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A description of the hydrogen occupation site in LiNbO3 and LiTaO3 is made based on theoretical structural models and validated by measured OH− stretching vibrational modes.

PrCa4O[BO3]3 was grown as a single crystal by the Czochralski method. The precursor material was synthesized by solid state reaction under O2 atmosphere and sintered after grinding under N2 atmosphere with a content of 20% H2. The Czochralski crystal growth proceeded under N2 atmosphere with an oriented GdCa4O[BO3]3 crystal seed. A PrCa4O[BO3]3 crystal with a cylinder length of 20 mm, a diameter of 15 mm, and weight of 23.1 g was obtained. The crystal structure was solved from single crystal X-ray diffraction data in the monoclinic crystal system with space group C1m1 (No. 8), the lattice parameters a = 8.1293(6) Å, b = 16.062(1) Å, c = 3.6023(2) Å, β = 101.371(2)°, formula units of 2 and final R- and wR2-values below 3.8%. A mixed occupancy between Pr and Ca of 6.0% was found.

Lithium tantalate (LiTaO3) has large technological importance due to numerous applications, especially for motion sensors, radiation detectors, and optical waveguides. In some manufacturing processes temperature treatments are applied to obtain the desired structure or composition. However, the temperature treatment influences the optical and electrical properties of the material due to the incorporation of defects. Unfortunately, the exact nature and concentration of these defects are only insufficiently known. In the present study, temperature-induced defect incorporation in congruent LiTaO3 crystals under vacuum at 1224 K is investigated. Inelastic neutron spectra are measured in situ to identify the point defects and separate them from further contributions of structural phase transition as well as ferroelectric domain structure. Inelastic neutron scattering data are analysed with respect to small defect concentrations by comparing with density functional theory results. The influence of ferroelectric domains is analysed by Raman and FT-IR spectroscopy. Combining the results of the different techniques, we will elucidate the role of and vacancies during the annealing of congruent LiTaO3.

Hydrogen incorporation during crystal growth or other treatment has attracted research interest for a long time, but the diffusion paths and the role of additional defect sites within the lithium metal oxides (LiMO3 with M = Nb, and Ta) are still not fully understood. We investigated the hydrogen diffusion by crystal orientation- and light polarization-resolved FT-IR spectroscopy. The OH− stretching vibration is modified by the out- and in-diffusion of hydrogen using appropriate temperature and atmosphere conditions. An isotropic out- and in-diffusion in the congruent as-grown materials was observed. For C-LiNbO3 a higher diffusion rate and a lower activation energy than in C-LiTaO3 were found. In comparison to C-LiTaO3, a possible reason could be the Ta interstitial defect cluster, which limits the diffusion through the empty octahedral sites. The in-diffusion coefficients of reduced crystals are almost two orders of magnitude higher compared to those of the as-grown materials. Obviously, the hydrogen diffusion is promoted by the presence of oxygen and lithium vacancies. Since the defect sites are decorated with hydrogen, the hydrogen saturation concentration depends on the defect concentration. Finally, the same diffusion rate is observed for reduced LiTaO3 and LiNbO3. Consequently, vacancy formation is lifting the diffusion blocking by Ta interstitial defects.

The ferroelectric phase transition of copolymers of vinylidene fluoride (VDF) with trifluoroethylene (TrFE) is well known and related to conformational changes in the polymer chain. Contrary to the expected paraelectric behaviour in the high temperature phase a pyroelectric investigation of the phase transition in the range from 0°C to 130°C combined with X-ray diffraction indicate the copolymer as ferroelectric when prepared and polarised in the high-temperature phase. Based on this finding the orthorhombic space-group Fmm2 is proposed for the polar high-temperature phase. Above the Curie temperature the material exhibits pyroelectricity with inverted sign, whose origin is interpreted as flexoelectricity.

Hydrogen incorporation into lithium niobate and lithium tantalate during crystal growth has attracted research interest for a long time, but the diffusion paths and defect sites within the materials are only partially understood. In the present study, the hydrogen defect is investigated by crystal orientation and light polarization resolved FT-IR spectroscopy. The OH − absorption band is splitting in two sub-bands, which are significantly affected by intrinsic defects. We attribute the line broadening of both band components to the antisite defect cluster, which is decorated by hydrogen. LiNbO3 and LiTaO3 although, isomorphic in crystal and defect structure, differ significantly in their antisite defect cluster concentration and, as we show, also in the occupation of hydrogen bonding sites in the structure.

Defect engineering is an effective tool to manipulate material properties and produce completely new ones that are symmetry-forbidden in a defect-free crystal. For example, single crystals of SrTiO 3 form, as a long-term reaction to external static electric fields, a strained near-surface layer through the migration of oxygen vacancies out of the area beneath the positively charged electrode. It was previously shown that this near-surface phase exhibits pyroelectric and piezoelectric properties, which are symmetry-forbidden in centrosymmetric bulk SrTiO 3. In the present paper, different approaches have been used to better understand the nature of this reaction. In situ XRD measurements were carried out to investigate the dynamics of the lattice distortion during the formation and relaxation of this phase. Interferometry measurements were carried out to determine the piezoelectric thickness change of the samples and to indirectly investigate the polar property of the unit cell before, during, and after electroformation. We observe the instantaneous formation of a polar strontium titanate unit cell at room temperature, explainable by electrostriction, and the expected additional contribution after a long-term field application.

Separators and electrolytes provide electronic blockage and ion permeability between the electrodes in electrochemical cells. Nowadays, their performance and cost is often even more crucial to the commercial use of common and future electrochemical cells than the chosen electrode materials. Hence, at the present, many efforts are directed towards finding safe and reliable solid electrolytes or liquid electrolyte/separator combinations. With this comprehensive review, the reader is provided with recent approaches on this field and the fundamental knowledge that can be helpful to understand and push forward the developments of new electrolytes for rechargeable batteries. After presenting different types of separators as well as the main hurdles that are associated with them, this work focuses on promising material classes and concepts for next-generation batteries. First, chemical and crystallographic concepts and models for the description and improvement of the ionic conductivity of bulk and composite solid electrolytes are outlined. To demonstrate recent perspectives, research highlights have been included in this work: magnesium borohydride-based complexes for solid-state Mg batteries as well as all-in-one rechargeable SrTiO3 single-crystal energy storage. Furthermore, ionic liquids pose a promising safe alternative for future battery cells. An overview on their basic principles and use is given, demonstrating their applicability for Li-ion systems as well as for so-called post-Li chemistries, such as Mg- and Al-ion batteries.

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.

The following compendium reviews the development and establishment of optical spectroscopy as an analytical method for battery material components and electrochemical reactions. The interaction of light with matter is a sensitive and non-destructive way to characterize any sample state, i.e. solids, liquids or gases. Special attention is devoted to infrared and ultraviolet spectroscopy, covering a wavelength range from 12 μm to 200 nm, as well as Raman scattering spectroscopy, in order to excite different vibrational/rotational lattice modes and transitions of valence electrons. This allows an insight into structural properties, chemical composition, oxidation states or kinetic processes. The development of spectroelectrochemical in situ cells allows the investigation of various battery components, e.g. working and counter electrode, separator, electrolyte as well as interfaces between these components. These powerful tools allow the evaluation of the functionality, stability and safety aspects of an electrochemical storage cell.

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.

Redistribution of oxygen vacancies in a strontium titanate single crystal is caused by an external electric field. We present electrical measurements during and directly after electroformation, showing that intrinsic defect separation establishes a non-equilibrium state in the transition metal oxide accompanied by an electromotive force. A comprehensive thermodynamic deduction in terms of theoretical energy and entropy calculations indicate an exergonic electrochemical reaction after the electric field is switched off. Based on that driving force the experimental and theoretical proof of concept of an all-in-one rechargeable SrTiO 3 single crystal energy storage is reported here.

Nitrogen ion implantation into strontium titanate single crystals causes a slight shift of the Ti-K edge position compared to pristine SrTiO3 and a strong increase of the second pre-edge peak in X-ray absorption near-edge spectroscopy (XANES) using grazing incidence geometry. Calculations by a finite difference method demonstrate that the strong increase of the second pre-edge feature in the defect distorted phase can be attributed to a static displacement of the Ti atom relative to the surrounding oxygen octahedron.

Strontium titanate is a promising insulator material in resistance switching random access memories. Strontium titanate thin films are prepared by atomic layer deposition from bis(tri-isopropylcyclopentadienyl)-strontium (Sr(iPr3Cp)2), Tetrakis-(dimethylamido)titanium(IV) (Ti[N(CH3)2]4) and water at a substrate temperature of 300 °C. The layer stoichiometry is analyzed by X-ray fluorescence spectroscopy for the main element composition and by X-ray photoelectron spectroscopy to detect light element contamination. A significant carbon contamination is found whereas nitrogen is not detected. These results are discussed with possible decomposition reactions of the Sr(iPr3Cp)2 molecule at the given deposition temperature. The film microstructure is characterized by grazing incidence X-ray diffraction. Optical and electrical characterizations show that the strontium titanate layers are transparent up to an optical gap of 3.85 eV and insulating.

Oxygen migration in perovskites is well known to occur via vacancies along the TiO

The time-resolved oxygen exchange rate of strontium titanate (SrTiO3) single crystals is studied by means of oxygen solid electrolyte coulometry (OSEC) and compared to model calculations. Experiments are performed on pure, ion implanted (Ni, Ag, O and N ions) and partially covered crystals with silver layer. In this work, a theoretical model is used, which is based on defect chemistry under equilibrium conditions. It is applied as a fit in order to determine the effective rate constants and activation energy of the oxygen exchange reaction on the crystal surface. OSEC is used for the first time to characterize kinetic parameters of oxygen exchange on single crystalline surfaces. Transmission electron microscopy and sputter X-ray photoelectron spectroscopy are performed to determine structural and chemical changes after ion implantation.

Subjecting strontium titanate single crystals to an electric field in the order of 10$^{6}$ V m$^{−1}$ is accompanied by a distortion of the cubic crystal structure, so that inversion symmetry vanishes and a polar phase is established. Since the polar nature of the migration-induced field-stabilized polar (MFP) phase is still unclear, the present work investigates and confirms the pyroelectric structure. We present measurements of thermally stimulated and pyroelectric currents that reveal a pyroelectric coefficient p$_{MFP}$ in the order of 30 μC K$^{−1}$m$^{−2}$. Therefore, a dielectric to pyroelectric phase transition in an originally centrosymmetric crystal structure with an inherent dipole moment is found, which is induced by defect migration. From symmetry considerations, we derive space group P$_{4mm}$ for the MFP phase of SrTiO$_{3}$. The entire electroformation cycle yields additional information about the directed movement and defect chemistry of oxygen vacancies.

Nonstoichiometric SrTiO3 thin films were fabricated by different thin-film deposition methods. The impact on the oxide/silcon interface stability as well as the crystallization onset temperature is investigated., Different physical vapor deposition methods have been used to fabricate strontium titanate thin films. Within the binary phase diagram of SrO and TiO2 the stoichiometry ranges from Ti rich to Sr rich, respectively. The crystallization of these amorphous SrTiO3 layers is investigated by in situ grazing-incidence X-ray diffraction using synchrotron radiation. The crystallization dynamics and evolution of the lattice constants as well as crystallite sizes of the SrTiO3 layers were determined for temperatures up to 1223 K under atmospheric conditions applying different heating rates. At approximately 473 K, crystallization of perovskite-type SrTiO3 is initiated for Sr-rich electron beam evaporated layers, whereas Sr-depleted sputter-deposited thin films crystallize at 739 K. During annealing, a significant diffusion of Si from the substrate into the SrTiO3 layers occurs in the case of Sr-rich composition. This leads to the formation of secondary silicate phases which are observed by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy.

The precise quantification of the pyroelectric coefficient p is indispensable for the characterization of pyroelectric materials and the development of pyroelectric-based devices, such as radiation sensors or energy harvesters. A summary of the variety of techniques to measure p is given in the present review. It provides a classification after the thermal excitation and an outline of capabilities and drawbacks of the individual techniques. The main selection criteria are: the possibility to separate different contributions to the pyroelectric coefficient, to exclude thermally stimulated currents, the capability to measure p locally, and the requirement for metallic electrodes. This overview should enable the reader to choose the technique best suited for specific samples.

Local reversible structural changes in ${\mathrm{SrTiO}}_{3}$ single crystals in an external electric field are induced by oxygen redistribution. We present in situ x-ray diffraction measurements during and immediately after electroformation. Several reflections are monitored and show an elongation of the cubic unit cell of strontium titanate. Raman investigations verify that the expansion of the unit cell involves a transition from the centrosymmetric to a lower symmetry phase. During a complete formation cycle, including the hold time of the electric field and relaxation time without field, two different dynamics are observed for the reversible transitions from cubic symmetry to tetragonal distortion: a slow one during the increase of the lattice constant in field direction and a fast one after switching off the electric field. Based on the experimental data, we propose the formation of a polar strontium titanate unit cell at room temperature stabilized by the electric field, which is referred to as migration-induced field-stabilized polar phase.

Defect engineering is an effective and powerful tool to control the existing material properties and produce completely new ones, which are symmetry-forbidden in a defect-free crystal. For example, the application of a static electric field to a single crystal of SrTiO3 forms a strained near-surface layer through the migration of oxygen vacancies out of the area beneath the positively charged electrode. While it was previously shown that this near-surface phase holds pyroelectric properties, which are symmetry-forbidden in centrosymmetric bulk SrTiO3, this paper reports that the same phase is strongly piezoelectric. We demonstrate the piezoelectricity of this phase through stroboscopic time-resolved X-ray diffraction under alternating electric field and show that the effective piezoelectric coefficient d33 ranges between 60 and 100 pC/N. The possible atomistic origins of the piezoelectric activity are discussed as a coupling between the electrostrictive effect and spontaneous polarization of this near-surface phase.

Pure strontium titanate exhibits a cubic perovskite-type structure at room temperature, but many approaches to break this high degree of symmetry are accessible. The present review summarizes some possible methods by discussing the effects of stoichiometry variation, ion implantation, temperature treatment and external electric fields. Since oxygen vacancies are the most prominent and most mobile defect species in strontium titanate, they play a crucial role also for structural changes due to external influences. Possible functionalities range from tuning of permittivity, band gap and conductivity to field-induced structure changes, which lead to applications as solid-state battery or switchable pyroelectric.

Pyroelectric materials have a broad spectrum for practical application. Apart from the established infrared sensor technology, recently the pyroelectric effect has been employed unconventionally in waste heat recovery, X-ray generation or water disinfection. This coupling phenomenon is the temperature dependence of a ferroelectric's spontaneous polarisation. A crystal structure that allows pyroelectricity cannot have an inversion centre, if it had a polar axis would not exist. Hence the well-known perovskite strontium titanate, crystallizing in the space group Pm-3m, is known to be dielectric. Nonetheless, under an external electric field of 1MV/m charged defects like oxygen vacancies redistribute in a strontium titanate single crystal, leading to a distortion of the unit cell and subsequently to the formation of a defect structure called the migration-induced field-stabilized polar (MFP) phase [1]. Raman scattering shows that the MFP phase of strontium titanate may exhibit broken centrosymmetry, suggesting the existence of a polar axis. Here, we investigate the pyroelectric properties of strontium titanate single crystals at room temperature during these electroformation cycles with a modified Sharp-Garn method [2]. Our frequency and field dependent measurements indicate the pyroelectricity of the MFP phase. Additionally the measurement method elucidates the kinetics of the oxygen vacancy migration as well as electric properties during electroformation. Inducing pyroelectricity in a centro-symmetric crystal structure opens the scope for a new class of pyroelectric materials.

Because of their broad range of applications, electrochemical energy storage devices are the subject of a growing field of science and technology. Their unique features of high practical energy and power densities and low prices allow mobile and stationary applications. A large variety of electrochemical systems has been tailored for specific applications: Lithium-ion batteries for example have been optimized for mobile applications ranging from mobile phones to electric vehicles. On the other hand, sodium-sulphur accumulators – among others – have been developed for stationary applications to account for the capricious nature of renewable energies. Chemistry, physics and materials science have led to the optimization of existing cell-chemistries and the development of new concepts such as all-liquid or all-solid state batteries as well as high-energy density metal-air batteries. The aim of the BMBF (Federal Ministry of Education and Research, Germany)-financed project "CryPhysConcept" is to develop new concepts for electrochemical energy storage applying a crystallographic approach. First, a categorization of the main solid components of batteries based on their underlying working principles is suggested. Second, an algorithm for the identification of suitable new materials and material combinations, based on economical, ecological and material properties as well as crystallographic parameters, is presented. Based on these results, new concepts using multi-valent metal ions are proposed. Theoretical as well as experimental results including an iron-ion approach are presented.

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 migration-induced 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.

Vacuum annealing is a widely used method to increase the electric conductivity of SrTiO3 single crystals. The induced oxygen vacancies act as intrinsic donors and lead to n-type conductivity. Apart from the changed electronic structure, however, also crystal structure modifications arise from this treatment. Hence, electronic properties are determined by the interplay between point defects and line defects. The present paper provides a survey of the real structure of commercially available SrTiO3 single crystals and the changes induced by reducing vacuum heat-treatment. Therefore, all investigations were performed ex situ, i.e., after the annealing process. Used characterization methods include atomic force microscopy, transmission electron microscopy, spectroscopic ellipsometry, infrared spectroscopy, and photoluminescence spectroscopy. Besides the expected variation of bulk properties, especially surface modifications have been detected. The intrinsic number of near-surface dislocations in the samples was reduced by vacuum annealing. X-ray photoelectron spectroscopy proves the existence of a layer of adsorbed molecules, which influences the SrTiO3 work function. Also, the interaction between adsorbates and surface point defects as well as laser annealing due to local oxygen absorption are discussed.