Cubic-tetragonal Phase Transition Research Articles

Structural and Electrical Properties of AgTaO3–modified (Na0.465K0.465Li0.07) NbO3 Lead-Free Ceramics

The (1−x)(Na0.465K0.465Li0.07)NbO3 - xAgTaO3 lead-free ceramics were fabricated by the solid-state reaction process. The densification enhanced with increasing AgTaO3 content, giving maximum relative density of 98% (x = 0.02). Both orthorhombic-tetragonal (T O–T ) and tetragonal-cubic (T C) phase transition temperature decreased gradually with the increasing AgTaO3 content. The ceramic with a composition of x = 0.02 exhibited excellent electrical properties: the dielectric constant (ϵr) = 470, dissipation factor (tan δ) = 0.022, piezoelectric constant d 33 = 160 pC/N, and Curie temperature T C = 432°C. These results clearly showed that the 0.98(Na0.465K0.465Li0.07)NbO3 – 0.02AgTaO3 ceramics are very promising lead-free piezoelectric candidates.

Atomistic Modeling and Ab Initio Calculations of Yttria-Stabilized Zirconia

Though a number of atomistic-model studies of yttria-stabilized zirconia (YSZ) have been reported to elucidate its properties, most of them have employed simple pairwise potential functions to express interactions between atoms, which limits the transferability of the models. We have developed a Tangney-Scandolo dipole model potential for YSZ. Energy, stress and forces on atoms calculated by the ab initio (first-principles) density functional theory are provided as reference data for potential fitting. The developed potential successfully reproduces cubic-tetragonal phase transition at a range of yttria concentration relevant with SOFC application. The potential can well reproduce the barrier energy of oxygen vacancy migration. Molecular dynamics simulations of oxygen diffusion in bulk and at grain boundaries are demonstrated.

Microstructure, calorimetric and dielectric investigation of hafnium doped barium titanate ceramics

Ferroelectric properties of BaTi1− x Hf x O3 ceramics with Hf content x = 0, 0.001, and 0.025 were examined. It was found that the substitution of Hf4+ for Ti4+ in BaTiO3 resulted in shifting the two low-temperature transitions (T 1, T 2) of the investigated samples to higher temperatures and decreasing the tetragonal-cubic phase transition temperature (T c). This is a well-known pinching effect. Thermal parameters of the transitions, such as the enthalpy ΔH and the entropy ΔS, were also estimated and discussed.

Temperature Correction for Ceramic Powder X-Ray Diffraction at a Higher Temperature

For powder XRD measurement at a higher temperature, there is a great difference (ΔT) between the temperature (T) detected by a thermocouple in a specimen stage and the actual temperature (Ta) of the ceramic powder surface irradiated by X-ray. The BaTiO3 ceramic powder was employed to make an temperature correction on heating in light of the change of the ~ 45º characteristic peak in the vicinity of its tetragonal-cubic phase transition point (TC = 130 °C). The thermal relaxation of BaTiO3 is considered. When the BaTiO3 ceramic powder was measured at TC, the phase transition occurred at T = 170 °C and ΔT was determined as 40 °C, which is 10 °C higher than that of the ceramic bulk. The error of temperature correction is less than ± 3 °C at TC. The approximation of a linear dependency Ta - T was given as a temperature correction line between 25 and 130 °C (Ta here).

Electric field modulation of the tetragonal domain orientation revealed in the magnetic ground state of quantum paraelectric EuTiO3

The magnetic properties of single-crystal EuTiO3 are suggestive of nanoscale disorder below its cubic-tetragonal phase transition. We demonstrate that electric field cooling acts to restore monocrystallinity, thus confirming that emergent structural disorder is an intrinsic low-temperature property of this material. Using torque magnetometry, we deduce that tetragonal EuTiO3 enters an easy-axis antiferromagnetic phase at 5.6 K, with a first-order transition to an easy-plane ground state below 3 K. Our data is reproduced by a 3D anisotropic Heisenberg spin model.

A variable temperature synchrotron X-ray diffraction study of the ferroelastic double perovskite Ba2GdMoO6

A study of the magnetic and structural properties of the double perovskite Ba2GdMoO6 has been performed. The crystal structure distorts from the ideal cubic (Fm3m) structure to the tetragonal space group I4/m at 220 K, before undergoing a second distortion to a triclinic system (I1) at 80 K. The phase transition to triclinic symmetry is also evident in magnetic susceptibility measurements. The variable temperature synchrotron powder X-ray diffraction results reveal that Ba2GdMoO6 is ferroelastic, with the onset of ferroelastic domain formation occurring at the cubic-tetragonal phase transition. A number of Rietveld refinement techniques for modelling diffuse scattering from ferroelastic domain boundaries have been explored.

Microwave dielectric properties and low sintering temperature of Ba0.5Sr0.5TiO3–Mg2TiO4 composites synthesized in situ by the hydrothermal method

Abstract (1− x )Ba 0.5 Sr 0.5 TiO 3 – x Mg 2 TiO 4 composite ceramics were synthesized in situ by the hydrothermal method and their structure and dielectric properties were investigated systematically. Results showed that a ferroelectric–dielectric composite containing Ba 0.5 Sr 0.5 TiO 3 and Mg 2 TiO 4 phases could be successfully synthesized at sintering temperatures below 1300 °C. As the sintering temperature increased, the dielectric peaks of the cubic-tetragonal phase transition were markedly enhanced and sharpened and shifted toward high temperature. As the concentration of Mg 2 TiO 4 increased, both permittivity ( e ) and tunability ( T ) decreased, whereas Q value firstly increased and then decreased. The sample with x =0.6 exhibited good microwave dielectric properties, with e of 136, Q of 342 (at 3.804 GHz) and T of 16.2%, which make it a potential candidate for the fabrication of tunable microwave devices.

Anomalous behavior of acoustic phonon mode and central peak in Pb(Zn1/3Nb2/3)0.85Ti0.15O3 single crystal studied using Brillouin scattering

Brillouin spectroscopic measurements have been carried out on relaxor ferroelectric Pb(Zn1/3Nb2/3)0.85Ti0.15O3 single crystal over the temperature range 300–873 K. The frequency of longitudinal acoustic phonon (LA) begins to exhibit a strong decrease below the Burn's temperature TB followed by an increase in the line-width below the characteristic intermediate temperature T* ∼ 525 K. The line-width of the LA phonon mode exhibits a sharp Landau-Khalatnikov-like maximum at 463 K, the tetragonal-cubic phase transition temperature (Ttc). In addition, a broad central peak, found below T* exhibits critical slowing down upon approaching Ttc indicating an order-disorder nature of the phase transition. The relaxation time of polar nanoregions estimated from the broad central peak is found to be same as that obtained from LA phonon mode suggesting a strong coupling between strain and polarization fluctuations.

Role of dynamic polar nanoregions in heterovalent perovskite relaxor: Inelastic light scattering study of ferroelectric Ti rich Pb(Zn1/3Nb2/3)O3-PbTiO3

The role of dynamic polar nanoregions (PNRs) of (1−x)Pb(Zn1/3Nb2/3)O3-xPbTiO3 (PZN-100xPT) single crystals with the PT content x = 0.15 (i.e., higher than above the morphotropic phase boundary composition x ∼ 0.09) has been investigated using inelastic light scattering. The remarkable anomaly of the longitudinal acoustic mode is clearly observed in the vicinity of the cubic-tetragonal phase transition temperature at the TC-T = 481 K. A broad central peak (CP) appears below the Burns temperature TB ∼ 700 K. Upon cooling, the relaxation time determined from the CP width clearly shows a critical slowing down when approaching TC-T in contrast with the suppressed slowing down previously observed in PZN-7PT [S. Tsukada and S. Kojima, Phys. Rev. B 78, 144106 (2008)]. These facts are due to the existence of dynamic PNRs and the disappearance of chemically ordered regions resulting from a higher PT content in the solid solution. In the cubic phase, the local symmetry breaking caused by the PNRs was observed by Raman scattering. The initial size of dynamic PNRs is about 3 nm just below TB, and it increases significantly upon cooling below the intermediate temperature T* ∼ 600 K, reaching 10 nm down to TC-T.

Dielectric properties and phase transitions of La2O3- and Sb2O3-doped barium strontium titanate ceramics

The dielectric properties and phase transition characteristics of La2O3- and Sb2O3-doped barium strontium titanate ceramics prepared by solid state route were investigated. The microstructure was identified by X-ray diffraction method and scanning electron microscope was also employed to observe the surface morphologies. It is found that (La, Sb)-codoped barium strontium titanate ceramics exhibit typical perovskite structure and the average grain size decreases dramatically with increasing the content of Sb2O3. Both La3+ ions and Sb3+ ions occupy the A-sites in perovskite lattice. The dielectric constant and dielectric loss of barium strontium titanate based ceramics are obviously influenced by La2O3 as well as Sb2O3 addition content. The tetragonal-cubic phase transition of La2O3 modified barium strontium titanate ceramics is of second order and the Curie temperature shifts to lower value with increasing the La2O3 doping content. The phase transition of (La, Sb)-codoped barium strontium titanate ceramics diffuses and the deviation from Curie-Weiss law becomes more obvious with the increase in Sb2O3 concentration. The temperature corresponding to the dielectric constant maximum of (La, Sb)-codoped barium strontium titanate ceramics decreases with increasing the Sb2O3 content, which is attributed to the replacement of host ions by the Sb3+ ions.

Dielectric and polarized Raman spectroscopic studies on 0.85Pb(Zn1/3Nb2/3)O3-0.15PbTiO3 single crystal

Tetragonal-cubic phase transition has been investigated in relaxor-ferroelectric 0.85Pb(Zn1/3Nb2/3)O3-0.15PbTiO3 single crystal using dielectric and Raman spectroscopy. A detailed analysis of the dielectric data suggests that the transition is of second order. In the tetragonal phase (P4mm), all the modes predicted by group theory were found in the Raman spectra and were assigned based on the symmetry and polarization configuration. Frequencies of several modes were found to disappear while a few modes exhibited discontinuous change across the phase transition temperature TC ∼473 K. While in the high temperature cubic phase (Pm3¯m) no first order Raman spectrum is expected, the presence of several Raman peaks at elevated temperature suggests substitutional disorder causing the appearance of symmetry-forbidden Raman bands. The line-width of A1(TO) mode at 273 cm−1 shows anomalies across TC and the intermediate temperature T*. Furthermore, based on the temperature dependence of total integrated intensities of all the modes in the polarized and depolarized spectra, the Burns temperature TB, T*, and TC are identified at 650, 525, and 473 K, respectively.

Off-center impurities in a robust ferroelectric material: Case of Li in KNbO3

The influence of Li impurities on the phase transition sequence of KNbO${}_{3}$ is investigated using an atomistic model with parameters fitted to first-principles calculations. We find that the replacement of K by Li ions increases local electric dipoles due to a large Li off-centering along [001] directions. Li doping strongly affects the transition temperatures of KNbO${}_{3}$ as a result of the coupling between the off-center Li relaxation dynamics and the macroscopic polarization of the host material. This coupling produces a decrease in the temperatures for the rhombohedral-orthorhombic and orthorhombic-tetragonal phase transitions, while the temperature of the cubic-tetragonal ferroelectric phase transition increases. The Li relaxation mechanism in the different phases of KNbO${}_{3}$ provides a simple framework to understand the transition temperature shifts.

Study of ferroelectric nanodomains and the effect of grain size in BaTiO3 ceramics

Nanodomains and grain-size effects in BaTiO3 have been studied theoretically. In this article, we have calculated the long-range Coulomb interaction between ferroelectric nanodomains in a single BaTiO3 grain, and obtained information on the domain structure at various temperatures and grain sizes. The relation between transition temperature and the grain size is obtained by incorporating the domain-wall energy, the surface energy, and the stress energy into the Landau–Ginzburg free-energy density. The results show that 180° domains exist in ferroelectric BaTiO3 nanoceramics. At room temperature, with the decreasing grain size, the domain width does not decrease monotonically; when the grain size reduces to 18 nm, the grain becomes a single domain, and when less than 12 nm, the ferroelectric phase disappears. With the decreasing grain size the temperature of the cubic-tetragonal phase transition is reduced, while the temperatures of the tetragonal–orthorhombic and orthorhombic–rhombohedral phase transitions reach a maximum when the grain sizes are in the vicinity of 200 nm. The theoretical results are compared with experimental data.

Piezoelectric properties and time stability of lead-free (Na0.52K0.44Li0.04)Nb1−x−ySbxTayO3 ceramics

(Na0.52K0.44Li0.04)Nb1−x−ySbxTayO3 (NKLNST-x/y) lead-free piezoelectric ceramics were prepared by conventional solid-state reaction method. Their crystal structure was determined by X-ray diffraction, and the temperature dependence of dielectric constants was measured. It is found that Sb5+ and Ta5+ diffuse into the (Na0.52K0.44Li0.04)NbO3 lattice to form a solid solution with a perovskite structure. The co-substitution of Sb5+ and Ta5+ for B-site Nb5+ decreases both the paraelectric cubic-tetragonal phase transition temperature (TC) and the tetragonal-orthorhombic phase transition temperature (TO-T) of the (Na0.52K0.44Li0.04)NbO3 ceramics, and results in significant improvements in piezoelectric properties. Enhanced electrical and electromechanical responses of d33=335pC/N and kp=0.46 are obtained in the ceramics with x/y=0.08/0.05. Meanwhile, piezoelectric properties of the NKLNST-0.08/0.05 ceramic show no obvious reduction and dielectric loss increases slightly when exposed in air for 30 day. The results indicate that NKLNST-x/y ceramic is a promising lead-free piezoelectric candidate material.

Piezoelectric properties of tetragonal K0.95Li0.05Ta x Nb1− x O3 lead-free single crystals near tetragonal-cubic phase transition boundary

Tetragonal K0.95Li0.05Ta x Nb1 − x O3 lead-free single crystals have been grown by top-seeded solution growth technique. Dielectric and piezoelectric properties of poled samples were investigated as a function of temperature and dc bias. Both orthorhombic–tetragonal and tetragonal–cubic phase transition temperatures shift to lower temperatures as the Ta content increased. High piezoelectric activity was found to the crystal near the tetragonal–cubic phase transition boundary, which is due to phase transition as well as domain instability. For K0.95Li0.05Ta0.59Nb0.41O3 single crystal, piezoelectric coefficient (d 31) and electromechanical coupling factor (k 31) achieved −340 pC N−1 and −40%, respectively, which were much larger than those of lead-based Pb(Zr, Ti)O3.

Phase transition behaviors in relaxor ferroelectric [001]-poled Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals studied by Brillouin light scattering and dielectric spectroscopies

Phase transition behaviors of [001]-oriented Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIN-PMN-PT) single crystals were studied under unpoled and [001]-poled conditions by Brillouin light scattering and dielectric spectroscopies. The unpoled crystal showed a diffused elastic anomaly accompanied by substantial dielectric dispersion, which were associated with the temperature evolution of polar nanoregions represented by the excitation of strong central peaks. The poled crystal exhibited two-step changes in both dielectric and elastic properties, which were attributed to the successive phase transitions from rhombohedral to tetragonal, and then from tetragonal to cubic phase upon heating. The high-temperature tetragonal-cubic phase transition remained diffused with dielectric dispersion due to local random fields inherent in relaxors. The transverse acoustic mode disappeared at the rhombohedral-tetragonal phase transition indicating a clear symmetry change.

The study of negative thermal expansion and magnetic evolution in antiperovskite compounds Cu0.8-xSnxMn0.2NMn3(0 ≤ x ≤ 0.3)

With increasing the substitution of Sn for Cu in Cu0.8-xSnxMn0.2NMn3, the initial cubic-tetragonal structural phase transition disappears for the samples x ≥ 0.10 and is replaced by a discontinuous lattice expansion with a cubic structure which has been confirmed by the measurements of variable temperature x-ray diffractions and specific heat. The discontinuous lattice expansion broadens with increasing the doping level x and the negative thermal expansion coefficient up to −64.54 ppm/K between 190 K and 235 K is found for the sample x = 0.3. Detailed magnetic measurements indicate that the magnetic ground state is meta-stable for the lower-doping level and transforms into spin-glass-like state owing to the enhancement of antiferromagnetic interaction when x is up to 0.3. Furthermore, the magnetization curves M(T) display abnormal behaviors for lower-x. For the samples x = 0.1 and 0.2, the jump of field-cooled magnetization curve MFC (defined as ΔMFC/MFC) around the lower-temperature magnetic transition is suppressed with increasing the magnetic field. These abnormalities of magnetizations are also discussed based on a simple model.

Diffusion phase transition and ferroelectric properties of 0.92K0.5Na0.5NbO3–(0.08 − x)Bi0.5Li0.5TiO3–xBaZrO3 ceramics

0.92K0.5Na0.5NbO3–(0.08−x)Bi0.5Li0.5TiO3–xBaZrO3 (KNN–BLT–BZ) lead-free piezoelectric ceramics were fabricated by traditional ceramics process. XRD results showed that complete solid solutions with mixed phases of rhombohedra and tetragonal were observed. Diffusion phase transitional behavior, dielectric properties and ferroelectric properties of the KNN–BLT–BZ ceramics were investigated. It was found that the cubic-tetragonal phase transition temperature (TC) slightly decreased as the amount of BaZrO3 was increased. A morphotropic phase boundary (MPB) between rhombohedral and tetragonal ferroelectric phases was identified in the composition range of 0.04≤x≤0.06. Furthermore, good ferroelectric properties were also found in this composition range, with Pr of 14.5μCcm−2 and Ec of 1.95kVmm−1 for the sample with x=0.04.

BCTZ Addition on the Microstructure, Piezoelectric/Dielectric Properties and Phase Transition of NKLN-AS Piezoelectric Ceramics

Presently, the most promising family of lead-free piezoelectric ceramics is based on <TEX>$K_{0.5}Na_{0.5}NbO_3$</TEX>(KNN). Lithium, silver and antimony co-doped KNN ceramics show high piezoelectric properties at room temperature, but often suffer from abnormal grain growth. In the present work, the <TEX>$(Ba_{0.85}Ca_{0.15})(Ti_{0.88}Zr_{0.12})O_3$</TEX> component, which has relaxor ferroelectric characteristics, was doped to suppress the abnormal grain growth. To investigate this effect, Lead-Free <TEX>$0.95(K_{0.5}Na_{0.5})_{0.95}Li_{0.05}NbO_3-(0.05-x)AgSbO_3-x(Ba_{0.85}Ca_{0.15})(Ti_{0.88}Zr_{0.12})O_3$</TEX>[KNLN-AS-xBCTZ] piezoelectric ceramics were synthesized by ball mill and nanosized-milling processes in lead-Free <TEX>$0.95(K_{0.5}Na_{0.5})_{0.95}Li_{0.05}NbO_3-(0.05-x)AgSbO_3$</TEX> in order to suppress the abnormal grain growth. The nanosized milling process of calcined powders enhanced the sintering density. The phase structure, microstructure, and ferroelectric and piezoelectric properties of the KNLN-AS ceramics were systematically investigated. XRD patterns for the doped and undoped samples showed perovskite phase while tetragonality was increased with increasing BCZT content, which increase was closely related to the decrease of TO-T. Dense and uniform microstructures were observed for all of the doped BCZT ceramics. After the addition of BCTZ, the tetragonal-cubic and orthorhombic-tetragonal phase transitions shifted to lower temperatures compared to those for the pure KNNL-AS. A coexistence of the orthorhombic and tetragonal phases was hence formed in the ceramics with x = 0.02 mol at room temperature, leading to a significant enhancement of the piezoelectric properties. For the composition with x = 0.02 mol, the piezoelectric properties showed optimum values of: <TEX>$d_{33}$</TEX> = 185 pC/N, <TEX>$k_P$</TEX> = 41%, <TEX>$T_C=325^{\circ}C$</TEX>, <TEX>$T_{O-T}=-4^{\circ}C$</TEX>.

Effects of Sb&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3 &lt;/sub&gt;Addition on Dielectric Properties of Barium Strontium Titanate Based Ceramics

The dielectric properties of Sb2O3-doped (Ba0.732Sr0.26Nd0.008)TiO3.004ceramics fabricated by conventional solid state method were investigated. The structure was identified by X-ray diffraction method and scanning electron microscope was also employed to observe the surface morphologies. It is confirmed that Sb2O3-doped (Ba0.732Sr0.26Nd0.008)TiO3.004ceramics exhibit perovskite structure and with increasing Sb2O3content Ba6Ti17O40(space group C2/c (15) ) forms as a secondary phase. The experimental results reveal that the addition of Sb2O3into (Ba0.732Sr0.26Nd0.008)TiO3.004ceramics reduce the dielectric constant and Curie temperature due to the substitution for A/B-sites with Sb3+ions. The standing voltage increases with increasing Sb2O3content while the dielectric loss and resistivity initially decrease and then increase with the increase of Sb2O3concentration. The temperature dependence of dielectric properties at different frequencies was studied as well. It indicates that the tetragonal-cubic phase transition peak is suppressed as the Sb2O3content increases and the peak value reduces with the increase of frequency.