Feature size is a natural determinant of material properties. Its design offers the technological perspectives for material improvement. Grain size, crystallite size, domain width, and structural defects of different nature constitute the classical design elements. Common ferroelectric ceramics contain micrometer grain sizes and submicrometer domain widths. Domain wall mobility is a major contribution to their macroscopic material properties providing approximately half of the macroscopic output in optimized materials. The extension into the dynamic nanoworld is provided by relaxor ferroelectrics. Ionic and nanoscale field disorders form the base to a state with natural nanometer-size polar structures even in bulk materials. These polar structures are highly mobile and can dynamically change over several orders of magnitude in time and space being extremely sensitive to external stimuli. This yields among the largest dielectric and piezoelectric constants known. In this feature article, we want to outline how lead-free relaxors will offer a route to an environmentally safer option in this outstanding material class. Properties of uniaxial, planar, and volumetric relaxor compositions will be discussed. They provide a broader and more interesting scope of physical properties and features than the classical lead-containing relaxor compositions.

Lead-Free Relaxor Ferroelectrics

Multiferroic bismuth ferrite-based materials for multifunctional applications: Ceramic bulks, thin films and nanostructures

We report discovery of collinear-magnetism-driven ferroelectricity in the Ising chain magnet Ca3Co2-xMn(x)O6 (x approximately 0.96). Neutron diffraction shows that Co2+ and Mn4+ ions alternating along the chains exhibit an up-up-down-down ( upward arrow upward arrow downward arrow downward arrow) magnetic order. The ferroelectricity results from the inversion symmetry breaking in the upward arrow upward arrow downward arrow downward arrow spin chain with an alternating charge order. Unlike in spiral magnetoelectrics where antisymmetric exchange coupling is active, the symmetry breaking in Ca3(Co,Mn)2O6 occurs through exchange striction associated with symmetric superexchange.

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Ferroelectricity in an Ising Chain Magnet

In this work, we have focused on the influence of Gd3+ substitution on the structural, magnetic and electrical properties of cobalt ferrite synthesized using a sol–gel auto-combustion method. The powder X-ray diffraction analysis reveals that the Gd-substituted cobalt ferrites crystallize in a single phase spinel structure for lower concentrations of Gd3+, while a trace of GdFeO3 appears as a minor phase for higher concentrations. Raman and Fourier transform infrared spectra confirm the formation of the spinel structure. Furthermore, Raman analysis shows that the inversion degree of cobalt ferrite decreases with Gd3+ doping. The field emission scanning electron microscopy images show that the substitution of small amounts of Gd3+ causes a considerable reduction of the grain size. Studies on the magnetic properties reveal that the coercivity of Gd-substituted cobalt ferrites enhances from 1265 Oe to 1635 Oe, the saturation magnetization decreases monotonically from 80 emu g−1 to 53.8 emu g−1 and the magnetocrystalline anisotropy constant increases from 5.8 × 105 erg cm−3 to 2.23 × 106 erg cm−3 at 300 K. The electrical properties show that the Gd3+ doped samples exhibit high dielectric constant (616 at 100 Hz) and ac conductivity (4.83 × 10−5 S cm−1 at 100 Hz) values at room temperature. The activation energy is found to decrease from 0.408 to 0.347 eV with the rise in Gd3+ content. The impedance study brings out the effect of the bulk grain and the grain boundary on the electrical resistance and capacitance of cobalt ferrite. Gd substitution and the nano-size of cobalt ferrite enhance the electrical and magnetic properties which could enable a higher memory storage capability.

Impact of Gd3+ substitution on the structural, magnetic and electrical properties of cobalt ferrite nanoparticles

Chemical doping is one of the major methods by which the properties of BaTiO3 are modified to induce various device characteristics. Doping strategies are generally separated into iso- and alio-valent (donors and acceptors) and into A- and B-site species within the perovskite structure. The A1g octahedral breathing mode at ∼800 cm−1 in BaTiO3 is Raman inactive for compositions with single B-site species. However, this mode becomes Raman active if more than one B-site species is present, including titanium vacancies, VT′′′′. Moreover, the relative intensity of the A1g breathing mode is qualitatively related to the concentration of the species replacing Ti in the B-site. This article illustrates some clear cases where the A1g octahedral breathing mode can be utilized to verify the site occupancy of dopants in BaTiO3 when used in conjunction with more conventional indirect methods such as x-ray diffraction (Vegard’s Law) and dielectric measurements as a function of temperature.

Use of Raman spectroscopy to determine the site occupancy of dopants in BaTiO3

BaTi1−xCaxO3−x ceramics, where 0⩽x⩽0.04, prepared by the mixed oxide route at 1500°C display a strong suppression of the Curie temperature and a significant increase in grain conductivity with increasing x. The ramifications of these results on the applicability of using Ca2+ as a dopant to achieve X8R specification in BaTiO3-based formulations are discussed.

Comment on the use of Calcium as a dopant in X8R BaTiO3-based ceramics

The Ba(Ti1−xZrx)O3 (BTZ) system with x≤0.35 has been studied using a combination of x-ray diffraction, dielectric measurements, Raman spectroscopy, and electron diffraction with a view to better understand the changes in structure and correlation length of polar order as a function of composition and temperature. Careful fitting of Raman spectra has confirmed stabilization of the orthorhombic phase at room temperature in x=0.05 with mixed phase (orthorhombic and rhombohedral) present at 80 K. Raman data suggest that the rhombohedral phase is most likely to be present at low temperature for x=0.15 and 0.25 and that only short range polar order evolves in x=0.35. Interpretation of the Raman spectra is in broad agreement with the dielectric data as a function of composition and temperature. Diffuse scatter at room temperature in high order zone axis electron diffraction remains essentially identical for all compositions irrespective of structure and correlation length of polar order. The diffuse maxima are t...

Polar order and diffuse scatter in Ba(Ti1-xZrx)O3 ceramics

Raman spectroscopy has been used to study the ferroelectric tetragonal to paraelectric cubic phase transition Tc in undoped BaTiO3 (BT) and (Ba0.92A0.08)TiO3 [where A=Ca (BCT), Sr (BST), and Pb (BPT)]. In BT and BPT, mode characteristic of the tetragonal phase at 303 and 710cm−1 persisted until >50°C above Tc, indicating the presence of local dynamic polar clusters well above the displacive phase transitions, ∼120 and ∼149°C, respectively. However, in BCT and BST, the Raman spectra exhibited only broad second order bands above Tc, suggesting that Ca and Sr, A-site dopants supress local dynamic polar clusters.

O. P. Thakur

Papers

Comment on the use of Calcium as a dopant in X8R BaTiO3-based ceramics

Polar order and diffuse scatter in Ba(Ti1-xZrx)O3 ceramics

In situ Raman spectroscopy of A-site doped barium titanate

Effect of Gd3+ substitution on dielectric properties of nano cobalt ferrite

Impedance spectroscopy and conductivity analysis of multiferroic BFO–BT solid solutions