemical cells similar to those that we use today started at the end of the eighteenth century with the experiments of Luigi Galvani. The following paragraphs will give an overview of the progress in electrochemistry from the very early reports to the state of the art. Additionally, some future perspectives from the recent years will be highlighted.

Electrodes: definitions and systematisation – a crystallographers view

Abstract Electrodes are, in combination with electrolytes and the active, reacting materials the function-giving materials in electrochemical energy storage devices. They are responsible for the transfer of electrons and provide the surface at which the electrochemical reactions take place. Those electrochemical reactions span the potential difference which drives the battery. We present a crystallographically inspired systematisation of all electrodes found in electrochemical storages that comprise inert and reactive electrodes, subdivided in active and passive electrodes, and solvation, mixed crystal, and phase transition electrodes, respectively. After the description of all electrode types we present a concise summary of battery chemistries and the applied electrode types.

Structure variations within certain rare earth disilicides

The dimorphism of the RSi2 and R2TSi3 compounds is a well known phenomenon (R is an alkaline earth metal, rare earth metal or actinoide, T is a transition metal). They crystallize in structures, which derive from hexagonal AlB2 or tetragonal ThSi2 prototypes. Despite their local similarities, both prototypes do not have a common root in the Bärnighausen diagram, which summarizes the symmetry relations between the high symmetrical basic structures and their lower symmetric variations. We performed an extensive literature research based on more than 400 structure reports of the RSi2 and R2TSi3 compounds. To gain an overview of the various structure reports within these compounds we summarized composition, lattice parameters a and c, ratios c/a, formula units per unit cell, and structure types in an extensive table. We performed DFT calculations on carefully chosen compounds to evaluate the probability of a successful synthesis. Finally, we discuss peculiarities of symmetry distribution among the RSi2 and R2TSi3 compounds and several correlations related to structural parameters. We found that the thermal treatment has a massive effect to the formation of superstructures. Furthermore, there are two different kinds of hexagonal R2TSi3 compounds being ionic or metallic, depending on the R element. Additionally, the main influence to the variation of the Si-T bonds is the electronic interplay between R element and Si lattice rather than the R radii.

Probing structural distortions with new high-precision resonant X-ray diffraction approach

Matthias Zschornak1, Carsten Richter2, Dmitri Novikov3, Erik Mehner1, Melanie Nentwich1, Semën Gorfman4, Juliane Hanzig1, Hartmut Stöcker1, Tilmann Leisegang1, Dirk C. Meyer1 1Institute Of Experimental Physics, TU Bergakademie Freiberg, Freiberg, Germany, 2European Synchrotron Radiation Facility (ESRF), Grenoble, France, 3Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany, 4Department of Physics, University of Siegen, Siegen, Germany E-mail: matthias.zschornak@physik.tu-freiberg.de

Analysis of modulated Ho2PdSi3 crystal structure at Pd K and Ho L absorption edges using resonant elastic X-scattering.

Replacing Si atoms with a transition metal in rare earth disilicides results in a family of intermetallic compounds with a variety of complex magnetic phase transitions. In particular, the family R 2PdSi3 shows interesting magnetic behavior arising from the electronic interaction of the R element with the transition metal in the Si network, inducing the specific structure of the related phase. Within this series, the highest degree of superstructural order was reported for the investigated representative Ho2PdSi3, although several competing superstructures have been proposed in literature. The diffraction anomalous fine structure (DAFS) method is highly sensitive to the local structure of chosen atoms at specific positions within the unit cell of a crystalline phase. In combination with x-ray absorption fine structure (XAFS), this sophisticated synchrotron method has been applied in the present work to several selected reflections, i.a. a satellite reflection. Extensive electronic modeling was used to test the most relevant structure proposals. The [Formula: see text] superstructure has been strongly confirmed, although a small amount of disorder in the modulation is very probable.

Evaluation of structure models of Ho2PdSi3 using DAFS, inter alia at a satellite reflection

The compounds R2PdSi3, with R = rare earth, exhibit a very interesting magnetic behavior with two phase transitions. Substituting one in four Si atoms by Pd in HoSi2 results in a modulation of the aristotype. There are several different variants discussed in literature about the nature of the modulation of this rare-earth compound. Two of the latest models were compared: a 2 × 2 × 1 layer and a 2 × 2 × 8 stack. The chosen method is Diffraction Anomalous Fine Structure (DAFS) and was applied both experimentally and by simulation at different absorption edges and reflections, i. a. a satellite reflection, aiming on finding the correct crystal structure.

Defect separation in strontium titanate: Formation of a polar phase

Stoichiometric perovskite-type strontium titanate acts as an insulator because of its wide electronic band gap and has therefore great potential as high-k dielectric and storage material in memory applications. Degradation phenomena of insulating properties of transition metal oxides occur during long time voltage application. From the defect chemistry point of view the question arises how mobile species react on an external electric field and which impact the redistribution has on the stability of the crystal structure. Here, we discuss near-surface reversible structural changes in SrTiO3 single crystals caused by oxygen vacancy redistribution in an external electric field. We present in-situ X-ray diffraction during and after electroformation. Several reflections are monitored and show a tetragonal elongation of the cubic unit cell. Raman investigations were carried out to verify that the expansion involves a transition from the centrosymmetric to a less symmetric structure. Regarding a whole formation cycle, two different time scales occur: a slow one during the increase of the lattice constant and a fast one after switching off the electric field. Based on the experimental data we suggest a model containing the formation of a polar SrTiO3 unit cell stabilized by the electric field, which is referred to as migrationinduced field-stabilized polar phase [1] at room temperature. As expected by a non-centrosymmetric crystal structure, pyroelectric properties will be presented in conjunction with temperature modulated electroformation cycles. Furthermore, we show that intrinsic defect separation establishes a non-equilibrium accompanied by an electromotive force. A comprehensive thermodynamic deduction in terms of theoretical energy and entropy calculations indicates an exergonic electrochemical reaction after the electric field is switched off. Based on that driving force the experimental and theoretical proof of concept of a solid-state SrTiO3 battery is reported.