Investigation of transitions between the M-phases in AgNbO3 based ceramics

Abstract

Ceramics based on AgNbO₃ show field-induced phase transitions from antiferroelectric (AFE) to ferroelectric (FE) phases which are of interest for high power energy storage in dielectric capacitors. AgNbO₃ shows complex phase behaviour, with the so-called M-phases showing little obvious differences in average structure, but significant differences in electrical behaviour. Here, through an A-/B-site co-doping approach, in the system Ag_(1−3x)Bi_xNb_0.8Ta_0.2O₃, we stabilise at room temperature the M₃ phase and the M₂ phase above the dipole freezing temperature (T_f); the latter we designate here as M_2b to distinguish it from the phase below T_f (M_2a). Investigations of dielectric and structural behaviour reveal that both M_2b and M₃ phases are AFE in nature, with an average structure that can be described in the orthorhombic space group Pbcm. However, local polar regions, which are evidenced by transmission electron microscopy, can be observed within the AFE matrix in both M_2b and M₃ phases. Within the M-phases, the AFE phase becomes increasingly more stable towards the field-induced AFE → FE transition in the order M₁ < M_2a < M_2b < M₃, which can be attributed to the relative extent of local polar regions in each of the phases.