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

Reverse Monte Carlo (RMC) modelling is a general method of structural modelling based on experimental data. RMC modelling can be applied to many different sorts of data, simultaneously if wished. Powder and single-crystal neutron diffraction (including isotopic substitution), x-ray diffraction (including anomalous scattering) and electron diffraction, extended x-ray absorption fine structure and nuclear magnetic resonance (magic angle spinning and second moment) have already been used to provide data. RMC modelling can also be applied to many different types of system - liquids, glasses, polymers, crystals and magnetic materials. This article outlines the RMC method and discusses some of the common misconceptions about it. It is stressed that RMC models are neither unique nor `correct'. However, they are often useful for aiding our understanding either of the structure itself, or of the relationships between local structure and other physical properties. Examples are given and the possibilities for further development of the RMC method are discussed.

http://slapper.apam.columbia.edu/bib/papers/mcgre_jpcm01.pdf

Reverse Monte Carlo modelling

Ferromagnets have been known since ancient times. Ferroelectrics were discovered 80 years ago, and both properties are important in many areas of technology and electronics. Materials displaying both ferroelectricity and ferromagnetism combine the potential applications of both parent phases, with a range of new applications in optics, electronic circuitry and multiple-state memory devices. Such materials are rare, but one described this week, the common sulpho-spinel CdCr2S4, shows promise. It combines reasonable ordering temperatures (the point at which magnetic properties disappear) with sizeable magnetization and polarization well suited for application. Materials in which magnetic and electric order coexist—termed ‘multiferroics’ or ‘magnetoelectrics’—have recently become the focus of much research1,2,3,4. In particular, the simultaneous occurrence of ferromagnetism and ferroelectricity, combined with an intimate coupling of magnetization and polarization via magnetocapacitive effects, holds promise for new generations of electronic devices. Here we present measurements on a simple cubic spinel compound with unusual, and potentially useful, magnetic and electric properties: it shows ferromagnetic order coexisting with relaxor ferroelectricity (a ferroelectric cluster state with a smeared-out phase transition), both having sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature, the magnetocapacitive coupling (characterized by a variation of the dielectric constant in an external magnetic field) reaches colossal values, approaching 500 per cent. We attribute the relaxor properties to geometric frustration, which is well known for magnetic moments but here is found to impede long-range order of the structural degrees of freedom that drive the formation of the ferroelectric state.

Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4.

The reasons for the lower piezoelectric properties in the most studied lead-free piezoelectrics, modified (K,Na)NbO3 and (Bi05Na05)TiO3, are discussed Contributions from domain wall motion and properties at the morphotropic phase boundary are considered and are compared to those in PZT Lead-free, non-piezoelectric solutions to electromechanical coupling are discussed

http://foresight.ifmo.ru/ict/shared/files/201310/1_80.pdf

What can be expected from lead-free piezoelectric materials?

Introduction.-Solution Chemistry Simple alkoxide based precursor systems Carboxylate based precursor systems Single-source precursors Acqueos Precursor Systems Solution Synthesis Strategies.-Analytical Methods Introduction Thermal Analysis NMR Sepctroscopy EXAFS Other Methods (XRM, SEM,TEM scattering methods at nanocrystalline films) Spin-Coating Dip Coating Inkjet Printing and Other Direct Writing Methods(dip point and imprint techniques) Chemical Bath Deposition Polymer Assisted Deposition.-Processing and Crystallization Thermodynamics and Heating Processes Material Systems Dominated by Heterogeneous Nucleation Material Systems Dominated by Homogeneous Nucleation Low Temperature Processing Epitaxial Films Powder Assisted Film Fabrication UV-and E-Beam Direct Patterning of Photosensitive CSD Films Template Controlled Growth.-Functions and Applications Introduction Integrated Capacitors Base Metal Electrodes Polar Oxide Films for MEMS Applications Conducting Films and Electrodes Transparent conducting oxides Superconducting Films Porous Films for Gas Sensors Luminescent Fims.-Appendix Synthesis for Standard material Systems.

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Chemical Solution Deposition of Functional Oxide Thin Films

The dielectric dispersion of the transparent relaxor ferroelectric ceramics PLZT 8/65/35 and 9.5/65/35 was determined in a wide frequency range including the microwave and infrared range. The number of observed polar phonons in infrared spectra gives evidence about the locally broken cubic symmetry and the presence of polar nanoclusters in the whole investigated temperature range up to 530 K. A single broad and symmetric dispersion that occurs below the polar phonon frequencies was fitted with the Cole-Cole formula and a uniform distribution of Debye relaxations. On decreasing temperature, the distribution of relaxation times becomes extremely broad which indicates increasing correlation among the clusters. The mean relaxation time diverges according to the Vogel-Fulcher law with the same freezing temperature 230±5 K for both ceramics, but different activation energies 1370 K and 1040 K for the 8/65/35 and 9.5/65/35 sample, respectively. The shortest relaxation time is about 10-12 s and remains almost temperature independent. Below room temperature, the loss spectra become essentially frequency independent and the permittivity increases linearly with decreasing logarithm of frequency. The slope of this dependence is proportional to T 4 in the investigated temperature range (above 210 K) which indicates appreciable anharmonicity of the potential for polarization fluctuations.

https://iopscience.iop.org/article/10.1088/0953-8984/12/4/309/pdf

Dielectric dispersion of the relaxor PLZT ceramics in the frequency range 20 Hz-100 THz

Dielectric properties of the relaxor ferroelectric ceramics PLZT 8/65/35 and 9.5/65/35 were studied in the broad frequency range of 100 Hz–1 THz at low temperatures below the freezing temperature. Nearly frequency-independent dielectric losses were observed up to 1 GHz on cooling down to 10 K. Their magnitude decreases exponentially with temperature, but remains remarkable high down to 10 K. A Landau-type thermodynamic model based on the perovskite structure near the morphotropic phase boundary is proposed for calculating the energy barriers for polarization reversal near the polar cluster boundaries and explaining the broad distribution function of relaxation times, which fits the observed frequency dependences of permittivity and losses below 1 GHz. High dielectric losses in the submillimetre region were explained by shear wave emission of vibrating polar cluster walls in an ac electric field and by piezoelectric resonances on polar clusters.

https://iopscience.iop.org/article/10.1088/0953-8984/15/35/310/pdf

Frequency-independent dielectric losses (1/f noise) in PLZT relaxors at low temperatures

Phonons in both ceramic samples KNbO3 (KN) and K0.5Na0.5NbO3 (KNN-50) were investigated from 10 to 900 K by means of Raman, infrared, and THz spectroscopy. First-order transitions from cubic to tetragonal phases were detected at about 755 K (in KN) and 710 K (in KNN-50), where the first component of the polarization appears. Transitions from the tetragonal to the orthorhombic phases take place around 510 and 475 K, respectively. The last transitions from orthorhombic to rhombohedral phases are strongly first-order type. TC is shifted from 200 K in KN to about 90 K in KNN-50. All Raman active modes below 200 cm-1. disappear in KN but not in KNN-50. The overdamped soft mode present at high temperatures in the THz range changes its dielectric strength at each phase transition (when the corresponding component jumps to higher frequencies) and abruptly disappears from THz spectra in the rhombohedral phase, because it stiffens up to 200 cm-1. This mode has lower frequency and higher dielectric strength in KNN-50.

Lattice dynamics and phase transitions in KNbO 3 and K 0.5 Na 0.5 NbO 3 ceramics

Infrared reflectivity and Raman spectra of hot-pressed PLZT 8/65/35, 9.5/65/35 and 2/95/5 ceramics show only weak temperature changes in the first two (ferroelectric) compositions but much richer spectra and pronounced temperature changes in the third (antiferroelectric) composition, including a soft phonon mode.

Infrared and raman spectroscopy on various PLZT ceramics

The dielectric behaviour of tetragonal PLZT x/40/60 ceramics doped with lanthanum (x = 5, 12 and 15%) has been investigated at 100–1014 Hz. Dielectric, THz and infrared spectra were measured from 500 to 20 K. For x ≥ 12 relaxor ferroelectric behaviour was observed. All three samples showed an additional relaxational mode below phonons. Analysis of the phonon modes for ferroelectric PLZT 5/40/60 is presented together with the complete dielectric behaviour.

M. Kosec

Papers

Dielectric dispersion of the relaxor PLZT ceramics in the frequency range 20 Hz-100 THz

Frequency-independent dielectric losses (1/f noise) in PLZT relaxors at low temperatures

Lattice dynamics and phase transitions in KNbO 3 and K 0.5 Na 0.5 NbO 3 ceramics

Infrared and raman spectroscopy on various PLZT ceramics

Broad-band dielectric spectroscopy of tetragonal PLZT x /40/60