23Na NMR Spectroscopic Quantification of the Antiferroelectric-Ferroelectric Phase Coexistence in Sodium Niobate

The irreversible field-induced phase transition between the antiferroelectric (P) and ferroelectric (Q) polymorphs of sodium niobate (NaNbO3) ceramics constitutes a focal point in improving the material's energy storage properties. The coexistence of P and Q phases can be verified by X-ray and electron diffraction methods, but its extent remains elusive. Two-dimensional solid-state nuclear magnetic resonance (NMR) spectroscopy allows the quantification of relative amounts of the coexisting polymorphs, but the analysis of ceramic sample pieces requires a trade-off between sufficient sensitivity (at higher magnetic fields) and separation of the overlapping P and Q signals (at lower magnetic fields). In this contribution, we apply the satellite transition magic angle spinning (STMAS) pulse sequence in a quantitative analysis of the antiferroelectric-ferroelectric phase transition in NaNbO3 ceramics. Both field- and grain size-induced transitions are investigated and the coexistence of the Q and P phases after the application of an electric field is quantified to be approximately 50%:50%. No indication is found that the local structure of the field-induced Q polymorph differs fundamentally from that induced in small-sized grains. Furthermore, the sensitivity and resolution of STMAS is compared to previously reported applications of the triple quantum magic angle spinning (3QMAS) sequence to the NaNbO3 system.