Na-23 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 (NaNbO 3) 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 thesatellite 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, thesensitivity and resolution of STMAS is compared to previously reported applications of the triple quantum magic angle spinning (3QMAS) sequence to the NaNbO3 system.Accepted Author ManuscriptRST/Storage of Electrochemical Energ

Design of lead-free antiferroelectric (1 − X)NaNbO₃−xSrSnO₃ compositions guided by first-principles calculations

Antiferroelectric materials exhibit a unique electric-field-induced phase transition, which enables their use in energy storage, electrocaloric cooling, and nonvolatile memory applications. However, in many prototype antiferroelectrics this transition is irreversible, which prevents their implementation. In this work, we demonstrate a general approach to promote the reversibility of this phase transition by targeted modification of the material's local structure. A new NaNbO3-based composition, namely (1− x)NaNbO3−xSrSnO3, was designed with a combination of first-principles calculations and experimental characterization. Our theoretical study predicts stabilization of the antiferroelectric state over the ferroelectric state with an energy difference of 1.4 meV/f.u. when 6.25 mol % of SrSnO3 is incorporated into NaNbO3. A series of samples was prepared using solid-state reactions, and the structural changes upon SrSnO3 incorporation were investigated using X-ray diffraction and 23Na solid-state nuclear magnetic resonance spectroscopy. The results revealed an increase in the unit cell volume and a more disordered, yet less distorted local Na environment, which were related to the stabilization of the antiferroelectric order. The SrSnO3-modified compositions exhibited well-defined double polarization loops and an eight times higher energy storage density as compared to unmodified NaNbO3. Our results indicate that this first-principles calculations based approach is of great potential for the design of new antiferroelectric compositions.Accepted Author ManuscriptRST/Storage of Electrochemical Energ

Tailoring high-energy storage NaNbO₃-based materials from antiferroelectric to relaxor states

Reversible field-induced phase transitions define antiferroelectric perovskite oxides and lay the foundation for high-energy storage density materials, required for future green technologies. However, promising new antiferroelectrics are hampered by transition´s irreversibility and low electrical resistivity. Here, we demonstrate an approach to overcome these problems by adjusting the local structure and defect chemistry, delivering NaNbO3-based antiferroelectrics with well-defined double polarization loops. The attending reversible phase transition and structural changes at different length scales are probed by in situ high-energy X-ray diffraction, total scattering, transmission electron microcopy, and nuclear magnetic resonance spectroscopy. We show that the energy-storage density of the antiferroelectric compositions can be increased by an order of magnitude, while increasing the chemical disorder transforms the material to a relaxor state with a high energy efficiency of 90%. The results provide guidelines for efficient design of (anti-)ferroelectrics and open the way for the development of new material systems for a sustainable future.RST/Storage of Electrochemical Energ