Relaxor ferroelectrics were discovered almost 50 years ago among the complex oxides with perovskite structure. In recent years this field of research has experienced a revival of interest. In this paper we review the progress achieved. We consider the crystal structure including quenched compositional disorder and polar nanoregions (PNR), the phase transitions including compositional order-disorder transition, transition to nonergodic (probably spherical cluster glass) state and to ferroelectric phase. We discuss the lattice dynamics and the peculiar (especially dielectric) relaxation in relaxors. Modern theoretical models for the mechanisms of PNR formation and freezing into nonergodic glassy state are also presented.

Recent progress in relaxor ferroelectrics with perovskite structure

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

Perovskite solid solution ceramics of (1 − x)BaTiO3–xBi(Mg2/3Nb1/3)O3 (BT–BMN) (x = 0.05–0.2) were synthesized by solid-state reaction technique. The results show that the BMN addition could lower the sintering temperature of BT-based ceramics. X-ray diffraction results reveal a pure perovskite structure for all studied samples. Dielectric measurements exhibit a relaxor-like characteristic for the BT–BMN ceramics, where broadened phase transition peaks change to a temperature-stable permittivity plateau (from −50°C to 300°C) with increasing the BMN content (x = 0.2), and slim polarization–electric field hysteresis loops were observed in samples with x ≥ 0.1. The dielectric breakdown strength and electrical resistivity of BT–BMN ceramics show their maxima of 287.7 kV/cm and 1.53 × 1013 Ω cm at x = 0.15, and an energy density of about 1.13 J/cm3 is achieved in the sample of x = 0.1.

Relaxor Ferroelectric BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for Energy Storage Application

Relaxor ferroelectrics were discovered in the 1950s but many of their properties are not understood. In this review, we shall concentrate on materials such as PMN (PbMg1/3Nb2/3O3), which crystallize in the cubic perovskite structure but with the Mg ion, charge 2+, and the Nb ion, charge 5+, randomly distributed over the B site of the perovskite structure. The peak of the dielectric susceptibility for relaxors is much broader in temperature than that of conventional ferroelectrics, while below the maximum of the susceptibility most relaxors remain cubic and show no electric polarization, unlike that observed for conventional ferroelectrics. Because of the large width of the susceptibility, relaxors are often used as capacitors. Recently, there have been many X-ray and neutron scattering studies of relaxors and the results have enabled a more detailed picture to be obtained. An important conclusion is that relaxors can exist in a random field state, as initially proposed by Westphal, Kleemann and Glinchuk, ...

Relaxing with relaxors: a review of relaxor ferroelectrics

Using neutron pair distribution function analysis over the temperature range from 1000 to 15 K, we demonstrate the existence of local polarization and the formation of medium-range, polar nanoregions (PNRs) with local rhombohedral order in a prototypical relaxor ferroelectric Pb(Mg(1/3)Nb(2/3))O3. We estimate the volume fraction of the PNRs as a function of temperature and show that this fraction steadily increases from 0% to a maximum of approximately 30% as the temperature decreases from 650 to 15 K. Below T approximately 200 K the volume fraction of the PNRs becomes significant, and PNRs freeze into the spin-glass-like state.

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Direct observation of the formation of polar nanoregions in Pb(Mg1/3Nb2/3)O3 using neutron pair distribution function analysis.

The perovskite structure of the relaxor PbMg1/3Nb2/3O3 (PMN) is studied using X-ray and neutron powder diffraction data. The static diffuse scattering (SDS) observed in the diffraction patterns at low temperatures is interpreted using a two-phase Rietveld analysis. The structural model is based on a long-range structure with an average cubic symmetry, and a short-range order due to atomic shifts involved by the formation of polar regions. The two-phase model provides a good improvement in reliability factors. The correlation length and the coexistence of different phases at low temperatures are discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/3/42/011/pdf

A structural model for the relaxor PbMg1/3Nb2/3O3 at 5 K

Modification of the B-site order of PbMg13Nb23O3 ceramics by thermal annealing or by La-doping

Structural study of a poled PbMg13Nb23O3 ceramic at low temperature

For pt. I see ibid., vol. 5, p.1261 (1993). An EXAFS study of lead magnesium niobate was performed at the Nb K edge and at the Pb LIII edge between 300 and 4.5 K. Two Nb-O bond lengths of 1.95 and 2.16 AA are observed and there is no significant modification of the local environment of the Nb atoms with temperature. The distribution of these two bonds in both polar nanodomains and 1-1 Nb-Mg ordered nanodomains is discussed in relation to results obtained previously by other techniques. It is shown that the ordered nanodomains could occupy about one third of the whole volume. The results obtained for the Nb-Nb and Nb-Mg distances confirm the structural results and particularly point to an increasing correlation of the Nb shift at their site, when the temperature is lowered to below 300 K. The weak EXAFS signal obtained at the Pb LIII edge and the small backscattered contribution of Pb at the Nb K edge reveal a strong static disorder of the Pb positions.

A. Morell

Papers

A structural model for the relaxor PbMg1/3Nb2/3O3 at 5 K

Modification of the B-site order of PbMg13Nb23O3 ceramics by thermal annealing or by La-doping

Structural study of a poled PbMg13Nb23O3 ceramic at low temperature

A low-temperature extended X-ray absorption study of the local order in simple and complex perovskites. II. PMN (PbMg1/3Nb2/3O3)

Modification of the nanostructure and dielectric properties of lead magnoniobate ceramics by doping