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Despite being applied with success in many fields of materials science, Raman spectroscopy is not yet determinant in the study of electroceramics. Recent experimental and theoretical developments, however, should increase the popularity of Raman spectroscopy in this class of materials. In this Review, we outline the fields of application of Raman spectroscopy and microscopy in various electroceramic systems, defining current key bottlenecks and explaining promising recent developments. We focus our attention to recent experimental developments, including coupling Raman spectroscopy with other methodologies, and modelling approaches involving both the model-based data interpretation and the ab initio calculation of realistic Raman spectra.

Ruthenium dioxide (RuO$_2$) thin films were synthesized by Chemical Solution Deposition (CSD) on silicon substrates using only water and acetic acid as solvents. The microstructure, phase-purity, electrical and optical properties as well as the thermal stability of the thin films have been characterized. The microstructure of the thin films strongly depends on the annealing temperature: A smooth thin film was achieved at an annealing temperature of 600$^\circ$C. Higher annealing temperatures (800$^\circ$C) led to radial grain growth and an inhomogeneous thin film. A very low resistivity of 0.89 {\Omega}m was measured for a 220 nm-thick thin film prepared at 600$^\circ$. The resistivity of the thin films increases with temperature, which indicates metallic behavior. Phase-purity of the thin films was confirmed with X-ray Diffraction (XRD) measurements, X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy. Transmission and reflectivity measurements indicate that RuO$_2$ efficiently blocks the UV-VIS and IR wavelengths. The optical constants determined via spectroscopic ellipsometry show high absorption in the near-IR region as well as a lower one in the UV-VIS region. The thermal stability was investigated by post-annealing, confirming that the thin films are stable up to 750$^\circ$C in synthetic air., Comment: Journal of Sol-Gel Science and Technology (2023)

The perovskite (1-x)YMnO$_3$-xBiFeO$_3$ binary system is very promising because of its multiferroic end members. Nanocrystalline phases have been recently obtained by mechanosynthesis across the system, and the perovskite structural evolution has been described. Two continuous solid solutions with orthorhombic Pnma and rhombohedral R3c symmetries were found, which coexist within a broad compositional interval of 0.5 < x < 0.9. This might be a polar-nonpolar morphotropic phase boundary region, at which strong phase-change magnetoelectric responses can be expected. A major issue is phase decomposition at moderate temperatures that highly complicates ceramic processing. This is required for carrying out a sound electrical characterization and also for their use in devices. We present here the application of Spark Plasma Sintering to the ceramic processing of YMnO$_3$-BiFeO$_3$ phases. This advanced technique, when combined with nanocrystalline powders, allowed densifying phases at reduced processing temperatures and times, so that perovskite decomposition was avoided. Electrical measurements were accomplished, and the response was shown to be mostly dominated by conduction. Nonetheless, the intrinsic dielectric permittivity was obtained, and a distinctive enhancement in the phase coexistence region was revealed. Besides, Rayleigh-type behaviour characteristic of ferroelectrics was also demonstrated for all rhombohedral phases. Magnetic characterization was performed in this region, and antiferromagnetism was shown.

It is well known that disordered relaxor ferroelectrics exhibit local polar correlations. The origin of localized fields that disrupt long range polar order for different substitution types, however, is unclear. Currently, it is known that substituents of the same valence as Ti4+ at the B-site of barium titanate lattice produce random disruption of Ti-O-Ti chains that induces relaxor behavior. On the other hand, investigating lattice disruption and relaxor behavior resulting from substituents of different valence at the B-site is more complex due to the simultaneous occurrence of charge imbalances and displacements of the substituent cation. The existence of an effective charge mediated mechanism for relaxor behavior appearing at low (< 10%) substituent contents in heterovalent modified barium titanate ceramics is presented in this work. These results will add credits to the current understanding of relaxor behavior in chemically modified ferroelectric materials and also acknowledge the critical role of defects (such as cation vacancies) in lattice disruption, paving the way for chemistry-based materials design in the field of dielectric and energy storage applications.

We report on the synthesis of complex lead-free perovskite-type (1-x)(Ba$_{0.8}$Ca$_{0.2}$)TiO$_3$-xBi(Mg$_{0.75}$W$_{0.25}$)O$_3$ (BCT-xBMW) and (1-x)(Ba$_{0.8}$Ca$_{0.2}$)TiO$_3$-xBi(Mg$_{0.75}$Mo$_{0.25}$)O$_3$ (BCT-xBMM) solid solutions via conventional solid-state reaction route. The sintering temperature was adjusted as a function of composition x to obtain dense samples (relative densities over 95%) at the same time minimizing bismuth evaporation. X-ray diffraction analysis shows formation of single-phase perovskites for $0 \le x \le 0.10$ in the BCT-xBMW series and increasing concentrations of impurity phases for $x \ge 0.15$ and for $x \ge 0.05$ in BCT-xBMM. A transition from a tetragonal to pseudo-cubic perovskite structure is observed in BCT-xBMW and BCT-xBMM at $x = 0.05$. The dielectric response has been characterized between -60 $^\circ$C and 300 $^\circ$C for BCT-xBMW, and between 30 $^\circ$C and 300 $^\circ$C for BCT-xBMM using impedance spectroscopy, showing a transition from ferroelectric to relaxor-like behavior at $x \ge 0.05$. Additional polarization and Raman spectroscopy measurements reveal the occurrence of highly disordered systems. Analysis of the Raman spectra indicates structural phase changes and lattice modifications caused by chemical substitution. For the composition 0.8Ba$_{0.8}$Ca$_{0.2}$TiO$_3$-0.2Bi(Mg$_{0.75}$W$_{0.25}$)O$_3$ a temperature-stable permittivity of about 600 ($\pm 15$% between 60 $^\circ$C and 300 $^\circ$C) and small losses of $\tan\delta < 0.02$ for $T \le 230$ $^\circ$C at 1 kHz are observed, making it a suitable dielectric material for high temperature capacitors.

Raman spectroscopy is an advantageous method for studying the local structure of materials, but the interpretation of measured spectra is complicated by the presence of oblique phonons in polycrystals of polar materials. Whilst group theory considerations and standard ab initio calculations are helpful, they are often valid only for single crystals. In this paper, we introduce a method for computing Raman spectra of polycrystalline materials from first principles. We start from the standard approach based on the (Placzek) rotation invariants of the Raman tensors and extend it to include the effect of the coupling between the lattice vibrations and the induced electric field, and the electro-optic contribution, relevant for polar materials like ferroelectrics. As exemplified by applying the method to rhombohedral BaTiO3, AlN, and LiNbO3, such an extension brings the simulated Raman spectrum to a much better correspondence with the experimental one. Additional advantages of the method are that it is general, permits automation, and thus can be used in high-throughput fashion., Comment: 23 pages, 4 figures

Electrical energy storage systems (EESSs) with high energy density and power density are essential for the effective miniaturization of future electronic devices. Among different EESSs available in the market, dielectric capacitors relying on swift electronic and ionic polarization-based mechanisms to store and deliver energy already demonstrate high power densities. However, different intrinsic and extrinsic contributions to energy dissipations prevent ceramic-based dielectric capacitors from reaching high recoverable energy density levels. Interestingly, relaxor ferroelectric-based dielectric capacitors, because of their low remnant polarization, show relatively high energy density and thus display great potential for applications requiring high energy density properties. Here, some of the main strategies to improve the energy density properties of perovskite lead-free relaxor systems are reviewed. This includes (i) chemical modification at different crystallographic sites, (ii) chemical additives that do not target lattice sites and (iii) novel processing approaches dedicated to bulk ceramics, thick and thin films, respectively. Recent advancements are summarized concerning the search for relaxor materials with superior energy density properties and the appropriate choice of both composition and processing route to match various needs in the application. Finally, future trends in computationally-aided materials design are presented., Comment: 47 pages, 11 figures, 2 tables

Ba{[Gax,Tax]Ti(1-2x)}O3 ceramics with x equal to 0, 0.0025, 0.005, 0.01, 0.025 and 0.05 have been prepared by conventional solid-state reaction. Structural and dielectric characterization have been performed to investigate the effect of dipole-pair substitution concentration on the macroscopic dielectric properties. Ba{[Gax,Tax]Ti(1-2x)}O3 evolves from a classic ferroelectric to a diffuse phase transition (DPT) as x increases. Ba{[Gax,Tax]Ti(1-2x)}O3 for x > or = 0.01 possesses diffuseness parameters comparable to Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) and recently reported (Ba0.97Pr0.03)(Ti0.9425Ce0.05)O3 (BPTC), yet it lacks the frequency and temperature dependence of Tm necessary to be a strictly defined relaxor ferroelectric. Additionally, Ba{[Ga0.05,Ta0.05]Ti0.9}O3 possesses a relative permittivity, {\epsilon}r, of 700+/-16% and dissipation factor less than 0.05 at 10 kHz within the temperature range [-75{\deg}C, 120{\deg}C]. In comparison to BaTiO3, Ba{[Gax,Tax]Ti(1-2x)}O3 possesses enhanced electrical resistivity at and above room temperature. In-situ XRD, including Rietveld refinement, have been performed to determine the lattice parameter, coefficient of thermal expansion and phase transition temperature (Tc) of each composition within the temperature range [RT, 1000{\deg}C], thus linking the dielectric properties with the materials structure. These studies have been corroborated by temperature dependent Raman spectroscopy to compare the Tc determined by electrical and structural characterization. The properties of Ba{[Gax,Tax]Ti(1-2x)}O3 are discussed in context with available models that describe donor and acceptor dopants spatially separated in the parent matrix, inter-relating lattice parameter, Curie temperature, and other material properties

The nature of the "forbidden" local- and long-range polar order in nominally nonpolar paraelectric phases of ferroelectric materials has been an open question since the discovery of ferroelectricity in oxide perovskites (ABO3). A currently considered model suggests locally correlated displacements of B-site atoms along a subset of <111> cubic directions. Such offsite displacements have been confirmed experimentally, however, being essentially dynamic in nature they cannot account for the static nature of the symmetry-forbidden polarization implied by the macroscopic experiments. Here, in an atomically resolved study by aberration corrected scanning transmission electron microscopy (STEM) complemented by Raman spectroscopy, we reveal, directly visualize and quantitatively describe static, 2-4 nm large polar nanoclusters in the nominally nonpolar cubic phases of (Ba,Sr)TiO3 and BaTiO3. These results have implications on understanding of the atomic-scale structure of disordered materials, the origin of precursor states in ferroelectrics, and may help answering ambiguities on the dynamic-versus-static nature of nano-sized clusters., Comment: the main text and supplementary information