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

Ag(Nb0.8Ta0.2)O-3 is used here as a model system to shed light on the nature of the low temperature phase behavior of the unsubstituted parent compound AgNbO3, which is an important material for high-power energy storage applications. The three dielectric anomalies previously identified as M-1 M-2, T-f and M-2 M-3 transitions in AgNbO3 ceramics are found to be intimately related to the polarization the behavior of the B-site cations. In particular, the M-1 M-2 transition is found to involve the disappearance of original ferroelectric polar structure in the M-1 phase. Analysis of weak-field and strong field hysteresis loops in the M-2 region below T-f suggests the presence of a weakly-polar structure exhibiting antipolar behavior (i.e., a non-compensated antiferroelectric), which can be considered as ferrielectric (FIE). Modeling of the permittivity data using the Curie-Weiss law indicates that the Curie temperature is close to the freezing temperature, T-f, which can be regarded as the Curie point of the FIE phase. Substitution by Ta5+ in this system enhances the stability of the weakly polar/antiferroelectric state, giving rise to an increased energy storage density of 3.7 J cm(-3) under an applied field of 27 MV m(-1), one of the highest values ever reported for a dielectric ceramic. Furthermore, the energy storage capability remains approximately constant at around 3 J cm(-3) up to 100 degrees C, at an applied field of 22 MV m(-1).