Abstract:High piezoelectricity of (K,Na)NbO 3 (KNN) lead-free materials benefits from a polymorphic phase transition (PPT) around room temperature, but its temperature sensitivity has been a bottleneck impeding their applications. We find that good thermal stability can be achieved in CaZrO 3 -modified KNN lead-free piezoceramics, in which the normalized strain d 33 * almost keeps constant from room temperature up to 140 o C. In situ synchrotron X-ray diffraction experiments combined with permitivity measurements disclose the occurrence of a new phase transformation under an electrical field, which extends the transition range between tetragonal and orthorhombic phases. It is revealed that such an electrically-enhanced diffused 2 polymorphic phase transition (EED-PPT) contributed to the boosted thermal stability of KNN based lead-free piezoceramics with high piezoelectricity. The present approach based on phase engineering should also be effective in endowing other lead-free piezoelectrics with high piezoelectricity and good temperature stability. IntroductionPiezoelectricity, a phenomenon whereby materials become electrically polarized upon the application of stress or deform in response to electrical stimuli, has been an active research topic since its discovery in 1880 by Pierre and Jacques Curie, because of its scientific interests and abundant applications. For the last half-century, the lead-contained materials, e.g., Pb(Zr,Ti)O 3 (PZT) and Pb(Mg,Nb)O 3 -PbTiO 3 (PMN-PT), have been the icons of piezoelectrics, exhibiting a morphotropic phase boundary (MPB), where plural phases with negligible difference in free energy coexist and strongly enhanced functional properties arise.[1] However, a possible toxicity of lead in PZT and PMN-PT has been raising intense health and environmental concerns; thus, the last decade has witnessed the surging dedication to viable lead-free alternatives. [2][3][4][5] Resembling the principle characteristics of MPB, [4][5][6][7][8][9][10] polymorphic phase transition (PPT) boundary has also been extensively pursued. [2, 11,12] Unfortunately, contrary to the nearly vertical MPB in the well-known PZT and PMN-PT systems, [1] the PPT in lead-free piezoelectrics is always tilted, resulting in unavoidable thermally unstable electromechanical properties. [13][14][15] Weak thermal stability is unacceptable for many industrial applications, even though lead-free piezoelectrics have competitive performance at ambient conditions. To address the issue, two approaches have been adopted so far, i.e., fabricating textured samples, [2] or shifting the PPT temperature T O-T well below room temperature. [13] However, the former confronts the poor reproducibility due to an excessively complex synthesis procedure; while the latter would inevitably sacrifice a large 3 portion of piezoelectric activity. Consequently, a barrier still exists in developing reliable lead-free piezomaterials as alternatives to currently market-dominating lead-based materials.Inspired by the nature of MPB in PZT and PMN-PT...

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Most of our knowledge of dislocation-mediated stress relaxation during epitaxial crystal growth comes from the study of inorganic heterostructures. Here we use Bragg coherent diffraction imaging to investigate a contrasting system, the epitaxial growth of calcite (CaCO3) crystals on organic self-assembled monolayers, where these are widely used as a model for biomineralization processes. The calcite crystals are imaged to simultaneously visualize the crystal morphology and internal strain fields. Our data reveal that each crystal possesses a single dislocation loop that occupies a common position in every crystal. The loops exhibit entirely different geometries to misfit dislocations generated in conventional epitaxial thin films and are suggested to form in response to the stress field, arising from interfacial defects and the nanoscale roughness of the substrate. This work provides unique insight into how self-assembled monolayers control the growth of inorganic crystals and demonstrates important differences as compared with inorganic substrates.

BiFeO 3 -PbTiO 3 (BF-PT) powders with mixed tetragonal and rhombohedral crystal structure (morphotropic phase boundary: MPB) were modified with lanthanum to provide a wide variation in tetragonal distortion. X-ray diffraction from both the powder and the corresponding bulk ceramic demonstrated that the MPB in the bulk is shifted from 1-5 mol % towards the tetragonal PT as compared to the powder. This shift was correlated with the degree of tetragonal distortion as quantified by c=a ratio.

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High-energy synchrotron diffraction has been employed to demonstrate a time-dependent structural phase transition in 0.2K1/2Bi1/2TiO3–0.8Na1/2Bi1/2TiO3. It is demonstrated that the rapid increase in electric field (≥0.25 kV mm−1 s−1) induces a transformation from rhombohedral to mixed phase rhombohedral and tetragonal symmetry. When the electric field is applied slowly (<0.25 kV mm−1 s−1), no transformation occurs, and rhombohedral symmetry is maintained. The extent of transformation increases with the rate of increase in electric field. High-speed capture of diffraction images (7 frames per second) has been used to show the evolution of the phase transformation as a function of time.

Herringbone domain structures have been mapped using electron backscatter diffraction (EBSD) in two tetragonal piezoelectrics, lead zirconate titanate, Pb(Zr,Ti)O 3 , and bismuth ferrite -lead titanate, (PbTi) 0.5 (BiFe) 0.5 O 3 . Analysis of the domain misorientations across the band junctions shows that the structures correspond very well to crystallographic models. High resolution mapping with a 20 nm step size allowed the crystal rotation across one of these band junctions in lead zirconate titanate to be studied in detail and allowed an improved estimation of the peak strain at the junction, of 0.56 GPa. The significance of this for crack nucleation and propagation in such materials is discussed.

Solid solution formation in the system (1−x) [(Na0.5K0.5NbO3)0.93–(LiTaO3)0.07]−x[BiScO3] has been investigated for compositions, 0≤x≤0.05. The tetragonal c/a ratio decreases with increasing BiScO3 content, with a discontinuity at x=0.02, X‐ray diffraction patterns appear pseudo‐cubic for x>0.02. There is a 10‐fold reduction in grain size between x=0.015 and x=0.02: dielectric peaks become suppressed and broadened at this point. Piezoelectric d33 charge coefficients decrease across the compositional range. Results are discussed in terms of chemical inhomogeneity and grain‐size effects.