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.

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High-resolution scanning transmission electron microscopy (STEM) enjoys great advantages for atomic-resolution visualization of the atomic structure, while failing to disclose structural information along the atomic columns. On the other hand, solid-state nuclear magnetic resonance (NMR) spectroscopy is highly sensitive to the three-dimensional, local structure around atoms in the bulk sample but typically cannot provide an intuitive visualization of the structure. Thus, the combination of atomic-resolution (S)TEM and solid-state NMR spectroscopy has the potential to establish an in-depth, multidimensional structural understanding. Here, we explore this novel strategy to probe the structure of antiferroelectric perovskite oxides PbZrO₃ and (Pb,La)(Zr,Sn,Ti)O₃. We combine complementary information regarding the in-plane displacement vector mapping from STEM with the analysis of local PbO₁₂ environments from ²⁰⁷Pb NMR spectroscopy to provide unprecedented insight into Pb displacements. For PbZrO₃, an ordered 4-fold in-plane displacement modulation is clearly revealed via STEM imaging; meanwhile, the out-of-plane information is provided by two discrete ²⁰⁷Pb NMR signals attributed to two crystallographic Pb sites in the 2D-PASS NMR spectrum. In the chemically modified (Pb,La)(Zr,Sn,Ti)O₃ system, disorder of the structure manifests in not only an inhomogeneous displacement modulation but also a broad distribution of ²⁰⁷Pb chemical shifts, related to significant disorder of displacement magnitudes and a favoring of larger displacements. We show that the displacement distribution depends on whether both in-plane and out-of-plane displacements or only out-of-plane displacements are considered. Our findings demonstrate the advantages in the structural analysis using combined TEM and NMR approaches, hence laying the foundation work for controlling and optimizing functional properties.

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Lead zirconate (PbZrO₃, PZ) is a prototype antiferroelectric (AFE) oxide from which state-of-the-art energy storage materials are derived by chemical substitutions. A thorough understanding of the structure-property relationships of PZ-based materials is essential for both performance improvement and the design of more environmentally friendly replacements. (Pb_1−xBa_x)ZrO₃ (PBZ) can serve as a model system for studying the effect of A-site substitution in the perovskite lattice, with barium destabilizing the AFE state. Here, the two-dimensional ²⁰⁷Pb solid-state NMR spectra of PZ and PBZ were recorded to analyze the local structural role of barium substitution. At low substitution levels, ²⁰⁷Pb NMR spectroscopy reveals the presence of Pb-O bond length disorder. Upon crossing the threshold value of x for the macroscopic phase transition into a ferroelectric (FE) state, the barium cations cause local-scale lattice expansions in their vicinity, resulting in the collapse of two lead lattice sites into one. The stabilization of the larger volume site coincides with the favoring of larger lead displacements. We also observed more covalent bonding environments which may originate from the lower polarizability of the barium cations, facilitating the formation of stronger Pb-O bonds in their vicinity. From the local structural point of view, we propose that the substitution-induced AFE → FE phase transition is therefore related to an increasing correlation of larger lead displacements in larger oxygen cavities as the barium content increases. Our results also highlight ²⁰⁷Pb NMR spectroscopy as a valuable method for the characterization of the structure-property relationships of PbZrO₃-based AFE and FE oxides.

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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, 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.@en

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.

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