Antiferroelectric (AFE) materials are highly interested in high-density energy storage and electrocaloric applications. PbZrO₃-based antiferroelectric materials have been widely studied. However, alternative lead-free AFE materials have become attractive to researchers due to toxicity and environmental concerns. Among them, alkali niobates with perovskites ABO₃ structures are a technologically important material with diverse applications in sensors, actuators, optoelectronics, solid-state cooling, energy harvesting, and energy storage. The fundamental principles that govern antiferroelectric (AFE) and ferroelectric (FE) transitions are not well understood for many solid solutions of perovskite compounds. In this work, we studied the local atomic mechanism of NaNbO₃-based solid solutions as the alternative to lead-free ferroelectric-antiferroelectric materials.

In NaNbO₃, the atomic structural mechanism for ferroelectric-antiferroelectric transformation was studied based on the time-of-flight neutron scattering and x-ray diffraction as a function of temperature. We have used combined x-ray and neutron powder diffraction and neutron pair distribution function analyses to characterize the relationship between the average and local structural phases of NaNbO₃, especially regarding transformations between the AFE and FE phases as a function of temperature. Here, based on an analysis of
X-ray and neutron diffraction data, we found that the coexistence of the FE Q (P2₁ma) and AFE P (Pbma) phases in the temperature range of 300 K ≤ T ≤ 615 K, while PDF analysis indicated that the local structure (r < 8 Å) is better described by a P2₁ma symmetry. Above 615 K, the average structure transitions to an AFE R phase (Pmmn or Pnma), while PDF analysis shows an increased disordering of the octahedral distortions and Na displacements at the local scale. The temperature-dependent changes in the average and local structure indicate that P to Q phase transitions in NaNbO₃ can be described as a result of the twinning of local P2₁ma structure blocks below 615 K. In contrast, the transition to R results from off-centered displacements of the Na atoms within the oxygen dodecahedron.

In K_xNa_1-xNbO₃ (x ≤ 0.02), we provide insights into how the relative local distortions around the A- and B-sites of the ABO₃ perovskite structure affect the AFE/FE order of the average crystallographic phases using neutron total scattering. The neutron Bragg diffraction data analysis indicated the coexistence of the AFE P(Pbma) and the FE Q (P2₁ma) phase at room temperature, and the Q phase fraction dramatically increased 95% for the composition of x ~ 0.02. Analysis of the neutron pair distribution function indicates that the P2₁ma structure is gradually stabilized over a longer interatomic distance with increasing substitution of K for Na at the A-site. Moreover, A-site substitution with a larger alkali ion, such as K, reduces the
distortion around the A-site distortion enough to tip the balance and stabilize the FE Q phase. Our analysis provides a rational basis to understand the composition-dependent changes in the volume fractions of AFE/FE phases in the (K_xNa_1-x) NbO₃ solid solutions, which is centered on a completion between the local atomic structural distortions around A- and B- sites of the ABO₃ perovskite structure. We show that a higher (lower) ratio of B-site centered distortions over A-site centered distortions drive transition towards a long-range FE (AFE) phase.

In (1-x) NaNbO_3-xCaZrO₃ (x ≤ 0.06), we explained the microscopic mechanism for AFE-like behavior and enhanced energy storage capacity on NNCZ ceramics based on their detailed structural characterization by neutron and Raman scattering experiments. We show that Ca/Zr doping increases the average AFE phase (Pbma) fraction compared to undoped NaNbO₃. However, the material remains as a composite of both FE (P2₁ma) and AFE phase regions. The apparent increase in the AFE phase percentage is explained as a result of a higher density of 180̊ twin boundaries, which are formed due to the introduction of microscopic charge disorder. Furthermore, we assert that the stability of the AFE-like behavior during electrical cycling in Ca/Zr-doped NaNbO₃ is principally due to the pinning of the twin boundaries by defect dipoles rather than due to an increase in the AFE crystallographic phase.

Our study provides a predictive tool for designing complex solid-solution perovskites with tunable (anti)ferroelectric polarization properties, which can be of interest for various energy-related applications such as high-density energy storage and solid-state cooling.