Egert, Sonja (2022)
Local Structure-Property Relations in Antiferroelectric Oxides.
Technische Universität Darmstadt
Dissertation, Bibliographie

Kurzbeschreibung (Abstract)

The unique, electric-field-induced functionality of antiferroelectrics justifies their potential as efficient capacitor materials in a broad range of applications. As such, they offer new solutions for the power electronics required in electric vehicles or the integration of renewable energies into the grid. A structural phase transition between two closely related phases is the key to the functional properties of antiferroelectrics. For application purposes, the relative stability of these phases is tailored by compositional modification so that a reversible transition results. While state-of-the-art materials are based on lead zirconate (PbZrO3) and contain toxic lead, environmental concerns increasingly promote the search for lead-free replacements based on sodium niobate (NaNbO3). In this work, solid-state nuclear magnetic resonance (NMR) spectroscopy provides insights into the relationships between compositional modification, the local structure, and the stability of antiferroelectric (AFE) and ferroelectric (FE) phases. The composition-induced phase transition from the AFE to the FE state in barium-modified PbZrO3 is studied using 207Pb 2D-PASS NMR spectroscopy via analysis of the isotropic chemical shift, its distribution, and the chemical shift anisotropy. The destabilizing influence of the barium cations is accompanied by considerable displacive disorder on the lead-sublattice despite the lack of changes to the global structure. The FE state is characterized by increased bond covalency, which is attributed to the lower polarizability of the barium cations and the formation of stronger Pb-O bonds in their vicinity. The AFE-to-FE phase transition is characterized by a lattice expansion and the collapse of two lattice sites into one, while larger displacements are simultaneously favoured. A local structural model is proposed wherein increasing correlation of larger lead displacements culminates in FE ordering. The role of individual substituents in the well-established class of PLZST materials is studied by means of a series of model compositions with increasing complexity using 207Pb and 119Sn nuclei as local probes. The stabilization of the FE state is accompanied by the emergence of an intermediate lead environment as well as broad parameter distributions. Three main effects of the lanthanum substituents are proposed: The promotion of displacive disorder, aided by the co-occurrence of vacancies, the promotion of a continuous variation from more to less covalent Pb-O bonds, brought forward by the presence of underbonded oxygen anions, and the promotion of local lattice compressions that allow less displacive freedom of the lead cations in smaller, more symmetric, and more ionic oxygen cavities. The tin-to-titanium ratio is found to influence the bond length disorder, the distortion of lead environments, and the unit cell volume. The estimation of bond length distribution widths via an empirical correlation allows for a direct comparison to polarization mapping. The electric-field induced phase transition of NaNbO3 is studied ex situ with quantitative 23Na STMAS spectroscopy. The irreversibility of the transition manifests in a phase coexistence after application of an electric field, with the phase fractions depending on the magnitude and polarity of the applied field. Indications of microstructural texturing suggest a preferred orientation of the local dipoles. The sensitivity of the STMAS experiment is found to be superior to that of the 3QMAS experiment at comparable resolution for the study of small-volume ceramic samples, and specifically more sensitive to small phase fractions. Thus, phase quantification is more reliably carried out even when one phase fraction is dominant. A reversible phase transition is achieved in a solid solution of NaNbO3 with SrSnO3. Analysis of 23Na chemical shift and quadrupolar parameters in combination with first-principles calculations reveal an increased distribution of Na-O distances compared to NaNbO3. At the same time, the variance of this distribution decreases, indicating a less distorted local environment of the sodium ions. These structural features are found to also occur in a series of NaNbO3-based solid solutions. It is proposed that the decrease of structural distortion, alongside the corresponding decrease of octahedral tilting in the perovskite structure, may facilitate the rotation of oxygen octahedra during the transition from the AFE to the FE state, hence influencing the energy landscape related to the transition. This study offers a local structural perspective on stabilizing and destabilizing influences on AFE and FE phases and the related reversibility of the field-induced phase transition. The role of various substituents is discussed in terms of local structural descriptors such as displacive disorder, distortions, bond lengths, bond covalency, and octahedral tilting, and allows for the elucidation of structureproperty relations even in highly disordered materials. Thus, the study promotes an advanced understanding of the stabilization of AFE ordering in perovskite oxides and, ultimately, to the design of next-generation materials based thereon.

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