Isolation of a ferroelectric intermediate phase in antiferroelectric dense sodium niobate ceramics

Switchable ferroelectric/antiferroelectric ceramics are of significant interest for high power energy storage applications. Grain size control of this switching is an interesting approach to controlling polarization and hence dielectric properties. However, the use of this approach in technologically relevant ceramics is hindered by difficulty in fabricating dense ceramics with small grain sizes. Here an intermediate polar ferroelectric phase (P2₁ma) has been isolated in dense bulk sodium niobate ceramics by grain size control through spark plasma sintering methods. Our findings, supported by XRD, DSC, P-E (I-E) loops and dielectric characterization, provide evidence that the phase transition from the antiferroelectric (AFE) R-phase, in space group Pnmm, above 300 °C, to the AFE P-phase, in space group Pbma, at room temperature, always involves the polar intermediate P2₁ma phase and that the P2₁ma → Pbma transition can be suppressed by reducing grain size.