Functional Materials Letters

Lead-free ferroelectric materials are currently subject to extensive research activities owing to environmental concerns and corresponding legislation which point to substitute lead-based ferroelectric materials. The paper provides an overview on recent research activities and trends in the field of lead-free ferroelectrics. The current work is grouped by contributions to atomistic-scale analysis of lead-free materials, structure and phase formation, micro structure and dopant effects on piezoelectric properties. The results and challenges to research in these fields are briefly sketched; so a framework is provided in which the contributions of the present topical issue to the major topics in lead-free ferroelectrics are related. The contributions in this issue are listed in Refs. 63–77. Finally, the current status of research and technology on lead-free materials is summarized.

The reasons for the lower piezoelectric properties in the most studied lead-free piezoelectrics, modified (K, Na)NbO₃ and (Bi_0.5Na_0.5)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.

In this review, the crystal structure and the synthesis of the sodium potassium niobate (K_0.5Na_0.5NbO₃) as a promising candidate for lead-free piezoelectrics are addressed. The sintering and the microstructure as prerequisites for obtaining ceramics with reliable and sufficiently high piezoelectric properties for selected applications are discussed.

The defect structure resulting from copper doping of potassium niobate (KNbO₃) ceramics was investigated by a combined analysis of electron paramagnetic resonance (EPR) spectroscopy and first-principles calculations based on density functional theory (DFT). The results indicate that under atmospheric oxygen partial pressure, Cu preferentially substitutes on the Nb sites where it can trap one or two oxygen vacancies. Correspondingly, for 0.25 mol% Cu doped KNbO₃ ceramics, two types of defect associates are formed — and . The association of Cu impurities and oxygen vacancies lowers the defect formation energy by 1.0 eV to 2.7 eV, depending on the Fermi level. Owing to the opposite charges of these two types of defects, overall charge neutrality is theoretically possible by mutual compensation of the defect associates, without formation of additional non-associated oxygen vacancies.

The effect of antimony (Sb) substitution for niobium (Nb) on potassium sodium niobate (KNN) ceramic was investigated with respect to the densification behaviour at different sintering temperatures, microstructure and electrical properties. A small amount of Sb⁵⁺ was added while simultaneously lowering the amount of Nb⁵⁺ and in this study of the (K_0.5Na_0.5)(Nb_1-xSb_x)O₃ system, x content was varied from 0 to 14 mol%. Our results show that Sb⁵⁺ slightly increased the optimum sintering temperature for KNN but above 8 mol%, its resistivity and piezoelectric properties decreased. As the amount of Sb⁵⁺ substituted is increased, the structure of the ceramic transformed from orthorhombic to pseudo-cubic which led to slight shrinkage in the unit cell volume. Microstructural examination revealed that above 10 mol%, a second phase (K₂NaSb₃O₉) was formed which segregated mainly to the grain boundary while the quantitative EDX analysis showed that there was A-site vacancy due to loss of the alkali elements. The two phase transitions points, Curie temperature (T_C) and the tetragonal to orthorhombic (T_T-O) shifted to lower temperature with increasing Sb⁵⁺ content and above 10 mol%, the T_T-O shifted to below room temperature. The dielectric loss slightly increases with increasing Sb⁵⁺ content up to 200°C. There was an improvement in the piezoelectric properties with ≤ 6 mol% Sb content while optimum properties were obtained with 4 mol% (K_P = 0.46, Q_m = 6.2, N_P = 2296).

The family of perovskite ferroelectrics based on sodium potassium niobate (NKN) ceramics exhibit useful piezoelectric properties and show potential for the replacement of conventional lead based piezoceramics. In the present study, iron-doped NKN ceramics were prepared by a solid state reaction method. Ferroelectric hysteresis measurements indicated that a remanent polarization value approaching 0.4Cm^-2 could be obtained by poling under a continuous AC electric field with amplitude of 3 MV m^-1. X-ray diffraction (XRD) patterns of these materials were recorded in transmission during poling, using high energy synchrotron X-rays with a wavelength of 1.55 × 10^-11m. Changes in crystallographic texture were observed during poling as a result of 90° and 60°/120° domain switching. The strain due to ferroelectric domain switching in orthorhombic NKN is described in terms of the differences in transformation strain between different domain variants, with reference to the high temperature cubic lattice.

We propose the design and synthesis of a polycrystalline sintered composite microstructure that leads to improved stability of piezoelectric response as a function of temperature in (K, Na)NbO₃ (KNN) based ceramics. The microstructure consists of matrix and island both of which have different composition and surface area. The matrix had much larger surface area and smaller orthorhombic to tetragonal transition temperature (T_O-T) than island. The composition corresponding to 0.99(K_0.48Na_0.48Li_0.04)(Nb_0.98Sb_0.02)O₃-0.01BaTiO₃ (KNLNS-BT, T_O-T ~ 33°C) was selected as matrix while (K_0.5Na_0.5)NbO₃ (KNN, T_O-T ~ 206°C) was selected as island. The results show the flat electromechanical coupling factor from room temperature until ~ 300°C.

The effect of an isovalent chemical variation in A site cations on the stability of ferroelectric order and consequent changes in the electrical properties of ferroelectric 0.93(Bi_1/2Na_1/2)TiO₃-0.05BaTiO₃-0.02(K_0.5Na_0.5)NbO₃ (0.93BNT-0.05BT-0.02KNN) lead-free piezoceramic at the morphotropic phase boundary (MPB) was investigated by substituting K for Na. X-ray diffraction analysis reveals that all the studied compositions undergo an irreversible phase transition from a pseudocubic to a mixture of rhombohedral and tetragonal phases during poling. In spite of the absence of an obvious structural difference, both electric-field-induced strain and polarization hysteresis loops show a clear distinction. It was found that the effect of substitution is so drastic that about 0.5 at% substitution renders the material softer and more than 1.0 at% results in a significant destabilization of the ferroelectric order. However, no notable difference in the temperature dependent permittivity measurements was detected, which suggests that the depoling of these materials has some other origin than a symmetry-breaking phase transition.

Although a great deal of work has been done to understand defect chemistry in "soft" and "hard" PZT-based materials, there is little understanding of how defect chemistry influences the properties of lead-free piezoelectric materials. This paper reports a systematic investigation of doping on the ferroelectric and piezoelectric properties in Na_0.5Bi_0.5TiO₃ (NBT)-based ceramics. NBT-based ceramics have been synthesized by traditional solid state routes using several different dopants including lanthanum, manganese, and iron in 1 mol%. The addition of iron and manganese lead to an increase in the coercive field (E_c), a decrease in the piezoelectric coefficient (d₃₃), and an increase in the thermal depoling temperature (T_depole), similar to the behavior of "hard" PZT. Lanthanum, on the other hand, leads to a decrease in the E_c, an increase in d₃₃, and a decrease in T_depole similar to that seen in "soft" PZT.

Lead-free (Bi_0.5Na_0.5)_1-x-y-zBa_x(Bi_0.5Li_0.5)_ySr_zTiO₃ ceramics were prepared by an ordinary sintering method and their structure and piezoelectric properties have been studied. X-ray diffraction patterns show that Ba²⁺, Li⁺ and Sr²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a solid solution with a pure perovskite structure and a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases is formed at 0.04 ≤ x ≤ 0.08 and z < 0.06. The content of Li⁺ has no obvious influence on the crystal structure of the ceramics. The ceramics situated within the MPB exhibit a lower coercive field E_c and a larger remanent polarization P_r. Because of the formation of the MPB, lower E_c and larger P_r, the ceramics with x = 0.06, y = 0.075 - 0.125 and z = 0-0.02 possess improved piezoelectric properties. The optimum d₃₃ (235 pC/N) and k_p (36.3%) are reached at x/y/z = 0.06/0.125/0.02 and 0.06/0.075/0.02, respectively. The deformed P–E loop is also observed near or above T_d.

In this letter, conclusive experimental evidence is presented for the coexistence of two phases within individual grains in the system (Bi_0.5Na_0.5TiO₃)_0.91-(BaTiO₃)_0.06-(K_0.5Na_0.5NbO₃)_0.03. Electron diffraction data unequivocally reveal the presence of a rhombohedral and a tetragonal phase with space group R3c and P4bm, respectively, both on the nanoscale level. Superlattice reflections, observed in the corresponding selected area electron diffraction, were used to identify the rhombohedral and tetragonal phase present at different regions within one grain. It is thought that the rhombohedral phase acts as a nucleation site for the phase transformation monitored under applied electrical field. The coexistence of these two phases in conjunction with the triggered tetragonal to rhombohedral phase transformation rationalizes the high bipolar and unipolar strain recorded for this lead-free ferroelectric material.

Barium titanate (BaTiO₃) was prepared by a solid state route from highly pure raw materials. Homogenization by attrition milling resulted in fine grained powders, which could be calcined to single phase BaTiO₃ at 875°C and sintered to high density ceramics at 1200–1250°C. Due to the relatively low sintering temperatures, grain size remains in the range below 2 μm. The material shows very good performance under 2 kV/mm electric high field cycling with 0.12% strain at ambient conditions. Field dependence of strain at 20°C shows moderate strain under subcoercive field conditions, high strain in the intermediate field range up to 2 kV/mm and saturation resulting in to low additional strain at higher fields. The temperature characteristics of the strain reveal a marked decrease by approximately one third when increasing the temperature to 60°C. At higher temperatures field induced strain stabilizes when approaching the Curie temperature. From the nonlinearity characteristics at ambient conditions and the temperature behavior, it is concluded that contributions from reversible domain switching to strain are a major source of the high strain performance.

The defect structure of barium titanate (BaTiO₃) heat-treated under reduced p_O2 has been investigated by electron paramagnetic resonance (EPR) spectroscopy. The results confirm the formation of -centers. Based on the relative size of the principal values of the electron g-matrix, g_⊥ > g_‖, the local distortion of the oxygen octahedron about the -centers was also analyzed and the formation of defect associates unambiguously established.

Aging, the gradual changing of materials properties with time, is a well observed but puzzling phenomenon in ferroelectrics. It lowers the reliability of ferroelectric devices, and is thus usually undesirable. However, based on a defect mediated reversible domain switching mechanism, we obtained large recoverable electro-strain in aged BaTiO₃-based ferroelectric crystals, which indicates aging is not always a detrimental effect; it might be beneficial for applications. Moreover, direct evidence for the reversible domain switching process was given by in situ polarizing microscope observation. The underlying microscopic origin was discussed in light of a symmetry-conforming short-range-order (SC-SRO) principle of point defects and electron paramagnetic resonance spectroscopy.

Flexoelectricity describes the physical phenomenon of the generation of electric polarization from mechanical strain gradient in solid insulators. In common dielectric materials, the flexoelectric coefficient is trivially small ~10^-10 C/m. In Ba(Sr,Ti)O₃ (BST) ceramics, flexoelectric coefficient up to 10^-4 C/m was observed. Such high coefficient makes it possible to design high piezoelectric response flexoelectric composites. In this letter, we will demonstrate that the newly designed flexoelectric composites could have piezoelectric properties better than conventional piezoelectric materials.

Bi₂FeCrO₆ thin films were epitaxially grown by pulsed laser deposition on (100)-oriented LaAlO₃, (LaAlO₃)_0.3(Sr₂LaTaO₆)_0.7 and SrTiO₃ single crystalline substrates with and without epitaxial CaRuO₃ buffered layer. The in-plane compressive strain induces monoclinic distortion of the Bi₂FeCrO₆ lattice cell. The strain originates from lattice mismatch between CaRuO₃ and single crystal substrates. The similar crystal structure of the substrate and the layer lead to coherent epitaxial growth of the heterostructures and avoid strain relaxation in particular for BFCO films deposited on LaAlO₃ substrates. The ferroelectric character is demonstrated for all grown BFCO films. The residual in-plane strain weakly affects the effective piezoelectric coefficient of BFCO layers.