Orthorhombic-tetragonal Phase Transition Research Articles

The Role of Adding Bi0.5A0.5ZrO3 in Affecting Orthorhombic-Tetragonal Phase Transition Temperature and Electrical Properties in Potassium Sodium Niobate Ceramics

By studying the alone and synergetic effects of Zr4+ and Bi3+ on potassium sodium niobate ((K, Na)NbO3, KNN) ceramics, we revealed how Bi0.5A0.5ZrO3 (A = K, Na, Ag, and (Na0.82K0.18)) reduces the orthorhombic-tetragonal phase transition temperature (TO–T) of KNN ceramics. Investigations into the alone effects reveal that aliovalent substitutions on K+/Na+ (Nb5+) with Bi3+ (Zr4+) inevitably destroy long-range ordering (LRO) and thus worsen piezo/ferroelectric properties. Despite this, Zr4+ can replace Nb5+ within a high content, remain an orthorhombic (O) phase, and slightly increase TO–T. Although substituting on K+/Na+ with Bi3+ decreases TO–T, it already significantly destroyed LRO before shifting TO–T to room temperature. Then, investigations into the synergetic effects show that Zr4+ acts as a buffer, Bi3+ is an accelerator, and A+ is a stabilizer. The buffer can exist in KNN ceramics within a high content and neutralizes the charges caused by the accelerator that concentrates on decreasing TO–T, and the stabilizer compensates for the stability of the perovskite phase. Their synergetic effects explain why Bi0.5A0.5ZrO3 can gradually reduce the TO–T of KNN ceramics without significantly destroying LRO. Therefore, this work helps understand how Bi0.5A0.5ZrO3 decreases TO–T and further design the phase boundary for KNN ceramics.

Correlation between Fermi surface reconstruction and superconductivity in pressurized FeTe0.55Se0.45

Here we report the first results of the high-pressure Hall coefficient (RH) measurements, combined with the high-pressure resistance measurements, at different temperatures on the putative topological superconductor FeTe0.55Se0.45. We find the intimate correlation of sign change of RH, a fingerprint to manifest the reconstruction of Fermi surface, with structural phase transition and superconductivity. Below the critical pressure (PC) of 2.7 GPa, our data reveal that the hole - electron carriers are thermally balanced (RH=0) at a critical temperature (T*), where RH changes its sign from positive to negative, and concurrently a tetragonal-orthorhombic phase transition takes place. Within the pressure range from 1bar to PC, T* is continuously suppressed by pressure, while TC increases monotonically. At about PC, T* is indistinguishable and TC reaches a maximum value. Moreover, a pressure-induced sign change of RH is found at ~PC where the orthorhombic-monoclinic phase transition occurs. With further compression, TC decreases and disappears at ~ 12 GPa. The correlation among the electron-hole balance, crystal structure and superconductivity found in the pressurized FeTe0.55Se0.45 implies that its nontrivial superconductivity is closely associated with its exotic normal state resulted from the interplay between the reconstruction of the Fermi surface and the change of the structural lattice.

Looking beyond the Surface: The Band Gap of Bulk Methylammonium Lead Iodide.

Despite the intense research on photovoltaic lead halide perovskites, reported optical properties as basic as the absorption onset and the optical band gap vary significantly. To unambiguously answer the question whether the discrepancies are a result of differences between bulk and "near-surface" material, we perform two nonlinear spectroscopies with drastically different information depths on single crystals of the prototypical (CH3NH3)PbI3 methylammonium lead iodide. Two-photon absorption, detected via the resulting generation of carriers and photocurrents (2PI-PC), probes the interband transitions with an information depth in the millimeter range relevant for bulk (single-crystal) material. In contrast, the transient magneto-optical Kerr effect (trMOKE) measured in a reflection geometry determines the excitonic transition energies in the region near (hundreds of nm) the surface which also determine the optical properties in typical thin films. To identify differences between structural phases, we sweep the sample temperature across the orthorhombic-tetragonal phase transition temperature. In the application-relevant room-temperature tetragonal phase (at 170 K), we find a bulk band gap of 1.55 ± 0.01 eV, whereas in the near-surface region excitonic transitions occur at 1.59 ± 0.01 eV. The latter value is consistent with previous reflectance measurements by other groups and considerably higher than the bulk band gap. The small band gap of the bulk material explains the extended infrared absorption of crystalline perovskite solar cells, the low-energy bands which carry optically driven spin-polarized currents, and the narrow bandwidth of crystalline perovskite photodetectors making use of the spectral filtering at the surface.

Effect of electric field orientation on ferroelectric phase transition and electrocaloric effect

The influence of electric field orientation on phase transitions and electrocaloric effects (ECEs) in BaTiO3 single crystals is studied by performing molecular dynamics simulation of first-principles-based effective Hamiltonian. The ECE is directly characterized via the adiabatic temperature change (ΔT) under the microcanonical ensemble. The simulation results demonstrate that different orientation relationship between the electric field and the polarization direction of ferroelectric phase leads to abundant phase structures and ECE behaviors. The electric-field-induced phase transitions produce remarkable ECE peaks/valleys, whereas the polarization rotation and/or extension without phase transition induced by the electric field produces small positive ECEs, which are insensitive to temperature and electric field direction. For the tetragonal-cubic phase transition, ECEs are always positive regardless of the applied electric field direction, and the value and width of ECE peak both increase with the reduction of the angle between electric field and crystallographic orientation. For the orthorhombic-tetragonal and rhombohedral-orthorhombic phase transitions, the sign of ECE gradually changes from negative to positive during the field direction rotates in planes between crystallographic orientations, and the coexistence of positive and negative appears when the field along non-crystallographic directions. Our simulated results shed light on a comprehensive physical understanding of the electric-field-orientation dependent ECE and offer instruction for the design of the refrigeration cycle by combining positive and negative ECEs.

Reduced degree of phase coexistence in KNN-Based ceramics by competing additives

This work reveals the role of Bi3+ in the controlling ability of (Bi0.5Ag0.5)ZrO3 to the phase structure of potassium sodium niobate based ceramics by introducing Ba2+. The substitution on (Bi0.5Ag0.5)2+ with Ba2+ results in the reduced degree of phase coexistence, accompanying the phase transition from coexisting orthorhombic-tetragonal (O–T) phases to an orthorhombic (O) phase, which is demonstrated by the systematic phase structure analysis and variations of electrical properties. The analysis reveals that the high electronegativity of Bi3+ results in the increased covalent bond character and then increases the stabilization of the T phase, leading to a decrease in orthorhombic-tetragonal phase transition temperature (TO-T). Therefore, this work not only compares the controlling ability of Ba2+ and (Bi0.5Ag0.5)2+ to TO-T but also reveals which and how elements cause the decreased TO-T, promoting the understanding of the effect of additives on the phase structure.

Photoluminescence signatures of thermal expansion, electron-phonon coupling and phase transitions in cesium lead bromide perovskite nanosheets.

In this article, photoluminescence (PL) behaviors of inorganic cesium lead bromide (CsPbBr3) nanosheets are investigated in a broad temperature range from 5 to 500 K. As can be seen from the temperature evolution of the PL peak position, the bandgap blueshift induced by thermal lattice expansion is found to be gradually compensated by the bandgap redshift caused by electron-phonon coupling for the temperature variation from 5 K to 360 K. As the temperature is further increased, the nearly completely compensated PL peak position turns to exhibit a rapid blueshift, again at ∼360 K. Such turning behavior is consistent with an orthorhombic-tetragonal (γ-β) phase transition at this critical temperature. For the PL linewidth, it shows a continuous broadening at temperatures beyond 40 K, suggesting the dominant role of phonon scattering, especially at high temperatures. These findings of temperature dependent photophysical properties of CsPbBr3 nanosheets may provide useful information of their further applications in the emerging nano photo-electronic devices.

Thermodynamic analyses of the orthorhombic-to-tetragonal phase transition in Pr2-xNdxNiO4+δ under controlled oxygen partial pressures.

The behavior of the structural orthorhombic-tetragonal phase transition of Pr2-xNdxNiO4+δ, a candidate material for solid oxide fuel cells and oxygen permeation membranes, was investigated by differential scanning calorimetry and thermogravimetry (TG), under controlled oxygen partial pressures, P(O2). The structural phase transition temperature, TP, of Pr2-xNdxNiO4+δ increased with increasing Nd content, x, or increasing P(O2). The phase transitions of all compositions involved discrete variations in the oxygen content, Δδ, which were observed in the TG curves under various P(O2) values. Δδ of Pr2-xNdxNiO4+δ with 0.5 ≤x≤ 1.5 were between those of Nd2NiO4+δ and Pr2NiO4+δ, regardless of P(O2), and were slightly increased with decreasing P(O2). It was proposed that the effect of the valence change of the Pr ion on Δδ was decreased with increasing Nd content. The standard enthalpy change, ΔH°, and entropy change, ΔS°, of the phase transition were estimated from the Ellingham diagrams and van't Hoff plots, which were prepared from the relationship between P(O2) and TP using an ideal solution model. ΔS° was decreased with increasing Nd content for the specimens with 0.0 ≤x≤ 1.5. The ΔH° of Pr2-xNdxNiO4+δ with 0.0 ≤x≤ 1.5 was almost constant for all Nd contents. The increase in the phase transition temperature of Pr2-xNdxNiO4+δ with increasing x from 0.0 to 1.5 was successfully explained using the calculated values of ΔH° and ΔS°.

Effect of manganese doping on ferroelectric and piezoelectric properties of KNbO<sub>3</sub> and (K<sub>0.5</sub>Na<sub>0.5</sub>)NbO<sub>3</sub> lead-free ceramics

Potassium sodium niobate ((K<sub>0.5</sub>Na<sub>0.5</sub>)NbO<sub>3</sub>)-based lead-free piezoelectric ceramics are excellent ferroelectric materials and have been demonstrated to have many practical applications. Recent studies have revealed that chemical doping plays a crucial role in optimizing the electromechanical coupling properties of (K<sub>0.5</sub>Na<sub>0.5</sub>)NbO<sub>3</sub>-based piezoelectric ceramics. In this paper, MnO<sub>2</sub> is doped into potassium niobate (KNbO<sub>3</sub>) and (K<sub>0.5</sub>Na<sub>0.5</sub>)NbO<sub>3</sub> piezoelectric ceramics prepared by the conventional solid-state reaction method. The influences of doped Mn cation on KNbO<sub>3</sub> and (K<sub>0.5</sub>Na<sub>0.5</sub>)NbO<sub>3</sub> piezoelectric ceramics including microstructure and macroscopic electrical properties are systematically investigated. The doping effects of Mn cation on the KNbO<sub>3</sub> and (K<sub>0.5</sub>Na<sub>0.5</sub>)NbO<sub>3</sub> piezoelectric ceramics are significantly different from each other. For the Mn-doped KNbO<sub>3</sub> piezoelectric ceramics, the sizes of ferroelectric domains are reduced. Meanwhile, the diffused orthorhombic-tetragonal phase transition is observed, which is accompanied by reducing dielectric loss and Curie temperature, and broadening vibration peaks in Raman spectrum. It is known that the oxygen vacancy can be formed to compensate for the charges created by the acceptor doping of Mn into the <i>B</i> site of perovskite, and thus forming a defect dipole with the acceptor center. From the ferroelectric measurement, a double hysteresis loop (<i>P</i>-<i>E</i> curve) and a recoverable electric-field-induced strain due to the formation of defect dipole are observed. On the contrary, for the Mn-doped (K<sub>0.5</sub>Na<sub>0.5</sub>)NbO<sub>3</sub> piezoelectric ceramics, the sizes of ferroelectric domains are not reduced. Meanwhile, the Curie temperature and vibration peaks in Raman spectrum are not changed. A rectangular hysteresis loop (<i>P</i>-<i>E</i> curve) and an unrecoverable electric-field-induced strain are observed in the ferroelectric measurement. The difference between these systems might originate from the greater ionic disorder and lattice distortion in (K<sub>0.5</sub>Na<sub>0.5</sub>)NbO<sub>3</sub> piezoelectric ceramics. The difference in ionic radius between Na<sup>+</sup> and K<sup>+</sup> can affect the migration and distribution of oxygen vacancies, which makes it difficult to form stable defect dipoles in the Mn-doped (K<sub>0.5</sub>Na<sub>0.5</sub>)NbO<sub>3</sub> piezoelectric ceramics. The results will serve as an important reference for preparing high-performance (K<sub>0.5</sub>Na<sub>0.5</sub>)NbO<sub>3</sub>-based piezoelectric ceramics via chemical doping.

Remarkably strong piezoelectricity, rhombohedral-orthorhombic-tetragonal phase coexistence and domain structure of (K,Na)(Nb,Sb)O3–(Bi,Na)ZrO3–BaZrO3 ceramics

Demands for lead-free piezoelectric ceramics increase rapidly owing to environmental concerns. Strong piezoelectricity in (K,Na)Nb-based ceramics was usually realized by taking the advantage of rhombohedral-tetragonal (R-T) phase coexistence associating with the R-T phase transition. In this paper, we report the study results on a series of dense (0.96-x)(K0.48Na0.52)(Nb0.96Sb0.04)O3−0.04(Bi0.50Na0.50)ZrO3−xBaZrO3 ceramics (abbreviated as KNNS-BNZ-xBZ ceramics), in which the one with x = 0.01 shows a giant piezoelectric coefficient d33 of 610 pC/N with quite large electromechanical coupling coefficients of kp = 0.58 and k33 = 0.69. Dielectric measurement indicated that the rhombohedral-orthorhombic (R–O) phase transition temperature TR-O, orthorhombic-tetragonal (O-T) phase transition temperature TO-T and Curie temperature TC of this ceramic are −29 °C, 43 °C and 241 °C, respectively. Meanwhile, XRD analysis unveiled that this ceramic is in rhombohedral-orthorhombic-tetragonal (R-O-T) phase coexistence at room temperature. Acid etching revealed that domain structure inside the polycrystalline grains consists of lamellar domain stacks with a large amount of hierarchical nanodomain structure existing in part of the lamellar domains. The observed strong piezoelectricity is ascribed to the R-O-T phase coexistence and the corresponding characteristic domain structure.

Cation effect on excitons in perovskite nanocrystals from single-dot photoluminescence of CH3NH3PbI3

The single-dot photoluminescence properties of $\ensuremath{\sim}7\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ perovskite $\mathrm{MAPb}{\mathrm{I}}_{3}(\mathrm{MA}=\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{{\mathrm{H}}_{3}}^{+})$ nanocrystals (NCs) were investigated in the $5\text{--}295\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ temperature range both in spectral and time domains. Repeatable single-dot measurements were facilitated by the use of a protective polymer, which stabilized the NCs. Temperature-induced phase transition and exciton-phonon interactions were revealed as well as the exciton fine structure. A pronounced spectral jump of the emission peak at $140\text{--}160\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, indicating a tetragonal-orthorhombic phase transition, was observed. In addition, the emission linewidth of $\ensuremath{\sim}0.6\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ was measured, which is the narrowest ever recorded for this perovskite material system. A $\ensuremath{\sim}4.0\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ phonon mode was identified for the NCs at 5 K, defining the linewidth thermal broadening. In general, the presence of $\mathrm{M}{\mathrm{A}}^{+}$ leads to broader spectra than for $\mathrm{C}{\mathrm{s}}^{+}$ or $\mathrm{F}{\mathrm{A}}^{+}$ containing perovskite NCs. It is attributed to higher polarity of this cation, thus it is more susceptible to spectral diffusion, which is clearly observed here. Photoluminescence decay measurements indicated that the recombination from the lowest energy state of the emission level manifold is partially forbidden. This is opposite to $\mathrm{C}{\mathrm{s}}^{+}$ cation NCs, highlighting the central role of the positive ion in the exchange interaction in perovskites. Finally, delayed luminescence was found to govern the recombination dynamics below room temperature, suggesting an involvement of trap sites for the orthorhombic phase. The reported photophysics of a quantum-confined exciton in this material, which is of interest for various light-converting applications, clarifies the role of the cation in perovskite nanocrystal optical properties.

Unusual Thermal Boundary Resistance in Halide Perovskites: A Way To Tune Ultralow Thermal Conductivity for Thermoelectrics.

Halide perovskites have emerged as promising candidates as the active material in photovoltaics and light-emitting diodes. They possess unusual bulk thermal transport properties that have been the focus of a number of studies, but there is much less understanding of thermal transport in thin films where a diverse range of structures and morphologies are accessible. Here, we report on the tuning of in-plane thermal conductivity in methylammonium lead iodide thin films by morphological control. Using 3-ω measurements, we find that the room temperature thermal conductivity of thermally evaporated methylammonium lead iodide perovskite films ranges from 0.31 to 0.59 W/(m K). We measure a discontinuity in thermal conductivity at the orthorhombic-tetragonal phase transition and explore this using density functional theory and attributing it to a collapse in the phonon group velocity along the c-axis of the tetragonal crystal. Moreover, we have quantified the thermal boundary resistance (Kapitza resistance) for thermally evaporated films, allowing us to estimate the Kapitza length, which is 36 ± 2 nm at room temperature and 15 ± 2 nm at 100 K. Curiously, the Kapitza resistance has a strong temperature dependence which we also explore using density functional theory, with these results suggesting an important role of methylammonium rotational modes in scattering phonons at the crystallite boundaries.

Effect of Electric Field Orientation on Ferroelectric Phase Transition and Electrocaloric Effect

This paper demonstrates the influence of electric field orientation on the phase transition and the electrocaloric effect (ECE) in BaTiO3 single crystals by performing molecular dynamics simulation of first-principles-based effective Hamiltonian, where the ECE is directly characterized through the adiabatic temperature change (ΔT) of the microcanonical ensemble. Different orientation relationship between the electric field and the polarization direction of ferroelectric phases leads to abundant polarization states and various phase structures. If the electric field direction is collinear to the polarization direction of a ferroelectric phase, the stability of the corresponding phase will be improved, and its phase area in the phase diagram is also enlarged; whereas the noncollinear electric field has reverse impacts and distorts the lattice to a low symmetry monoclinic phase. The phase transitions produce large ECEs far beyond those caused by the domain switching. The tetragonal-cubic phase transition produces a positive ECE regardless of the electric field directions, while the orthorhombic-tetragonal and rhombohedral-orthorhombic phase transitions induce positive or negative ECE, or even their coexistence, depending on the field-induced phase transitions under different electric field directions. The signs of ECE are determined by the lattice symmetry before and after the field-induced phase transition. Our simulated results help reveal a more comprehensive physical understanding of ECE and offer an instruction for the design of the refrigeration cycle by combining positive and negative ECEs.

Thermo-mechanical behaviours of Li3xLa1/3-xMO3 (M = Ta and Nb)

The present study investigated thermo-mechanical behaviour of A-site deficient perovskites Li3xLa1/3-xTaO3 (LLTaO) and Li3xLa1/3-xNbO3 (LLNbO) with x = 0.00 to 0.06 as a function of temperature by means of thermo-mechanical analysis. LLTaO for all compositions exhibits an orthorhombic-tetragonal phase transition around 200 K, above which it has a tetragonal phase. As the temperature increases from 200 K to 400 K, the linear expansion coefficient shows a slight decrease for all the compositions, followed by a constant value of ~3 × 10−6 K−1 above 400 K regardless of x. On the other hand, LLNbO has an orthorhombic phase for x ≤ 0.02 and the tetragonal for x ≥ 0.04 at room temperature. The linear expansion coefficient of LLNbO with x ≤ 0.02 shows a peak around 470 K, which can be attributed to an orthorhombic-tetragonal phase transition. In addition, the expansion coefficient of LLNbO with x ≥ 0.02 significantly increases as the temperature rises from 200 K to 400 K, followed by a plateau and a relatively abrupt decrease after reaching a temperature above 600 K. Additional ionic conductivity measurement of LLNbO suggested that the inconstant expansion behaviour could be attributed to the lithium-ion motion.

Giant piezoelectricity, rhombohedral-orthorhombic-tetragonal phase coexistence and domain configurations of (K,Na)(Nb,Sb)O3–BiFeO3–(Bi, Na)ZrO3 ceramics

Research on lead-free piezoelectric ceramics has been an important subject in recent years due to increasingly strong environmental concerns. In this paper, we report the piezoelectric performance, phase transitions and domain structure for a series of dense (0.994-x)(K0.40Na0.60)(Nb0.955Sb0.045)O3–0.006BiFeO3–x(Bi0.50Na0.50)ZrO3 ceramics prepared by two step-sintering through solid-state reaction. Among these ceramics, the one with x = 0.03 shows a giant piezoelectricity with remarkably high piezoelectric coefficient d33 of 550 pC/N at room temperature. Concurrent measurements of ε′ and ε′′ vs. temperature dependences and X-ray diffraction analysis showed that ceramics with x ≤ 0.04 are in rhombohedral-orthorhombic-tetragonal (R-O-T) phase coexistence at room temperature. More intriguingly, the rhombohedral-orthorhombic phase transition temperature, TR-O, is almost independent of the (Bi0.50Na0.50)ZrO3 content, while orthorhombic-tetragonal phase transition temperature, TO-T, decreases largely with increasing the (Bi0.50Na0.50)ZrO3 content. Domain configurations of the ceramic with x = 0.03 were investigated by acid-etching. Complicated domain patterns consisting of watermark-shaped domains of long parallel stripes separated by irregularly shaped boundaries are seen before poling. In contrast, relatively simple domain patterns of long parallel stripes with some nanodomains appearing in a part of broad stripes are observed after poling. The obtained excellent piezoelectric properties are ascribed to the R-O-T phase coexistence and the corresponding characteristic domain structure.

Enhanced thermal reliability of Mn-doped (K, Na)NbO3-based piezoelectric ceramics

Improving temperature stability of (K, Na)NbO3-based ceramics can facilitate their development in practical application. In this work, 0.5 mol% Mn elements doped 0.95(Na0.5K0.5)NbO3–0.05(Bi0.5Na0.5)HfO3 (abbreviated as KNN) piezoelectric ceramics are reported. Permittivity–temperature curves reveal that the Mn-doped ceramics have higher Curie temperature and more dispersive orthorhombic–tetragonal phase transition compared with undoped ceramics, which improves the temperature stability of the KNN-based ceramics. The electric-field-induced strain varies less than 22% over a wide temperature range from 20 to 200 °C for the Mn-doped KNN-based ceramics, which is much better than the undoped KNN-based ceramics (variation: 38%). Therefore, our work provided a new promising candidate for temperature-insensitive piezoelectric ceramic devices.

Synergistic effect of terbium and calcium ions on the temperature stability and dielectric loss of BaTiO3-based ceramics

To improve permittivity-temperature stability for BaTiO3 ceramics containing terbium dopant, a new dielectric system (Ba1–xTbx)(Ti1–x/2Cax/2)O3 (x = 0.01–0.20) (BTTC) was prepared at 1400 °C using the conventional mixed-oxide method. The solubility limit of Tb/Ca in BTTC was determined by XRD to be x = 0.12 and BTTC exhibited tetragonal perovskite structures. An abnormal phenomenon was discovered that with increasing x from 0.01 to 0.04 the dielectric peak associated with the orthorhombic-tetragonal phase transition point shifted to low temperatures. The mixed valence states of Tb3+/Tb4+ and mixed A and B occupancy of Tb/Ca2+ in the perovskite lattice were confirmed. BTTC with x = 0.03 and 0.04 satisfied X5R and X4P specifications, respectively, and exhibited some advantageous properties such as higher relative density (ρr = 91 and 93%), lower loss (tan δ = 0.016 and 0.014), higher permittivity (ε′RT = 1820 and 1360), and frequency stability in the range of 1–107 Hz. The point defect chemistry is discussed and the real formula of BTTC is suggested. By comparing BaTiO3 ceramics containing Tb or co-doped with Tb and other rare-earth ions (La, Dy) in many existed works in the dielectric field, the synergistic effect of Tb and Ca in BaTiO3 is more significant and helpful to improve the temperature stability of Tb-related BaTiO3 ceramics or to reduce the dielectric loss in the working temperature region. A small number of B-site Tb4+ in BTTC played a decisive role in the low dielectric loss.

Structural, dielectric, and piezoelectric properties of lead-free (1 − x)K1/2Na1/2NbO3 − xCa(Zn1/3Ta2/3)O3 perovskite solid solution

Polycrystalline ceramics in (1 − x)K1/2Na1/2NbO3 − xCaZn1/3Ta2/3O3 (abbreviated as KNN-CZT) solid solution have been fabricated by using conventional solid-state synthesis route and the effect of CZT addition on the crystal structure, dielectric, and piezoelectric behaviour of KNN has been studied. Rietveld refinement of the room temperature X-ray diffraction data confirmed the crystal structure to be a pure perovskite phase for the compositions in the range x = 0 − 0.10. Further, the crystal structure gradually changed from orthorhombic to cubic via the formation of rhombohedral and tetragonal mixed phases with the increase in x. Raman spectroscopy suggested damping of phonon modes and a strong anharmonicity in the crystal arising due to the increased disorder in the structure as a consequence of multivalent cations occupying the A and B sites in CZT substituted compositions. Dielectric behaviour indicated the shifting of both tetragonal-cubic and orthorhombic–tetragonal phase transitions towards room temperature. The temperature dependent dielectric constant was modelled by Lorentz quadratic law, and the fitted value of diffuseness parameters confirmed an increase in diffuseness of phase transition with increasing substitution. Sample with the composition corresponding to x = 0.02 showed an improved piezoelectric coefficient d33 ~ 125 pC/N and electromechanical coupling coefficient kp ~ 30% at room temperature and d33 ~ 61 pC/N and kp ~ 24% at 300 °C which make this material a potential candidate for high-temperature piezoelectric applications. Variation of voltage coefficient (g33) with the change in compositions are also reported. Improvement in the piezoelectric properties is attributed to the reduced oxygen vacancies.

Co-Doping Effect of BiGaO3 and (Bi,Na,K,Li)ZrO3 on Multi-Phase Structure and Piezoelectric Properties of (K,Na)NbO3 Lead-Free Ceramics

The phase boundary structure of (K,Na)NbO3 piezoelectric ceramic was modified by doping with Bi(Na,K,Li)ZrO3 and BiGaO3 through normal solid-state sintering. Rietveld refinements by X-ray diffraction revealed that the Bi(Na,K,Li)ZrO3/BiGaO3 co-doping in (K,Na)NbO3 led to a multi-phase structure at room-temperature, effectively moving the rhombohedral-orthorhombic (R-O) and orthorhombic-tetragonal (O-T) polymorphic phase transition temperatures close to the room temperature region. Increased levels of doping also generated a structural transition, i.e., triphasic R-O-T to diphasic R-T (T-rich) and finally to R-T (R-rich), contributing to shrinkage of the O phase as well as the increase of R phase fraction. A sensitive influence of the BiGaO3 doping (0.001 mole fraction level) on the structural properties such as the phase and microstructure was shown, resulting from the effect of the super-tetragonal structure of BiGaO3. The d33 property was strongly dependent on the phase and its volume fraction, in addition to the grain sizes. Eventually, enhanced and balanced properties of the piezoelectric coefficient and Curie temperature (d33 = 309 pC/N, TC = 343 °C) were obtained when the doped ceramic had a T-rich (86%) R-T structure.

Structural evolution and electrical properties of lead-free (1-x) (K0.4Na0.6)Nb0.96Sb0.04O3-xBa0.1(Bi0.5Na0.5)0.9ZrO3 ceramics

New lead-free piezoelectric ceramic system based on (1-x) (K0.4Na0.6) Nb0.96Sb0.04O3-xBa0.1(Bi0.5Na0.5)0.9ZrO3 was designed and fabricated by using conventional ceramic processing techniques. An investigation was carried out with a focus on the effects of Ba0.1(Bi0.5Na0.5)0.9ZrO3 content on the phase structure, dielectric and piezoelectric properties of the ceramics. It was found that adding Ba0.1(Bi0.5Na0.5)0.9ZrO3 could move the rhombohedral-orthorhombic phase transition temperature upwards and simultaneously shift the orthorhombic-tetragonal phase transition temperature downwards, and thereby resulted in the formation of a rhombohedral-tetragonal phase boundary with 0.045 ≤ x ≤ 0.05. The piezoelectric properties were obviously enhanced for the ceramics with compositions near the rhombohedral-tetragonal phase boundary. Furthermore, an analysis was as well conducted on the main characteristics of the ferroelectric-paraelectric phase transition of the ceramics. Our experimental results suggest that the Ba0.1(Bi0.5Na0.5)0.9ZrO3 is an effective additive for the construction of phase boundaries in (K,Na)NbO3-based ceramics, therefore benefiting the improvement of piezoelectric activity.

Effect of processing conditions on (Ba1-xCax)(Ti0.9Sn0.1)O3 lead-free ceramics for the enhancement of structural, humidity sensing and dielectric properties

The present study reports detailed guidelines for the preparation of high-quality perovskite (Ba1-xCax)(Ti0.9Sn0.1)O3 (BCTS) (x = 0.0–0.1) lead-free ceramics by solid state reaction. The compositions (x = 0.0–0.04) exhibit orthorhombic–tetragonal phase transition (TO-T), except x ≥ 0.06 that shows a pure tetragonal structure phase which conformed by X-ray diffraction (XRD). The microstructure and purity of the sintered ceramics were examined using scanning electron microscope equipped with an energy dispersive spectrometer (SEM-EDS). Some pores existing in the grain boundary were observed at high concentrations of Ca content. Field emission scanning electron microscope (FE-SEM) was used to examine the morphology of sensing film of the calcined powder and it was prepared as a humidity sensor using screen-printing technique. All the compositions exhibited poor sensitivity toward the humidity sensing in the range of 0–98% RH at room temperature. Hot-stage microscope (HSM) has been used to investigate the sintering curve of the pure calcined powder and it was found that the suitable sintering temperature for obtaining a fully dense microstructure is 1400 °C. The highest values of permittivity (er = 46,515, at 10 kHz) and piezoelectric coefficient (d33 = 510 pC/N) were achieved in the composition x = 0.02. The difference between alumina and platinum crucibles for the processing of the powders has been introduced, and by the aid of dispersive spectrometer analysis and it was indicated that use of alumina crucibles leads to the undesired presence of Al in the ceramics, which can be prevented by using a capped platinum crucibles.