Phase transitions in bismuth-modified silver niobate ceramics for high power energy storage

Abstract

Ceramics of composition Ag_1−3xBi_xNbO₃ (0.005 ≤ x ≤ 0.040) were prepared by solid state methods and their structure and electrical behavior were characterized with a view to their potential use as high power energy storage materials. All compositions exhibited an average orthorhombic non-polar structure. The low temperature phase transitions M₁ ↔ M₂ and M₂ ↔ M₃ and the freezing temperature T_f found in AgNbO₃ are increasingly shifted to lower temperatures with increasing x-value. Similarly to AgNbO₃, the structure of the M₂ phase above T_f is antiferroelectric. In the M₂ phase, below T_f it is proposed that the structure possesses local polar regions, which can expand during electrical loading within an average non-polar antiferroelectric matrix. The polar domains found in the M₁ phase of AgNbO₃ diminish with increasing bismuth content, as confirmed by the suppression of domain switching peaks in the current–polarization–electric field hysteresis loops. The antiferroelectric to ferroelectric electric field-induced transformation is progressively hindered, as the structure evolves towards long-range antiferroelectric order with increasing bismuth content. Moreover, up to a certain substitution level, bismuth addition is seen to enhance energy storage properties compared to unsubstituted AgNbO₃, with a high energy storage density of 2.6 J cm⁻³ and high energy efficiency of 0.86 achieved. These values make these materials amongst the best performing energy storage lead-free ceramics currently known.