Revealing the solid-state processing mechanisms of antiferroelectric AgNbO3 for energy storage

Abstract

AgNbO3 is one of the prominent lead-free antiferroelectric (AFE) oxides, which readily exhibits a field-induced AFE to ferroelectric phase transition and thus a high energy storage density. The solid-state synthesis of AgNbO3 is considered difficult and an oxidizing atmosphere is typically employed during AgNbO3 processing, on the premise that oxygen can prevent possible decomposition of the silver oxide at high temperatures. However, details about the influence of processing parameters on the functional properties of AFE AgNbO3 are insufficiently understood. In this work, the solid-state reaction of a stoichiometric AgO and Nb2O5 mixture was investigated. We found that ball milling can convert AgO into metallic Ag, which is beneficial for lowering the reaction temperature for the formation of the perovskite phase to 500‒600℃. Moreover, the influence of the processing atmosphere (air, O2, and N2) was investigated by thermal analysis and in situ X-ray diffraction. Since the reaction between Ag and Nb2O5 to form AgNbO3 requires oxygen uptake, AgNbO3 was only found to form in air and O2, whereby the kinetics were faster in the latter case. All the sintered AgNbO3 samples exhibited a similar crystallographic structure, although the samples processed in O2 had a lower oxygen vacancy concentration. Despite this, well-defined AFE double polarization loops were obtained in all cases. Our results indicate that decomposition of sliver oxide during ball milling is beneficial for the solid-state reaction, while a pure O2 atmosphere is not essential for the synthesis of high-quality AgNbO3. These findings may simplify the processing and facilitate further research of AgNbO3-based antiferroelectrics.