Rietveld neutron powder profile analysis of the compound Na0.5Bi0.5TiO3 (NBT) is reported over the temperature range 5–873 K. The sequence of phase transitions from the high-temperature prototypic cubic structure (above 813 K), to one of tetragonal (673–773 K) and then rhombohedral structures (5–528 K) has been established. Coexisting tetragonal/cubic (773–813 K) and rhombohedral/tetragonal (with an upper temperature limit of 145 K between 528 and 673 K) phases have also been observed. Refinements have revealed that the rhombohedral phase, space group R3c, with aH = 5.4887 (2), cH = 13.5048 (8) A, V = 352.33 (3) A3, Z = 6 and Dx = 5.99 Mg m−3, exhibits an antiphase, a−a−a− oxygen tilt system, ω = 8.24 (4)°, with parallel cation displacements at room temperature. The tetragonal phase, space group P4bm, with aT = 5.5179 (2), cT = 3.9073 (2) A, V = 118.96 (1) A3, Z = 2 and Dx = 5.91 Mg m−3, possesses an unusual combination of in-phase, a0a0c+ oxygen octahedra tilts, ω = 3.06 (2)°, and antiparallel cation displacements along the polar axis. General trends of cation displacements and the various deviations of the octahedral network from the prototypic cubic perovskite structure have been established and their systematic behaviour with temperature is reported. An investigation of phase transition behaviour using second harmonic generation (SHG) to establish the centrosymmetric or non-centrosymmetric nature of the various phases is also reported.

Investigation of the structure and phase transitions in the novel A-site substituted distorted perovskite compound Na0.5Bi0.5TiO3

In response to the current environmental regulations against the use of lead in daily electronic devices, a number of investigations have been performed worldwide in search for alternative piezoelectric ceramics that can replace the market-dominating lead-based ones, representatively Pb(Zr
 x
 Ti1-x
 )O3 (PZT)-based solid solutions. Selected systems of potential importance such as chemically modified and/or crystallographically textured (K, Na)NbO3 and (Bi1/2Na1/2)TiO3-based solid solutions have been developed. Nevertheless, only few achievements have so far been introduced to the marketplace. A recent discovery has greatly extended our tool box for material design by furnishing (Bi1/2Na1/2)TiO3-based ceramics with a reversible phase transition between an ergodic relaxor state and a ferroelectric with the application of electric field. This paired the piezoelectric effect with a strain-generating phase transition and extended opportunities for actuator applications in a completely new manner. In this contribution, we will present the status and perspectives of this new class of actuator ceramics, aiming at covering a wide spectrum of topics, i.e., from fundamentals to practice.

Giant electric-field-induced strains in lead-free ceramics for actuator applications – status and perspective

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

Temperature-dependent dielectric permittivity of 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 (BNT-6BT) lead-free piezoceramics was studied to disentangle the existing unclear issues over the crystallographic aspects and phase stability of the system. Application of existing phenomenological relaxor models enabled the relaxor contribution to the entire dielectric permittivity spectra to be deconvoluted. The deconvoluted data in comparison with the temperature-dependent dielectric permittivity of a classical perovskite relaxor, La-modified lead zirconate titanate, clearly suggest that BNT-6BT belongs to the same relaxor category, which was also confirmed by a comparative study on the temperature- dependent polarization hysteresis loops of both materials. Based on these results, we propose that the low-temperature dielectric anomaly does not involve any phase transition such as ferroelectric- to-antiferroelectric. Supported by transmission electron microscopy and X-ray diffraction experiments at ambient temperature, we propose that the commonly observed two dielectric anomalies are attributed to thermal evolution of ferroelectric polar nanoregions of R3c and P4bm symmetry, which coexist nearly throughout the entire temperature range and reversibly transform into each other with temperature.

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On the phase identity and its thermal evolution of lead free (Bi1/2Na1/2)TiO3-6 mol% BaTiO3

Lead-free Na 1/2 Bi 1/2 TiO 3 –BaTiO 3 piezoelectric ceramics with perovskite structure were studied. The piezoelectric properties of (1− x )Na 1/2 Bi 1/2 TiO 3 – x BaTiO 3 ( x =0.02, 0.04, 0.06, 0.08, 0.10) ceramics were reported and their piezoelectric properties reach extreme values near the MPB (about x =0.06). Influences of nonstoichiometry and doping on the structures and piezoelectric properties of (Na 1/2 Bi 1/2 ) 0.92 Ba 0.08 TiO 3 ceramics were further studied. The relationship between piezoelectric constant and depolarization temperature of nonstoichiometric and doped ceramics was also investigated. It was found that in most of Na 1/2 Bi 1/2 TiO 3 –BaTiO 3 ceramics studied, their depolarization temperatures were much lower than maximum temperatures of dielectric constant–temperature curves; the origin of this phenomenon was discussed.

Electrical properties of Na1/2Bi1/2TiO3–BaTiO3 ceramics

The results of experimental studies of the crystal and domain structures of NaO, 5BiO, 5TiO3 as well as its lattice dynamics, dielectric, optical and other physical properties are considered. Nature of peculiar temperature points is discussed.

Phase transitions and soft modes in sodium bismuth titanate

Phase transitions in sodium-bismuth titanate

The evolution of the structure of K1−x
 LixTa1−y
 NbyO3 single crystals with x = 0.001, y = 0.026, and 1900 ppm Cu (KLTN277) and with x = 0.0014 and y = 0.024 (KLTN123), which exhibit an extremely high permittivity (up to 4 × 105 in the quasi-static regime for the KLTN277 crystal), is investigated in the range from room temperature to 20 K. It is demonstrated that, upon cooling to the lowest temperatures, both crystals retain their cubic structure, but the lattice parameters pass through a minimum at the temperature of the observed anomalies of the dielectric response (∼50 K). In the neutron diffraction pattern of the KLTN123 sample, satellites appear in the vicinity of the (hhh) reflections at temperatures below ∼50 K. These satellites can be associated with the nucleation of the rhombohedral phase.

Investigation into the evolution of the structure of K 1-x Li x Ta 1-y Nb y O 3 single crystals under variations in temperature

Controlled generation of coherent spin waves with the highest possible frequencies and the shortest possible wavelengths is a cornerstone of spintronics and magnonics. Here, using the Heisenberg antiferromagnet RbMnF$_3$, we demonstrate that laser-induced THz spin dynamics corresponding to pairs of the mutually coherent counter propagating spin waves with the wavevectors up to the edge of the Brillouin zone cannot be understood in the terms of magnetization and antiferromagnetic vectors, conventionally used to describe spin waves in the classical macrospin approximation. Instead, we propose to model such a spin dynamics using the spin correlations function. We derive quantum-mechanical equation of motion for the latter and emphasize that unlike the magnetization and the antiferromagnetic vectors the spin correlations in antiferromagnets do not have an inertia.

Coherent THz Spin Dynamics in Antiferromagnets Beyond the Macrospin Approximation

Abstract We have studied the dielectric polarization induced by periodic spherical bending in KTaO3 thin (140 μm) single-crystal plates. It has been shown that the induced polarization can be attributed to the direct flexoelectric effect and space redistribution of charges. An effective transverse flexoelectric tensor has been estimated. We have studied the relaxation of the polarization and inhomogeneous deformation and obtained their relaxation times.

P. P. Syrnikov

Papers

Phase transitions and soft modes in sodium bismuth titanate

Phase transitions in sodium-bismuth titanate

Investigation into the evolution of the structure of K 1-x Li x Ta 1-y Nb y O 3 single crystals under variations in temperature

Coherent THz Spin Dynamics in Antiferromagnets Beyond the Macrospin Approximation

Study of static direct flexoelectric effect in KTaO3 single crystals