Ferroelectric Materials

The field of ferroelectric materials is continuously expanding towards new realms of extraordinary properties and applications. New materials have been synthesized, and more advanced properties have been obtained, opening the way to more and more cutting-edge applications. Different methods for the design of new materials through chemical and epitaxial strain have been employed. Strong progress has also been registered in the synthesis and characterization of ferroelectric materials with constrained geometries, like thin film heterostructures and nanomaterials. Epitaxial strain engineering has been employed to stabilize non-bulk phases in thin films and heterostructures. Potentialities arising from the coupling of the ferroelectric order parameter with magnetic or elastic order parameters, such as in multiferroics, have been explored, opening up new application windows. In this Special Issue, we have collected eight articles (two review papers and six original research articles) devoted to research on ferroelectric materials in a wide range of topics: piezoelectric and multiferroic single crystals and ceramics, molecular piezoelectrics, perovskite epitaxial thin films, flexible piezo-composite thick films, additive manufacturing, and sustainable “green” synthesis. In the following, we will briefly evidence of some of the important points in these papers.

Xu et al. [1] investigated frustrated magnetism and ferroelectricity in a Dy³⁺-based triangular lattice. By using the laser floating zone technique, they successfully grew a DyInO₃ crystal, which contains Dy³⁺-based triangular layers. As evidenced by the authors, such triangular lattice magnets can combine geometrically frustrated magnetism and quantum fluctuations, being potential platforms for different emergent phenomena. The desired hexagonal phase has been stabilized via the fine-tuning of Indium stoichiometry, and the crystal structure has been confirmed via an X-ray diffraction study, which revealed a non-centrosymmetric P63mc space group. The authors demonstrated switchable ferroelectric polarization at room temperature, together with strongly frustrated magnetism.

In an article devoted to multiferroic doped strontium hexaferrite [2], Nikolic et al. investigated the ferroelectric, magnetic, and dielectric properties of SrCo_0.2Zn_0.2Fe_11.6O_18.8 hexaferrite obtained via green sol–gel synthesis, by using eco-friendly organic reactants. The influence of cobalt and zinc co-doping, as well as that of synthesis process parameters on the structure and morphology and magnetic, dielectric, and ferroelectric properties, have been analyzed.

In a review paper [3], Xiao et al. explored the preparation, properties, and applications of near-stoichiometric lithium tantalate (NSLT) crystals. Piezoelectric LiTaO₃ single crystals display good piezoelectric, acousto-optic, and nonlinear optical properties, used in infrared detectors and optical and surface acoustic wave (SAW) devices. In NSLT crystals, the Li content is higher than that of congruent lithium tantalate crystal, improving the acoustic performance for the application of SAW filters in 5G communication. The article reviews the current research status in the physical properties and preparation methods, as well as the acoustics and optics of NSLT crystals.

Cordero et al. [4] studied the phase transition and dynamics of defects in the molecular ferroelectric TMCM-MnCl₃ and the effect of partial substitutions of Mn ion with Cu, Fe, or Ni (TMCM = trimethylchlorometylammonium). Dielectric and anelastic spectroscopy and PFM measurements have been performed on the molecular ferroelectric powders pressed in bars or deposited as films on Si via Matrix-Assisted Pulsed Laser Evaporation (MAPLE). The measured dielectric and elastic anomalies evidenced an improper ferroelectric transition. Moreover, two thermally activated relaxation processes have been evidenced below room temperature in the dielectric and anelastic spectra. The microscopic parameters obtained from the analysis of these peaks showed that they are due to point defects, related to Cl vacancies and their complexes with TMCM vacancies. The partial substitution of Mn with 5% Cu or Fe produces a large loss enhancement due to domain wall relaxation, while Ni substitution has little influence. The ferroelectric properties of the films below the transition temperature have been investigated via PFM.

In recent years, strong interest has been raised in unconventional fabrication techniques, which would allow for application-oriented designed components, impossible to achieve with traditional processes. Binder jetting is such an emergent additive manufacturing technique for ceramic materials. This has been employed by Mariani et al. [5] to print ceramic bodies based on niobium-doped lead zirconate titanate (PZT-N) powder. The binder jetting process and sintering parameters have been selected to obtain adequate grain size distribution and residual porosity. The piezoelectric charge and voltage constants were evaluated to assess the possible use of the printed parts as porous piezoelectric components for various applications.

Ferroelectric composite materials have opened the field of applications for devices that combine high flexibility with good piezoelectric properties. Bobic et al. [6] investigated the properties and potential application of thick piezo-composite films obtained via the hot-pressing of mixtures of lead-free Ba(Zr,Ti)O₃ or lead-based Pb(Zr,Ti)O₃ ferroelectric ceramic powders with various amounts of PVDF polymer. The ferroelectric properties were investigated, and the evaluation of storage energies and output voltage revealed an increasing trend, with the increase in the active phase content in the composite. Moreover, the piezoelectric output voltage was evaluated, and the potential use of these films as energy storage and energy harvesting systems was analyzed.

Ferroelectric thin films deposited on various substrates have been widely investigated, as well as their deposition methods and parameters. Controlling the surface morphology and composition of the substrates is a critical aspect in tuning the final properties of the deposited films and their interfaces. In Ref. [7], Leca presents a chemical etching method developed for (110) and (001) NdGaO₃ single crystal substrates to obtain a well-defined GaO_2−x-terminated surface. The results indicate that the chemically etched substrate surface has the desired GaO_2−x-terminated surface, which has a lower surface energy than the NdO_1+x termination.

Finally, Andrei et al. [8] reviewed the role that the strain engineering effects induced by different means (e.g., the substrate lattice mismatch and/or chemical doping) play in determining the functional properties of perovskite ferroelectric thin films. The effect of the strain state of the structure for the lead-free perovskite ferrite-based materials (BiFeO₃ –BFO, LaFeO₃–LFO, and related compounds) on their functional properties has been highlighted. It has been evidenced that the dielectric properties of BFO epitaxial thin films are strongly influenced by the film thickness and by the epitaxial strain induced by the lattice mismatch between the substrate and film. Doping the BFO ferroelectric perovskite with rare-earth elements or inducing a high level of structural deformation into the crystalline structure of LFO thin films have allowed for the tuning of the functional properties of these materials, such as dielectric, optical, or photocatalytic ones.

To conclude, the results presented in the original articles and review papers belonging to this volume confirm the remarkable and various advancements in this research domain. We hope that this Special Issue will be useful for future research in the field of ferroelectric materials and their applications.