Orthorhombic-tetragonal Phase Transition Research Articles

Properties of (Na 0.53K 0.4Li 0.07)Nb 1− x% Sb x% O 3 ceramics with high lithium and antimony content

(Na 0.53K 0.4Li 0.07)Nb 1− x% Sb x% O 3 (NKLNS x) piezoelectric ceramics with high content of lithium and antimony have been prepared by the solid-state reaction technique. The effect of Sb content on the crystallographic structure, phase transitions, piezoelectric and ferroelectric properties of NKLNS x ceramics is studied. The X-ray diffraction results suggest that the structure of the ceramics changes from pure tetragonal phase to the coexistence of orthorhombic and tetragonal phases with the increasing x, and that a little amount of second phase, LiSbO 3, is formed. The increase of Sb content induces a decrease of the Curie temperature T C and an increase of the orthorhombic–tetragonal phase transition temperature T O-T. The two compositions, NKLNS8.8 and NKLNS9.1, with T O-T closest to room temperature, possess optimum properties, with d 33 ∼ 290 pC/N, ɛ r ∼ 1340, and P r ∼ 24.2 μC/cm 2. These properties are superior to those of previously reported (K, Na, Li)(Nb, Sb)O 3 systems with low content of lithium and antimony. The thermal stability investigations show that the change of k p is minor for the ceramics that do not undergo a phase transition in the examined temperature range.

Laser sintering effects on the physical properties and structure of 0.94(K0.5Na0.5)NbO3-0.06LiTaO3 lead-free piezoelectric ceramics

Taking laser as the heat source for ceramic sintering, 0.94K0.5Na0.5NbO3-0.06LiTaO3 ceramics with unique physical properties were prepared. Fine ferroelectric properties with remnant polarization of 18 μC/cm2 and coercive field of 18.8 kV/cm were obtained. The piezoelectric properties and density of the ceramics were d33∼120 pC/N, kp∼36.6%, TC∼420 °C, and ρ∼4.125 g/cm3. The shift of the two phase transition temperatures (tetragonal-orthorhombic phase transition temperature TO-T shifting to 100 °C from 190 °C while Curie temperature Tc shifting to 420 from 394 °C) in laser-sintered ceramics reflects large structure distortion of phase transition and Li+ substitution of high temperature phase. Orientation degree of the texture structure based on anisometric grains of submicron diameter in the ceramics was 23%. The texture structure corresponding to the fine ferroelectric properties was resulted from the mass transform in liquid phase and strictly heating directivity of laser irradiation. O2 discharge through coaxial nozzle takes some actions like O2 annealing and effectively suppression on the volatilization of Na2O during laser sintering. Lager substitution of Li+ leads to the presence of small amount of K3Li2Nb5O15 phase damaging in d33. By analysis of the sintering effects on the properties and structure evolution, laser sintering was shown as a promising method in preparation of functional ceramics with excellent and applicable physical properties, compared to a traditional furnace sintering method.

Polymorphic phase transition and excellent piezoelectric performance of (K0.55Na0.45)0.965Li0.035Nb0.80Ta0.20O3 lead-free ceramics

A lead-free ceramic with the composition (K0.55Na0.45)0.965Li0.035Nb0.80Ta0.20O3 was found having an outstanding piezoelectric performance. It possesses high piezoelectric properties of d33=262 pC/N, kp=0.53, k33=0.63, and k31=0.31 with ε′=1290 and tan δ=0.019 at room temperature. In spite of the orthorhombic-tetragonal polymorphic phase transition around 30 °C, temperature stability of the electromechanical coupling coefficients is very good over the common usage temperature interval between −40 and 85 °C. Furthermore, the piezoelectric properties remain almost unchanged in the severe thermal aging test down to −150 °C and up to about 300 °C. It is suggested that the outstanding piezoelectric performance of this ceramic can be largely ascribed to the phase coexistence in a wide temperature range and the Ta-rich composition.

Effect of Sb2O5 Addition on Phase Formation and Characterization of (Bi1/2Na1/2)TiO3-(K1/2Na1/2)NbO3 Ceramics

In this study, Sb 2 O 5 doped (Bi 0.5 Na 0.5 )TiO 3 -(K 0.5 Na 0.5 )NbO 3 ceramics have been prepared by a conventional mixed-oxide method via vibro-milling technique. The phase formation behavior, microstructure and electrical properties have been investigated as a function of Sb 2 O 5 content. It is shown that the addition of Sb 2 O 5 can improve both density and electrical properties in BNT-KNN ceramics. While the dielectric constant and ferroelectric properties improve with addition of Sb 2 O 5 , the dielectric loss tangent of the ceramics does not change significantly. Though at high doping level of 1.0 mol % the density of ceramics decreases, the dielectric constant increases. Improved electrical properties obtained in Sb 2 O 5 -doped KNN-BNT are likely due to the coexistence of tetragonal and orthorhombic phases near the orthorhombic-tetragonal polymorphic phase transition at room temperature.

Dielectric Properties of Ba(Ti0.91Zr0.09)O3 Ceramics Doped with CuO

The structure and dielectric properties of Ba(Ti 0.91 Zr 0.09 )O 3 ceramics with CuO doped were investigated. XRD indicates that compositions have single tetragonal phase for x (CuO) ≤ 0.38wt% at room temperature and shows the increase in the distance of (110) lattice plane. A decrease in grain size is observed when the content of CuO increase. For x = 0, only one dielectric constant peak in ϵ -T curve is observed at TC. It appears a normal ferroelectric behavior. With increasing CuO, another dielectric constant peaks in ϵ -T curve is observed at the transition point (T 1 ) from tetragonal to orthorhombic. At Orthorhombic-Tetragonal phase transition point, larger diffuseness of the phase transition (▵Tdif), the frequency dispersion (▵Trelax) and γ are observed and a diffuse phase transition at Tetragonal-Cubic phase transition point. The physical mechanism of the relaxation process is discussed.

Structure and piezoelectric properties of lead-free (Na0.52K0.44−x )(Nb0.95−x Sb0.05)O3-xLiTaO3 ceramics

Lead-free (Na0.52K0.48−x )(Nb0.95−x Sb0.05)O3-xLiTaO3 (x = 0.025–0.05) piezoelectric ceramics in which the Sb content is kept constant, have been specially designed and successfully fabricated by a conventional solid state reaction method. The (Na0.52K0.48)(Nb0.95Sb0.05)O3 ceramics can be well sintered after A-site and B-site cations are replaced by Li+ and Ta5+, respectively. A single-phase perovskite structure remains within the studied substitution concentration. An orthorhombic-tetragonal phase transition occurs with gradually increasing the content of Li+ and Ta5+, and was identified in the composition range of 0.0375 < x < 0.0425 where two kinds of ferroelectric phases may coexist and simultaneously a strong compositional dependence of electrical properties was found out. An appropriate content of Sb effectively enhanced the piezoelectric properties of the materials. The optimum overall properties with a piezoelectric constant d 33 of 321 pC/N, a dielectric constant $$ \varepsilon_{33}^{T} $$ of 1,780, a planar electromechanical coupling coefficient k p of 0.52 and a Curie temperature T c of 315 °C were obtained in the composition with x = 0.0425, indicating the ceramics studied have potentials for replacing lead-containing ceramics for device applications.

Phase structure and electrical properties of strontium modified (Na0.48-x K0.48-x Li0.04 Sr x )Nb0.89 Ta0.05 Sb0.06 O3 ceramics

(Na0.48–xK0.48–xLi0.04Srx)Nb0.89Ta0.05Sb0.06O3 (abbreviated as NKLSrx NST) lead-free ceramics are fabricated by conventional solid state reaction. The material with perovskite structure contains both tetragonal and orthorhombic phases at room temperature when x ≤ 0.02, and it changes into cubic phase gradually as x increases. A second phase KSr2Nb5O15 begins to appear at x = 0.03 and becomes obvious with increasing Sr content. Dielectric studies show that the Sr2+ modification lowers the Curie temperature and induces the diffuseness of cubic–tetragonal phase transition. The addition of Sr2+ improves the density and piezoelectric properties of NKLNST. The ceramic modified with 0.008 mol SrO shows the optimum piezoelectric properties (d33 = 278 pC/N, kp = 0.504, tan δ = 0.025). Degradation of kp is found after the first thermal cycling due to the tetragonal–orthorhombic phase transition near room temperature. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Frustrated magnetic interactions, giant magneto–elastic coupling, and magnetic phonons in iron–pnictides

We present a detailed first-principles study of Fe-pnictides with particular emphasis on competing magnetic interactions, structural phase transition, giant magneto–elastic coupling and its effect on phonons. The exchange interactions J i, j ( R) are calculated up to ≈12 Å from two different approaches based on direct spin-flip and infinitesimal spin-rotation. We find that J i, j ( R) has an oscillatory character with an envelop decaying as 1/ R 3 along the stripe-direction while it is very short range along the diagonal direction and antiferromagnetic. A brief discussion of the neutron scattering determination of these exchange constants from a single crystal sample with orthorhombic-twinning is given. The lattice parameter dependence of the exchange constants, dJ i, j / da are calculated for a simple spin-Peierls like model to explain the fine details of the tetragonal–orthorhombic phase transition. We then discuss giant magneto–elastic effects in these systems. We show that when the Fe-spin is turned off the optimized c-values are shorter than experimental values by 1.4 Å for CaFe 2As 2, by 0.4 Å for BaFe 2As 2, and by 0.13 Å for LaOFeAs. We explain this strange behavior by unraveling surprisingly strong interactions between arsenic ions, the strength of which is controlled by the Fe-spin state through Fe–As hybridization. Reducing the Fe-magnetic moment, weakens the Fe–As bonding, and in turn, increases As–As interactions, causing a giant reduction in the c-axis. These findings also explain why the Fe-moment is so tightly coupled to the As-z position. Finally, we show that Fe-spin is also required to obtain the right phonon energies, in particular As c-polarized and Fe–Fe in-plane modes that have been recently observed by inelastic X-ray and neutron scattering but cannot be explained based on non-magnetic phonon calculations. Since treating iron as magnetic ion always gives much better results than non-magnetic ones and since there is no large c-axis reduction during the normal to superconducting phase transition, the iron magnetic moment should be present in Fe-pnictides at all times. We discuss the implications of our results on the mechanism of superconductivity in these fascinating Fe-pnictide systems.

Piezoelectric and Dielectric Properties of (Na0.5K0.5)NbO3–(Bi0.5Na0.5)0.94Ba0.06TiO3 Lead-Free Piezoelectric Ceramics

Lead-free piezoelectric ceramics (1-x)(Na0.5K0.5)NbO3–x(Bi0.5Na0.5)0.94Ba0.06TiO3 (abbreviated as NKN–BNBT100x, x = 0–0.10) were fabricated by a conventional mixed oxide route, and the effects of BNBT on electrical properties and crystal structures were investigated. X-ray diffraction analysis results indicate that BNBT diffuses into the NKN lattice to form a solid solution during sintering. All samples show pure perovskite structures and an orthorhombic-tetragonal phase transition with increasing x. The tetragonality, however, declined with further increasing x. The dielectric and piezoelectric properties are enhanced for the composition near the orthorhombic-tetragonal morphotropic phase boundary. The NKN–BNBT4 ceramics show good performance with piezoelectric constant d33 = 256 pC/N, planar electromechanical coupling factor kp = 43%, and dielectric constant ε33T/ε0 = 875. The results reveal that (1-x)(Na0.5K0.5)NbO3–x(Bi0.5Na0.5)0.94Ba0.06TiO3 ceramics are promising candidate materials for lead-free piezoelectric application.

Piezoelectric properties and stabilities of CuO-modified Ba(Ti,Zr)O3 ceramics

Due to the orthorhombic-tetragonal polymorphic phase transition near room temperature, undesirable large temperature dependence of piezoelectric properties is observed over common usage temperature range in BaTiO3 ceramics with high d33 values. Whereas shifting the phase transition temperature upward by partially substituting Ti with Zr is effective in reducing the piezoelectric temperature dependence, serious long-term degradation occurs. However, it is found that this could be overcome by incorporating a small amount of CuO additive. CuO-modified Ba(Ti0.9625Zr0.0375)O3 ceramics possess excellent piezoelectric properties of d33=300 pC/N, kp=0.493, and k33=0.651 with tan δ=0.011, and its kp remains larger than 0.40 in the broad temperature range from −43 to 73 °C and is almost constant between −25 and 55 °C. The results indicate that CuO-modified Ba(Ti,Zr)O3 ceramics are a promising low-cost lead-free material for practical applications.

Magnetic ordering and negative thermal expansion in PrFeAsO

We report the structure and magnetism of PrFeAsO, one of the parent phases of the Fe-As family of superconductors, as measured by neutron powder diffraction. In common with other REFeAsO materials, a tetragonal-orthorhombic phase transition is found on cooling below 136 K and striped Fe magnetism with $k=(1,0,1)$ is detected below $\ensuremath{\sim}85\text{ }\text{K}$. Our magnetic order-parameter measurements show that the ordered Fe moment along the $a$ axis reaches a maximum at $\ensuremath{\sim}40\text{ }\text{K}$, below which an anomalous expansion of the $c$ axis sets in that results in a negative thermal volume expansion of 0.015% at 2 K. We propose that this effect, which is suppressed in superconducting samples, is driven by a delicate interplay between Fe and Pr ordered moments.

Phase transitions in LaFeAsO: Structural, magnetic, elastic, and transport properties, heat capacity and Mössbauer spectra

We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of ``FeAs'' based oxypnictide superconductors. Upon cooling, this material undergoes a tetragonal-orthorhombic crystallographic phase transition at $\ensuremath{\sim}160\text{ }\text{K}$ followed closely by an antiferromagnetic ordering near 145 K. Analysis of these phase transitions using temperature dependent powder x-ray and neutron-diffraction measurements is presented. A magnetic moment of $\ensuremath{\sim}0.35{\ensuremath{\mu}}_{B}$ per iron is derived from M\"ossbauer spectra in the low-temperature phase. Evidence of the structural transition is observed at temperatures well above the transition temperature (up to near 200 K) in the diffraction data as well as the polycrystalline elastic moduli probed by resonant ultrasound spectroscopy measurements. The effects of the two phase transitions on the transport properties (resistivity, thermal conductivity, Seebeck coefficient, and Hall coefficient), heat capacity, and magnetization of LaFeAsO are also reported, including a dramatic increase in the magnitude of the Hall coefficient below 160 K. The results suggest that the structural distortion leads to a localization of carriers on Fe, producing small local magnetic moments which subsequently order antiferromagnetically upon further cooling. Evidence of strong electron-phonon interactions in the high-temperature tetragonal phase is also observed.

Tantalum influence on physical properties of (K 0.5Na 0.5)(Nb 1− xTa x)O 3 ceramics

Lead-free (K 0.5Na 0.5)(Nb 1− x Ta x )O 3 ceramics with x = 0.00–0.30 were prepared by the solid-state reaction technique. The effects of Ta on microstructure, crystallographic structure, phase transition and piezoelectric properties have been investigated. It has been shown that the substitution of Ta decreases Curie temperature T C and orthorhombic–tetragonal phase transition temperature T O–T, while increasing the rhombohedral–orthorhombic phase transition temperature T R–O. In addition, piezoelectric activity is enhanced with the increase of Ta content. The ceramics with x = 0.30 have the high value of piezoelectric coefficient d 33 = 205 pC/N. Moreover, k p shows little temperature dependence between −75° C and 75 °C, and d 33 exhibits very good thermal stability over the range from −196 °C to 75 °C in the aging test.

Structure and relaxor behavior of BaTiO3–CaTiO3–SrTiO3 ternary system ceramics

This study prepared (1−x)BaTiO3–xCaTiO3 (x=0–1) (denoted as BCT), (0.95−x)BaTiO3–xCaTiO3–0.05SrTiO3 (x=0.00–0.95) (denoted as BS5CT), and (0.90–x)BaTiO3–xCaTiO3–0.10SrTiO3 (x=0.00–0.90) (denoted as BS10CT) ceramics. Their preparation and structural and dielectric properties were characterized. The results show that the x-ray diffraction (XRD) peaks of the BCT, BS5CT, and BS10CT ceramics shift toward higher angles with increasing Ca. All samples were uniform and dense by 1400 °C sintering and the grain size decreased with the Ca addition. With the Sr and Ca cosubstitutions, the phase transition temperature (TC) decreased and the rhombohedral-orthorhombic and orthorhombic-tetragonal phase transition points disappeared. The dielectric peaks became increasingly broad and the relative dielectric constant of the BCT, BS5CT, and BS10CT samples decreased with increasing Ca. The replacement of Ba2+ ions by Sr2+ and Ca2+ ions reduced the lattice constant with increasing hydrostatic pressure and moved the Curie point downwards. The index of relaxation (γ) and the broadening parameter were estimated from a linear fit of the modified Curie–Weiss law.

Pr 3 + photoluminescence in ferroelectric (Ba0.77Ca0.23)TiO3 ceramics: Sensitive to polarization and phase transitions

We reported in this study that the photoluminescence (PL) spectra of (Ba0.77Ca0.23)TiO3:Pr3+ were sensitive to both polarization and phase transitions of the ferroelectric ceramics. Comparing with the unpoled sample, all the red emissions under different temperatures of 50to300K increase about 30% for the poled 0.1mol% Pr3+-doped ceramic. Obvious peaks around 100K for the red and blue emission intensities of Pr3+ ions were found when the ceramic passed through the orthorhombic-tetragonal phase transition, and a step decrease in the red emission intensity occurs around the tetragonal-cubic transition. Both polarization and phase transition effects on the Pr3+ PL were ascribed to local environmental changes of Pr3+ ions in the (Ba0.77Ca0.23)TiO3 ceramic.

Microstructure and electrical properties of (Li, Ag, Ta, Sb)-modified (K0.50Na0.50)NbO3 lead-free ceramics with good temperature stability

0.98[(K0.4725Na0.4725Li0.055)NbO3]–0.02Ag(Ta1−xSbx)O3 lead-free piezoelectric ceramics were prepared by the conventional mixed-oxide method. The effects of the Sb content on the microstructure and the electrical properties of the ceramics were systematically investigated. The experimental results show that Sb strongly affects the microstructure and the electrical properties of the ceramics and that the partial substitution of Sb slightly decreases the Curie temperature (Tc) and shifts the orthorhombic–tetragonal phase transition temperature (TO–T) to below room temperature. The thermal-depoling behaviour, age characteristics and temperature stability of the ceramics with x = 0.40 were also studied. The ceramics with x = 0.40 exhibit optimum properties (d33 = 250 pC N−1, kp = 44%, Tc = 444 °C, εr ∼ 1186, tan δ ∼ 2.7%, Pr ∼ 32.0 µC cm−2, Ec ∼ 14.8 kV cm−1), good thermal-depoling behaviour and age characteristics and improved temperature stability. These results show that the ceramic with x = 0.40 is a promising candidate material for high-temperature lead-free piezoelectric ceramics.

Mitigation of thermal and fatigue behavior in K0.5Na0.5NbO3-based lead free piezoceramics

K(0.5)Na(0.5)NbO(3)-(KNN) based lead free materials have been found to exhibit good piezoelectric properties (d(33) approximately 250 pCN) due to the orthorhombic-tetragonal polymorphic phase transition (PPT) temperature compositionally shifted downward to near room temperature. However, associated with the PPT are issues of temperature and domain instability, making them impractical for applications. In this work, CaTiO(3) (CT) was used to effectively shift the PPT downward in effort to mitigate these issues. As expected, CT modified KNN based materials exhibited nearly temperature independent properties (-50 approximately 200 degrees C) and fatigue-free behavior, together with its relatively high d(33) value of approximately 200 pCN, make the CT modified KNN based materials excellent candidates for lead free actuators and transducers.

BiFeO3-doped (Na0.5K0.5)NbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1−x)(Na0.5K0.5)NbO3-xBiFeO3 (x=0∼0.07) were synthesized by the solid-state reaction. Differential scanning calorimetry (DSC) measurements revealed that an increase in the amount of BiFeO3 dopant resulted in a decrease in the orthorhombic-tetragonal and tetragonal-cubic phase transition temperature of the material. One percent BiFeO3 additive suppressed grain growth, which not only benefits the sintering of ceramics but also enhances the piezoelectric and ferroelectric properties, where d33=145pC/N, kp=0.31, Qm=80, Pr=11.3 μC cm−2 and Ec=16.5 kV cm−1. As xBF>0.01, both piezoelectric and ferroelectric properties decreased rapidly with an increasing amount of dopant.

Crystalline structural phase boundaries in (K,Na,Li)NbO 3 ceramics

Lead-free (K x Na 1− x ) 1− y Li y NbO 3 ceramics (with x = 0.50 , y = 0 – 0.08 and x = 0.30 – 0.70 , y = 0.06 , respectively) were synthesized using the conventional solid-state reaction technique. Compositional influences of K, Na and Li constituents on microstructures, crystalline structures, dielectric and piezoelectric properties were investigated. It has been known that microstructures change largely with the alkali constituents and that there exist three orthorhombic-tetragonal phase boundaries at room temperature. One occurs in (K 0.50Na 0.50) 1− y Li y NbO 3 ceramics at y = 0.05 – 0.06 , which corresponds to the previously reported morphotropic phase boundary (MPB). The other two appear in (K x Na 1− x ) 0.94Li 0.06NbO 3 ceramics near x = 0.40 and x = 0.60 , respectively. (K 0.50Na 0.50) 0.935Li 0.065NbO 3 and (K 0.45Na 0.55) 0.94Li 0.06NbO 3 ceramics show high piezoelectric properties with the d 33 values over 200 pC/N and the k p values around 45% at room temperature. It is thought that the observed high piezoelectric properties are largely affected by the temperature-driven orthorhombic-tetragonal phase transition.

Piezoelectric properties and hardening behavior of K5.4Cu1.3Ta10O29-doped K0.5Na0.5NbO3 ceramics

Lead-free piezoelectric ceramics K0.5Na0.5NbO3+xmol% K5.4Cu1.3Ta10O29 have been prepared by a conventional ceramic fabrication technique. All the ceramics possess a perovskite structure with orthorhombic symmetry. Our results reveal that the addition of K5.4Cu1.3Ta10O29 is effective in improving the densification of the ceramics. Besides, after the addition of K5.4Cu1.3Ta10O29, the Curie temperature and the tetragonal-orthorhombic phase transition temperature decrease and the P-E loop becomes constricted, in particular, for the ceramic with x=0.75. Based on the symmetry-conforming principle of point defects, it is suggested that defect dipoles are formed by the acceptor dopant ions Cu2+ and O2− vacancies along the polarization direction. As a result of the low migration rate of defects, the defect dipoles remain in the original orientation during the P-E loop measurement and thus provide restoring forces to reverse the switched polarizations. Similarly, the defect dipoles do not response along with the polarization in the normal piezoelectric activities and thus provide “pinning” to the deformed polarization, making the ceramics become “hardened.” For the ceramic with x=0.75, the mechanical quality factor Qm becomes maximum at a value of 1530, while the other piezoelectric properties remain reasonably high: piezoelectric coefficient d33=90pC∕N, planar and thickness mode electromechanical coupling factors kP=41 and kt=46%.