Local Structure and Disorder in PbZrO3- and NaNbO3-based Antiferroelectrics probed with NMR Spectroscopy
Dr Sonja Egert University of St Andrews School of Chemistry
The sensitivity of solid-state NMR spectroscopy to octahedral tilting, distortions, and disorder makes it a powerful technique for the characterization of the local structure in perovskite oxides. In this talk, I will show how features of the local structures of antiferroelectric (AFE) oxides can be characterised with NMR techniques. Selected examples will be accompanied by an overview on the information attainable with NMR spectroscopy, as well as guidance on how to read the discussed spectra tailored towards the non-spectroscopist.
In the work presented, the 207Pb and 23Na nuclei are used as local probes to elucidate relationships between the compositional modification, local structure, and phase stability of AFE oxides based on the analysis of two-dimensional NMR line shapes. The composition-induced phase transition from the AFE to the FE state in barium-modified PbZrO3 is studied via the analysis of the isotropic chemical shift, its distribution, and its anisotropy, and a local structural model is proposed. 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. Meanwhile, the FE state exhibits increased bond covalency. The phase transition is then characterised by a lattice expansion and the collapse of two sites into one, while larger displacements are simultaneously favoured.
Ex-situ NMR studies shed light on the effects of electric fields in NaNbO3-based AFE, specifically the field-induced phase transition. The irreversibility of the phase transition in pristine NaNbO3 is accompanied by a phase coexistence which can be accurately quantified using NMR spectroscopy. In NaNbO3-based solid solutions, the analysis of chemical shift and quadrupolar parameters in combination with first-principles calculations reveals an increased distribution of Na-O distances. At the same time, the variance of this distribution decreases, indicating a less distorted local environment of the sodium ions.
Sonja Egert is a postdoctoral research fellow at the University of St Andrews, UK. Her research focuses on elucidating structure-property relations by solid-state NMR spectroscopy, with a particular interest in disordered materials. Sonja received a master's degree in Chemistry in 2018 and a PhD in 2022 from the Technical University of Darmstadt, Germany. During their doctoral studies as a FLAME member in the group of Prof Gerd Buntkowsky, they established methods for the study of structure-property relations in perovskite oxides with NMR spectroscopy, providing a local structural perspective on stabilizing and destabilizing influences on (anti)ferroelectric phases. Sonja is currently working in the field of oxide catalysts.
About the FLAME-inars
The FLAME-inars are organized by the collaborative project FLAME at TU Darmstadt, in which electronic-structure-property relationships are being developed and exploited to realize novel lead-free antiferroelectric compounds. The seminars will gather experts in processing, characterization and theory to discuss materials and applications, bulk and thin films, fundamental properties, electronic structure & defects, and related aspects.