F. Zimmer

Bio: F. Zimmer is an academic researcher from Max Planck Society. The author has contributed to research in topics: Ionic conductivity & Phase transition. The author has an hindex of 2, co-authored 2 publications receiving 36 citations.

The conductivity anomaly in PbF2: a numerical investigation by classical MD and MC simulations

Abstract: Recently Hainovsky and Maier (N. Hainovsky, J. Maier, Phys. Rev. B 51 (1995) 15789) showed that for the Frenkel disordered material PbF2 as well as for AgI, AgCl and AgBr, the anomalous conductivity increase at high temperature can be described by a cube root term in the chemical potential of the defects, which reflects their mutual interaction in a mean field sense. For PbF2 it has been assumed that only one charge carrier is responsible for the conduction. In the case of PbF2, which undergoes a higher order phase transition, the description also includes conductivity behaviour at and above the phase transition. In this work we investigate the above model by molecular dynamics and Monte Carlo simulations. The defect concentrations and the defect energies, including excess energies, are computed as a function of temperature based on a classical semi-empirical potential that was successfully developed and applied to PbF2 by Walker et al. [A.B. Walker, M. Dixon, M.J. Gillan, J. Phys. C: Solid State Phys. 15 (1982) 4061] in an earlier work. We compute the concentration of fluoride defects as a function of temperature and investigate the mechanism of conduction. We estimate the mobility of the fluoride ions by comparing the defect concentrations with the experimental conductivity data. The results show that the conductivity anomaly is indeed essentially caused by an anomalous increase in defect concentrations and the cube-root approximation is reasonably well fulfilled. The computation also indicates a perceptible contribution of the interstitial defect conductivity for T>600 K.

Charge carrier interactions in ionic conductors: A classical molecular-dynamics and Monte Carlo study on AgI

Abstract: The equilibrium concentration of ionic and electronic charge carriers in ionic crystals as a function of temperature, concentration of dopants, and chemical environment is phenomenologically well understood as long as these point defects can be considered sufficiently dilute. However, there are cases, usually at temperatures close to the melting point, where the defects appear in higher concentrations. In these cases interactions come into play and cause anomalous increases in the conductivity or even phase transitions. Recently Hainovsky and Maier showed that for various Frenkel disordered materials this anomalous conductivity increase at high temperature can be described by a cube root term in the chemical potential of the defects. This quasi-Madelung approach does not only allow ionic conductivities and heat capacities to be computed, it also leads to a phenomenological understanding of the solid–liquid or superionic transition temperatures. In the present study we analyze this approach on the atomistic level for AgI: The defect concentrations as well as defect energies, including excess energies, are computed as a function of temperature by molecular-dynamics and Monte Carlo simulations based on a classical semiempirical potential. The simulations support the cube-root model, yield approximately the same interaction constants and show that the corrections in the chemical potential are of an energetic nature. In agreement with structural expectations, the simulations reveal that two different kinds of interstitials are present: Octahedral interstitials, which essentially determine the ionic transport at higher temperature, and tetrahedral ones, which remain substantially associated with the vacancies. It is shown how these refinements have to be introduced into the cube root.

Superionics: crystal structures and conduction processes

Abstract: Superionic conductors are compounds that exhibit exceptionally high values of ionic conductivity within the solid state. Indeed, their conductivities often reach values of the order of 1 Ω−1 cm−1, which are comparable to those observed in the molten state. Following Faraday's first observation of high ionic conductivity within the solids β-PbF2 and Ag2S in 1836, a fundamental understanding of the nature of the superionic state has provided one of the major challenges in the field of condensed matter science. However, experimental and theoretical approaches to their study are often made difficult by the extensive dynamic structural disorder which characterizes superionic conduction and the inapplicability of many of the commonly used approximations in solid state physics. Nevertheless, a clearer picture of the nature of the superionic state at the ionic level has emerged within the past few decades. Many different techniques have contributed to these advances, but the most significant insights have been provided by neutron scattering experiments and molecular dynamics simulations. This review will summarize the state of current knowledge concerning the crystal structures and conduction processes of superionic conductors, beginning with a comparison of the behaviour of two of the most widely studied binary compounds, AgI and β-PbF2. Each can be considered a parent of two larger families of highly conducting compounds which are related by either chemical or structural means. These include perovskite-structured oxides and Li+ containing spinel-structured compounds, which have important commercial applications in fuel cells and lightweight batteries, respectively. In parallel with these discussions, the relative importance of factors such as bonding character and the properties of the mobile and immobile ions (charge, size, polarizability, etc) in promoting the extensive lattice disorder which characterizes superionic behaviour will be assessed and the possibilities for predicting a priori which compounds will display high ionic conductivity discussed.

Solid‐State Ionics: Roots, Status, and Future Prospects

Abstract: This review represents the authors' view of the evolution of solid-state ionics over approximately the past 100 years. A brief history, introducing milestones of the development of this discipline, is followed by a short summary of the theory of ionic conduction in the bulk and the more recently developed theory of ionic conduction at interfaces. The central part of the article gives examples of ionic-conducting materials systems with structures ranging from one- to three-dimensional disorder. Important experimental techniques for analyzing ionic conduction, including alternating-current impedance spectroscopy, direct-current coulometry, and direct-current current-voltage measurements with blocking electrodes, are also summarized. The main technological applications, that is, batteries, solid-oxide fuel cells, electrochemical sensors, electrochromic windows, and oxygen-separation membranes, are reviewed. Finally, new concepts in solid-state ionics are presented, including the investigation of new materials (such as nanostructured phases), the study of boundaries (for example, using microelectrodes), the development of computational techniques, and the connections with other classes of materials (notably magnetic and semiconducting materials).

Ionic conduction in the solid state

Abstract: Solid state ionic conductors are important from an industrial viewpoint. A variety of such conductors have been found. In order to understand the reasons for high ionic conductivity in these solids, there have been a number of experimental, theoretical and computational studies in the literature. We provide here a survey of these investigations with focus on what is known and elaborate on issues that still remain unresolved. Conductivity depends on a number of factors such as presence of interstitial sites, ion size, temperature, crystal structure etc. We discuss the recent results from atomistic computer simulations on the dependence of conductivity in NASICONs as a function of composition, temperature, phase change and cation among others. A new potential for modelling of NASICON structure that has been proposed is also discussed.

Defect association in acceptor-doped SrTiO3: case study for Fe′TiV˙˙O and Mn″TiV˙˙O

Abstract: The association of acceptor cations (Fe′Ti and Mn″Ti) with oxygen vacancies in Fe- and Mn-doped SrTiO3 single crystals is investigated using in-situ EPR spectroscopy. Effective association enthalpies ΔassH0eff and entropies ΔassS0eff are determined, and ΔassH0eff is found to depend strongly on the dopant concentration. This dependence can be roughly described by a correction term in the chemical potential that is proportional to the cube root of the defect concentration. Extrapolation to infinite dilution yields an association enthalpy of −26 kJ mol−1. The association of oxygen vacancies can significantly reduce the ionic conductivity of these materials at temperatures up to ca. 200 °C (Fe′Ti) or even higher (Mn″Ti), and must therefore be taken into account in the respective defect chemical models at moderate or low temperatures.

Lead difluoride and related systems

Abstract: Properties of lead difluoride are considered. Phase equilibria in PbF2–MFn systems (n = 1–5) and data on compounds and solid solutions formed in these systems are analysed. The characteristic features of phase diagrams of systems involving lead difluoride are discussed. Composites of PbF2 with other inorganic compounds hold promise as a basis for the design of new functional materials. The bibliography includes 331 references.