Cubic-tetragonal Phase Transition Research Articles

Effects of Sb<sub>2</sub>O<sub>3 </sub>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.

Electrical properties of B site substituted (K0.48Na0.52)(W2/3Bi1/3)xNb1−xO3 piezoceramics

Lead-free (K0.48Na0.52)(W2/3Bi1/3)xNb1−xO3 (KNN-WBi) piezoceramics with x ranging from 0.004 to 0.010 were synthesized by conventional ceramic processing. The sintered KNN-WBi ceramics showed perovskite structure without detectable secondary phase containing W and Bi. With increasing x, the orthorhombic-tetragonal phase transition temperature (T O-T) decreased from 200 to 184 °C whereas, the tetragonal-cubic phase transition temperature (T C) decreased slightly. With the doping of (W2/3Bi1/3), the piezoelectric properties were greatly improved and the piezoelectric constants d 33, k p, Q m exhibited maximum values of 136 pC/N, 43.3% and 175, respectively at x = 0.008. The KNN-WBi ceramics also exhibited good ferroelectric properties with remnant polarizations P r higher than 25 μC/cm2 and coercive fields E c lower than 1,000 V/mm. The results strongly suggest that the B site doping of constructed quinquevalent element is an effective method for the investigation of potassium sodium niobate system.

Theoretical predictions of morphotropic phase boundary in (1−x)Na1/2Bi1/2TiO3-xBaTiO3 by first-principle calculations

A morphotropic phase boundary (MPB) in a solid solution of (1−x)Na1/2Bi1/2TiO3-xBaTiO3 (0≤x≤1.0) (NBT-BT) is directly demonstrated by first-principle calculations. The results show that in the NBT-BT system, a structural transition from rhombohedral to tetragonal occurs when x is changed from 0.06 to 0.12, and the MPB appears at compositions of around 0.05≤x≤0.07. It can be verified by the stronger hybridization orbital of p-d based on the analysis of the partial density of states (PDOS) in the Ti–O atoms. The theoretical predictions agree well with experimental observations which indicate that the best ferroelectric properties can be attained in the vicinity of around 0.05≤x≤0.07 in the MPB of the NBT-BT system. Another tetragonal-cubic phase transition which needs further experimental verification is also found from the system with compositions of 0.475<x<0.65.

Ferroelectric Behavior of CuO Doped Ba (Ti0.91Zr0.09) O3 Ceramics

Ba (Ti0.91Zr0.09) O3 ceramics doped with CuO were prepared by a solid phase reaction. The ferroelectric properties of Ba (Ti0.91Zr0.09) O3 ceramics were investigated. The results revealed that the materials are of relaxor type at Orthorhombic-Tetragonal phase transition point and undergoes a diffuse phase transition at Tetragonal-Cubic phase transition point. A non-linear P-E loop and narrow loop are observed, which exhibit a typical ferroelectric relaxor characteristic at the room temperature. Remanent polarization Pr and coercive electric field EC decrease with increasing dopant concentration, A remanent polarization of 0.20 μC/cm2 and a coercive filed of 0.81 KV/cm for BTZC38 at the room temperature were obtained.

Effect of Li on the microstructure and electrical properties of (K 0.17Na 0.83)NbO 3 lead-free piezoceramics

Lead-free (K 0.17Na 0.83−xLi x)NbO 3 piezoceramics were synthesized by conventional solid state sintering and the effect of Li on the microstructure and electrical properties was investigated. The experimental results show that 4 mol% Li doping facilitates the grain growth and increases the density of the ceramics. The measurement of dielectric properties reveals that the orthorhombic-tetragonal ( T O-T) and tetragonal-cubic ( T C) phase transition temperatures change linearly with increasing Li content and the sample with x = 0.06 exhibits the highest T C of 429 °C and lowest T O-T of 181 °C. Because of the wide apart of T O-T from room temperature, no polymorphic phase structures were obtained and the piezoelectric strain coefficient d 33 and piezoelectric coupling constant k p varied between low values of 57, 61 pC/N and 0.18, 0.24, respectively with less than 6 mol% Li doping.

Discharge Current Measurement and Depolarization Property of Lead-Free Niobate Piezoceramics

Lead-free Na0.5K0.5NbO3 (NKN) piezoelectric ceramics was synthesized by an ordinal solid state reaction method. The ceramics was electrically poled under 3 kV/mm at 150 oC for 30 min. The discharge current-flow was measured for the poled ceramics at the heating rate of 1 oC/min from room temperature up to 500 oC. The two sharp peaks were observed at 209 and 417 oC. These temperatures were corresponded to orthorhombic-tetragonal (TO-T) and tetragonal-cubic (TC) phase transition temperatures, respectively. The sharp peaks were mainly originated from the decrease of the spontaneous polarization. Additionally, the several large broad patterns were also observed at 280, 363 and 375 oC, According to the electromechanical coupling factor kp, which is the indicative parameter of the polarization, mainly decreased at TO-T and TC. These broad peaks were not associated with the spontaneous polarization. The peaks seem to have been originated from the discharge current of the oxygen ion migration on elevating temperature inside the poled NKN ceramics.

Pressure investigation of superconductivity of V3Si

V3Si is one of the superconducting A15 compounds and has relatively high Tc of T = 16.7 K at ambient pressure. A15 compounds have another feature the martensitic cubic-tetragonal structural phase transition at TM comparatively close to Tc; TM of V3Si is 21.5 K at ambient pressure. Interestingly, there are two types of crystals of V3Si showing transformation (T) and non-transformation (NT). Here, we have revealed the pressure dependence of Tc in NT V3Si under hydrostatic pressures up to 14 GPa. Tc monotonically increases up to 6 GPa and saturates on further compression, which is different behavior from previous report.

Lead-free (K0.5Na0.5)0.95(LiSb)0.05Nb0.95O3-BaTiO3 piezoceramics

Lead-free (1-x)(K0.5Na0.5)0.95(LiSb)0.05Nb0.95O3-xBaTiO3 (abbreviated as (1-x)KNNLS-xBT) piezoceramics were synthesized by conventional solid state sintering and the effect of BaTiO3 on the microstructure, dielectric and piezoelectric properties was investigated. It was found that both orthorhombic-tetragonal (T O-T) and tetragonal-cubic (T C) phase transition temperatures decreased obviously with increasing BaTiO3 content. Although proper amount of BaTiO3 facilitated the sintering of (1-x)KNNLS-xBT ceramics, the addition of BaTiO3 affected the relaxor behavior slightly and it was not beneficial to improve piezoelectric strain coefficient d 33, remnant polarization P r and piezoelectric coupling constant k p.

Structure and piezoelectric properties of new ternary K0.5Na0.5NbO3–LiSbO3–CaTiO3 lead-free piezoceramics

New ternary (1−x)K0.5Na0.5NbO3–x(0.80LiSbO3–0.20CaTiO3) lead-free ceramics were fabricated by a conventional ceramic technique and their structure and piezoelectric properties were studied. The results of X-ray diffraction reveal that LiSbO3 and CaTiO3 diffuse into the K0.5Na0.5NbO3 lattices to form a new solid solution with a perovskite structure. After the addition of LiSbO3 and CaTiO3, the cubic-tetragonal and tetragonal-orthorhombic phase transitions shift to lower temperatures. Coexistence of the orthorhombic and tetragonal phases is hence formed in the ceramics with 0.03 < x < 0.07 at room temperature, leading to a significant enhancement of the piezoelectric properties. For the ceramics with x = 0.04–0.06, the piezoelectric properties become optimum: d33 = 172–253 pC/N, kP = 49.9–55.5%, kt = 49.2–52.1% and TC = 348–373 °C. The ceramic with x = 0.04 also exhibits a good thermal stability of piezoelectric properties.

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.

Phase transition and thermal expansion coefficient of leucite ceramics with addition of SiO2

In this work, the influence of SiO2 additions in leucite ceramics on the bulk linear thermal expansion coefficient (TEC) especially during the phase transition, has been studied. Thermal expansion and x-ray diffraction measurements at high temperatures were carried out to characterize the tetragonal-cubic phase transition. TEC for reference and SiO2-added leucite samples exhibited similar behavior as a function of temperature. Before and after the phase transition, the TEC values were similar to those observed in non-SiO2-added samples, whereas during the phase transition, a maximum TEC value was observed and it tends to decrease as the SiO2 addition increases. This behavior could be caused by the formation of an intermediate phase with an extremely high TEC (70 × 10–6 °C−1) during the phase transformation. Furthermore, the results suggest that as the intermediate phase is partially suppressed via SiO2 addition, the cubic phase can be partially stabilized at temperatures as low as 200 °C.

Origin of ferroelastic domains in free-standing single-crystal ferroelectric films

The origin of the unusual $90\ifmmode^\circ\else\textdegree\fi{}$ ferroelectric/ferroelastic domains, consistently observed in recent studies on mesoscale and nanoscale free-standing single crystals of ${\text{BaTiO}}_{3}$ [Schilling et al., Phys. Rev. B 74, 024115 (2006); Schilling et al., Nano Lett. 7, 3787 (2007)], has been considered. A model has been developed which postulates that the domains form as a response to elastic stress induced by a surface layer which does not undergo the paraelectric-ferroelectric cubic-tetragonal phase transition. This model was found to accurately account for the changes in domain periodicity as a function of size that had been observed experimentally. The physical origin of the surface layer might readily be associated with patterning damage, seen in experiment; however, when all evidence of physical damage is removed from the ${\text{BaTiO}}_{3}$ surfaces by thermal annealing, the domain configuration remains practically unchanged. This suggests a more intrinsic origin, such as the increased importance of surface tension at small dimensions. The effect of surface tension is also shown to be proportional to the difference in hardness between the surface and the interior of the ferroelectric. The present model for surface-tension induced twinning should also be relevant for finely grained or core-shell structured ceramics.

Study of monoclinic-tetragonal-cubic phase transition in Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 single crystals by micro-Raman spectroscopy

The relaxor ferroelectric Pb(Zn1/3Nb2/3)O3–0.08PbTiO3 (PZN–0.08PT) single crystal has been investigated using polarized micro-Raman scattering technique in a wide temperature range from −150 to 350 °C. Raman spectra of the crystal at temperatures lower than 90 °C show 11 modes over a frequency range of 100–1000 cm−1. There are 12 Raman active vibration modes in the monoclinic structure of relaxor ferroelectrics according to the group theory analysis. Thus the structure of PZN-0.08PT single crystal at low temperatures is confirmed as monoclinic. Two structural phase transitions in the crystal were observed when the temperature was increasing. First, the monoclinic-tetragonal (M-T) phase transition happens at 90 °C. Second, the tetragonal-cubic (T-C) phase transition takes place at 170 °C.

Anomalous behavior of acoustic phonon and central peak in Pb[(In1/2Nb1/2)0.65Ti0.35]O3 single crystal

The temperature dependences of acoustic phonon mode and the central peak have been investigated in Pb[(In1/2Nb1/2)0.65Ti0.35]O3 single crystal by Brillouin scattering. Longitudinal acoustic phonon begins to show marked softening below the Burns temperature TB∼700 K. The longitudinal acoustic phonon mode exhibits clear anomalies at the cubic-tetragonal phase transition temperature of 540 K and at the tetragonal-rhombohedral one of 460 K. Below 600 K, the relaxation time calculated from the phonon mode agrees well with that of the broad central peak, suggesting a coupling between the local polarization and strain fluctuations of polar nanoregions. This temperature has been identified as another characteristic temperature T∗ besides TB, where the formation of long-lived polar nanoregions accompanied by the local strain fields governs the relaxation dynamics.

Elastic softening and central peaks in BaTiO3 single crystals above the cubic-tetragonal phase-transition temperature

A strong relaxation mode was observed in the paraelectric phase of barium titanate (BaTiO3) single crystals by Brillouin scattering study and was found to correlate with the softening of the longitudinal acoustic mode and the increase in the hypersonic damping. These observations support the existence of polar percursors and their electrostrictive coupling with the strain caused by the acoustic waves, consistent with former studies evidencing off-centered Ti ions in the high-symmetry cubic phase. A critical slowing down has been clearly observed in the vicinity of the cubic-tetragonal phase transition, indicating order-disorder component contributes to the phase transition of BaTiO3.

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.

High-temperature X-ray powder diffraction study of the tetragonal-cubic phase transition in nanocrystalline, compositionally homogeneous ZrO2–CeO2 solid solutions

Crystal structure of compositionally homogeneous, nanocrystalline ZrO2–CeO2 solutions was investigated by X-ray powder diffraction as a function of temperature for compositions between 50 and 65 mol % CeO2. ZrO2-50 and 60 mol % CeO2 solid solutions, which exhibit the t′-form of the tetragonal phase at room temperature, transform into the cubic phase in two steps: t′-to-t″ followed by t″-to-cubic. But the ZrO2-65 mol % CeO2, which exhibits the t″-form, transforms directly to the cubic phase. The results suggest that t′-to-t″ transition is of first order, but t″-to-cubic seems to be of second order.

Synchrotron X-ray powder diffraction study of the tetragonal-cubic phase transition in nanostructured ZrO2-Sc2O3 solid solutions

The transition between tetragonal and cubic phases in nanostructured ZrO2-Sc2O3 solid solutions by high-temperature X-ray powder diffraction using synchrotron radiation is presented. ZrO2-8 and 11 mol% Sc2O3 nanopowders that exhibit the t′- and t″-forms of the tetragonal phase, respectively, were synthesized by a stoichiometric nitrate-lysine gel-combustion route. The average crystallite size treated at 900°C was about 25 nm for both compositions. Our results showed that t′-t″ and t″-cubic transitions take place for the 8 and 11 mol% Sc2O3 samples, respectively.

Polarized Micro-Raman Study of the Temperature-Induced Phase Transition In 0.67 Pb(Mg1/3Nb2/3)O3-0.33PbTiO3

Polarized Raman spectra of ferroelectric relaxor 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 (0.67PMN-0.33PT) single crystal are systematically investigated in a wide temperature range from -196 to 600°C by micro-Raman scattering technique. The results clearly reveal that there are two structural phase transitions in such composite ferroelectric relaxor: the rhombohedral-tetragonal (R - T) phase transition and the tetragonal-cubic (T - C) phase transition. The former occurs at about TR-T = 34°C, corresponding to the vanishing of the soft A1 mode at 106 cm−1 recorded in the parallel polarization. The latter appears at about TT-C = 144°C, which can be verified with the vanishing of mode at 780 cm−1 measured in the crossed polarization.

Structural Fluctuations in the Spin-Liquid State ofTb2Ti2O7

High-resolution x-ray scattering measurements on single crystal Tb2Ti2O7 reveal finite structural correlations at low temperatures. This geometrically frustrated pyrochlore is known to exhibit a spin-liquid or cooperative paramagnetic state at temperatures below approximately 20 K. Parametric studies of structural Bragg peaks appropriate to the Fd3[over ]m space group of Tb2Ti2O7 reveal substantial broadening and peak intensity reduction in the temperature regime 20 K to 300 mK. We also observe a small, anomalous lattice expansion on cooling below a density maximum at approximately 18 K. These measurements are consistent with the development of fluctuations above a cooperative Jahn-Teller, cubic-tetragonal phase transition at very low temperatures.