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

Ag(Nb _0.8 Ta _0.2 )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 AgNbO ₃ , 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 AgNbO ₃ 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 Ta ⁵⁺ 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 °C, at an applied field of 22 MV m ^-1 .