Field-Induced Phase Transition of Lead-Free Antiferroelectric Niobates

Field: Materials Science (Physical Sciences)

Antiferroelectric materials represent a new generation of capacitor materials whose capacitance increases with increasing bias voltage. Such materials offer a new solution for the snubber and DC link capacitors of future power switches with wide bandgap semiconductors and are also needed in electric and hybrid vehicles. The functionality of antiferroelectrics is intimately related to their unique property of changing between the AFE and FE states using an electric field. However, very few AFE systems are known and the mechanisms of the field-induced phase transition in polycrystalline materials remain insufficiently understood. Here, we show how the irreversible AFE to FE phase transition in the lead-free AFE prototype, NaNbO3, is tailored to be reversible via compositional modification and defect chemistry engineering. Several questions regarding the mechanisms of the field-induced phase transition process in NaNbO3-based antiferroelectrics have been addressed in this work. Pure polycrystalline NaNbO3 ceramic material with 100% AFE phase was prepared, which is the basis to study the field-induced phase transition. The AFE−FE phase transition could be triggered by applying a sufficiently high electric field and the attending structural and microstructural changes were investigated by a combination of ex situ XRD, TEM, and NMR characterizations. Notably, the phase transition is of irreversible nature. To gain further insight into the phase transition process, in situ high-energy XRD was employed. A field-induced phase transition from AFE to FE first occurred without significant polarization and strain changes. Subsequently, a microstructural change and a large change in both electric charge and volume were recorded simultaneously, which hints at a domain switching process that is clearly decoupled from the phase transition. The presence of the two successive processes was also corroborated by the latent heat measurement. To achieve a reversible phase transition, a solid solution based on SrSnO3 was proposed with reference to lead-based antiferroelectrics and the Goldschmidt tolerance factor, and then validated by first-principles calculations. Based on this strategy, a series of SrSnO3-substituted ceramic materials were prepared and double polarization hysteresis loops characteristic of a reversible phase transition were obtained. The recorded remanent polarization and energy storage density are lower by a factor of 3 and higher by a factor of 7, respectively, compared to pure NaNbO3. The influence of a less distorted local structure of the sodium site and a lower orthorhombic lattice distortion on the reversibility of the phase transition is discussed. Despite the reversibility, the large remanence of the solid solution and the associated low electrical breakdown strength of the solid solution are not desirable for high density energy storage applications. To this end, the system was modified with different amounts of MnO2, resulting in 10-fold lower remanent polarization and 15-fold higher energy storage density compared to pure NaNbO3. In addition, the excellent temperature stability up to 150 °C renders the system a potential candidate for high temperature capacitor applications. The addition of Mn is believed to suppress the formation of oxygen vacancies and thus the amount of mobile defects upon application of an electric field. The structural basis for the reversibility of the AFE−FE phase transition is revealed using in situ high-energy XRD. The rigid structures with almost 70% lower atomic displacement of sodium ions compared to pure NaNbO3 are assumed to be the driving force for the restoration of the AFE order. This study has laid the foundation for penetrating the intricate and delicate interplay of composition, crystal structure, microstructure, and defect chemistry to understand how it defines the functional properties of antiferroic materials. It is expected that the strategies and approaches contained in this work will open the door to a new paradigm for the development of next-generation AFE materials.