The macroscopic dielectric behaviour of barium titanate (BaTiO3, BT) based perovskites is highly dependent on the local polar order (i.e. at a length scale of 10 unit cells or below), which greatly impacts the frequency dependence and temperature stability of the relative dielectric permittivity, the tunability, and the recoverable energy density. In particular, there is a strong difference whether BT is substituted at either the A- or B-site of the perovskite lattice, and also whether the substituent is homovalent or heterovalent. On both perovskite sites, heterovalent substituents demonstrate to cause stronger effects in terms of the disruption of the long-range ferroelectric order, which is at the basis of the aforementioned modification of dielectric properties.

In this work, we review these fascinating structure-property relationships by changing the viewpoint: Instead of focusing on the correlations of local dipoles, we investigate how substitution gives rise to local regions breaking these correlations, which we name Polarization Decorrelation Regions (POLDERs). POLDERs are intended as the defect-related entities disrupting the spatial long-range correlation of B-site displacements, which are at the root of ferroelectricity in BT and related solid solutions. We show that difference in substituents’ valence or ionic radii, and the related emergence of diffuse phase transition, relaxor, or dipolar glass behaviour, can be associated to the size and distribution of POLDERs. We present here results from literature and own work on multiple materials, including A-site and B-site homovalent (Sr2+ and Zr4+, respectively) and heterovalent (La3+ and Nb5+, respectively) substituents, using various experimental and computational methodologies. We show that, in heterovalent-substituted BT, the charge-compensating defects (Ti vacancies) strongly influence the lattice volume and polar order, which is thus the main driving force for the observed strong lattice disorder of that system. Furthermore, we show how, using large-scale Molecular Dynamics (MD) simulations, a correlation between POLDERs and macroscopic dielectric properties (permittivity, energy density) can be found.

Lead-free ferroelectric oxides, heralded as eco-friendly alternatives to lead containing materials, find versatile applications in electronic devices, sensors, and energy storage systems. This study investigates lead-free BaTiO3 perovskite ceramics, focusing on the synergistic effects of simultaneous homovalent (Zr4+) and heterovalent (Nb5+) substitution. With a focus on piezoelectric, ferroelectric, and relaxor behavior, our comprehensive analyses demonstrate that 5% Zr and 3% Nb co-substitution induce room-temperature relaxor behavior, resulting in elevated permittivity and maximum polarization compared to single-element substitutes. Bipolar strain measurements reveal substantial large-signal d33* values (~250 pm/V) across a wide temperature range (–50 °C to 30 °C) for ceramics with 5% Zr and 2% Nb co-substitution, which is associated with the presence of a diffuse phase transition. Additional insights from frequency-dependent permittivity measurements, modified Curie-Weiss law analyses, and Raman spectroscopy contribute to advancing our understanding of homovalent (Zr4+) and heterovalent (Nb5+) substitution in BaTiO3 ceramics. This research opens avenues for tailoring properties in applications requiring high recoverable energy storage density and stable piezoelectric performance over a certain temperature range, making these materials attractive for underwater or in-body applications, given the biocompatibility of BaTiO3-based compositions.

The stability of the field-induced ferroelectric state in relaxor Na₀.₅Bi₀.₅TiO₃ (NBT) and NBT-based ceramics has been a long-standing problem. It has been widely assumed that depolarization temperature and depolarization process are related to the grain size in these materials. We report a detailed piezoresponse force microscopy (PFM) study of two NBT ceramic samples with substantially different grain sizes due to different sintering temperature. Non-poled, macroscopically poled, and locally PFM-poled states were studied, paying close attention to the decay of the induced ferroelectric state at the depolarization temperature. Although the observed domain morphology is grain size-dependent, room temperature measurements show no correlation between grain size and PFM hysteresis loop parameters. We show that, in spite of dependence of the depolarization temperature on sintering temperature, it is not directly related to the grain size. The significant asymmetry of the PFM hysteresis loops at elevated temperatures is attributed to the internal field created by oxygen vacancies accumulated at the internal boundary of the polarized region.

We use X-ray linear dichroism at the Ti L2,3 edge in Energy filtered PhotoEmission Electron Microscopy (XLD-PEEM) to identify the ferroelastic strain states at the surface of a CaTiO3 single crystal.
Below 1300°C, CaTiO3 is ferroelastic with an orthorhombic structure. The oxygen octahedra defines the strain state in a given ferroelastic domain tilts. Ferroelastic domains are separated by twin walls where one of the tilts goes to zero, allowing the central Ti cation to off-center, creating polarity in the plane of the wall. At the surface, the ferroelastic ordering gives rise to a physical topography characterized by the twin angle, which is intimately connected to the domain strain state and determines the twin orientation and polarity. Knowledge of the strain state in each domain is crucial to understanding and possibly controlling the twin wall polarity.
PEEM is a non-destructive surface-sensitive imaging technique with both physical and electrical topography giving rise to contrast. Although the strain states in domain twins are symmetry-related, the tilt angle changes the d orbital orientation. It is then possible to use the difference between vertically and horizontally polarized light to probe these angular differences using linear dichroism. We image microscopic regions of the sample surface containing several ferroelastic domains in XLD-PEEM and map out the domain by domain strain state at the surface.
From symmetry conditions, there are six possible ferroelastic strain states, described by strain tensors. We have simulated the XLD spectra using the FDMNES package to identify the strain state in each ferroelastic domain of our XLD-PEEM observations.
Our results demonstrate the observation of the surface strain state via linear dichroic analysis of the octahedral tilts in pure ferroelastic CaTiO3. The XLD-PEEM analysis allows us to evaluate the possible correlation with the magnitude of the twin polarity.