Unlike metals where dislocations carry strain singularity but no charge, dislocations in oxide ceramics are characterized by both a strain field and a local charge with a compensating charge envelope. Oxide ceramics with their deliberate engineering and manipulation are pivotal in numerous modern technologies such as semiconductors, superconductors, solar cells, and ferroics. Dislocations facilitate plastic deformation in metals and lead to a monotonous increase in the strength of metallic materials in accordance with the widely recognized Taylor hardening law. However, achieving the objective of tailoring the functionality of oxide ceramics by dislocation density still remains elusive. Here a strategy to imprint dislocations with {100}<100> slip systems and a tenfold change in dislocation density of BaTiO₃ single crystals using high-temperature uniaxial compression are reported. Through a dislocation density-based approach, dielectric permittivity, converse piezoelectric coefficient, and alternating current conductivity are tailored, exhibiting a peak at medium dislocation density. Combined with phase-field simulations and domain wall potential energy analyses, the dislocation-density-based design in bulk ferroelectrics is mechanistically rationalized. These findings may provide a new dimension for employing plastic strain engineering to tune the electrical properties of ferroics, potentially paving the way for advancing dislocation technology in functional ceramics.

@en

Tailoring the electromechanical properties of a material without altering the original composition is an emerging phenomenon for the optimization of functional properties. Post-sintering annealing with varying maximum temperatures, cooling rates, and atmospheres can influence the crystallographic phases, domain structures, conductivity, mechanical properties, and the temperature stability of the electromechanical properties. K_1/2Bi_1/2TiO₃ (KBT) is a high-temperature stable >280 °C A-site complex perovskite piezoelectric and is critical for high-temperature applications. However, the influence of annealing conditions on crystal structure, domain structure, and functional properties is not well-known. This work demonstrates the effect of annealing cooling rate and maximum temperature on the macroscopic electromechanical response as well as the crystal and domain structure. It is shown that the room-temperature state of KBT can be reversibly switched between the ferroelectric and relaxor state, where the slow cooling from 900 °C favors the stabilization of the relaxor state and quenching induces the ferroelectric state. Importantly, the quenched sample showed a stable piezoelectric coefficient up to 368 °C in the depolarization temperature, an increase of 78 °C. The origin of ferroelectric-relaxor state change is proposed to be related to the A-site cation redistribution and the associated change in the crystal structure and domain structure.

@en

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.

@en

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.

@en

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

The formation of associated defects (e.g.[Al _Ti-V _O]˙) upon acceptor doping is commonly seen as a reason for trapping of mobile vacancies in perovskite ionic conductors and electromechanical hardening in piezoelectric perovskites. In order to clarify the presence of associated defects in Al-doped (Na _1/2,Bi _1/2)TiO ₃(NBT-Al) and Al-substituted ((Na,K) _1/2Bi _1/2)TiO ₃-BiAlO ₃(NKBT-BA), we employ a combination of impedance spectroscopy, ²⁷Al NMR spectroscopy, and electronic structure calculations. Our results indicate that associated defects between and oxygen vacancies can only be found in case of low acceptor doping concentrations. This suggests a decreased driving force for defect association at high doping concentrations as the reason for the non-linear dependence between acceptor concentration and oxygen ionic conductivity for NBT-based ceramics. Furthermore, the combination of experimental and theoretical techniques provides clear evidence for the successive occupation of the B-site, the A-site, and finally the formation of a secondary phase with increasing Al ³⁺content. Altogether, these results call for a new evaluation of the interaction between aliovalent dopants and O ²⁻vacancies in acceptor-doped functional oxides, with implications for the design of ionic conductors as well as ferroelectric materials.

@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.

@en