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

Ceramics of composition Ag 1−3x Bi x NbO 3 (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 M 1 ↔ M 2 and M 2 ↔ M 3 and the freezing temperature T f found in AgNbO 3 are increasingly shifted to lower temperatures with increasing x -value. Similarly to AgNbO 3 , the structure of the M 2 phase above T f is antiferroelectric. In the M 2 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 M 1 phase of AgNbO 3 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 AgNbO 3 , 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.