Rhombohedral-orthorhombic Phase Transition Research Articles

Direct photoluminescence manipulation in Er-doped AgNbO3 lead-free antiferroelectrics via reversible electric-field-induced symmetry modulations

The electric field-modulated photoluminescent (E-PL) effect has received much attention because of its potential applications in optical data storage and communication. Based on the E-induced antiferroelectric-ferroelectric (AFE-FE) phase transition, a large and reversible modulation of the PL intensity was previously realized in lanthanum-doped Pb(Zr0.9Ti0.1)O3 (PLZT) ceramics. However, such systems contain abundant leads and are thus toxic to humans and the environment. In this work, Er-doped AgNbO3 (ANO) lead-free ceramics were prepared, and their E-PL effect was investigated. From the perspective of the crystallographic symmetry, the reversibly E-induced orthorhombic Pbcm to orthorhombic Pmc21 transition results in a negative PL intensity change of 25%, which is different from the positive 30% PL modulation in the PLZT system triggered by the AFE-FE orthorhombic-rhombohedral phase transition. This symmetry-dependent negative PL modulation in ANO ceramics enriches the understanding of the E-PL effect in AFEs and provides an environmentally friendly alternative for multifunctional optoelectronics.

Structural, Vibrational, and Magnetic Characterization of Orthoferrite LaFeO3 Ceramic Prepared by Reaction Flash Sintering.

LaFeO3 perovskite ceramics have been prepared via reaction flash technique using Fe2O3 and La2O3 as precursors. The obtained pellets have been investigated using several techniques. The formation of LaFeO3 has been clearly confirmed by X-ray diffraction. The scanning electron microscopy micrographs have shown the microporous character of the obtained pellets due to the low temperature and dwell time used in the synthesis process (10 min at 1173 K). The orthorhombic-rhombohedral phase transition has been observed at approximately 1273 K in differential thermal analysis measurements, which also allows us to determine the Néel temperature at 742 K. The fitted Mössbauer spectra exposed the presence of a single sextet ascribed to the Fe+3 ions in the tetrahedral site. Finally, magnetic measurements at room temperature indicate the antiferromagnetic character of the sample.

Piezoelectric properties, phase transitions and domain structure of (Bi,Na)HfO3−modified (K,Na)(Nb,Sb)O3 ceramics

Piezoelectric performances of (K,Na)NbO3-based ceramics are often greatly affected by phase transitions and domain structure, which are in turn substantially determined by chemical compositions. Systematical studies from this perspective are important for the good understanding about piezoelectric characters. Further, whether rhombohedral-tetragonal (R-T) or rhombohedral-orthorhombic-tetragonal (R-O-T) phase coexistence is really the essential precondition for obtaining high piezoelectric coefficient d33 is also a fundamental issue. A series of (1-x)(K0.48Na0.52)Nb0.955Sb0.045O3–x(Bi0.50Na0.50)HfO3 ceramics with 0.01≦x≦0.04 were chosen in this work to examine the compositional effects on piezoelectric properties, phase transitions and domain structure. It was found that piezoelectric coefficient d33, orthorhombic-tetragonal (O-T) phase transition and domain structure are sensitive to the chemical composition. The d33 increases first, reaches a maximum of 512 pC/N at x = 0.037 and then decreases with x. Dielectric measurement showed that the O-T phase transition temperature TO-T decreases monotonically with x while the rhombohedral-orthorhombic phase transition temperature TR-O remains almost unchanged when x ≥ 0.03. Domain structure was investigated by acid etching. All the compositional ceramics exhibit quite complex domain patterns consisting of long parallel stripes and irregularly-shaped domain boundaries before poling. Domain patterns become less complicated upon poling due to the collective polarization reversals of domain clusters. The ceramic with x = 0.037 differs significantly from the others, showing its TO-T close to room temperature and possessing the unique domain structure that contains a large amount of hierarchical nanodomain structure. Its outstanding piezoelectric properties should be ascribed to the O-T phase coexistence and the characteristic domain structure.

A highly accurate interatomic potential for LaMnO3 perovskites with temperature-dependence of structure and thermal properties

ABO3-type perovskites LaMnO3 is a colossal magnetoresistance material that undergoes an orthorhombic-rhombohedral phase transition as the temperature increases. Phase transition determines its structural properties and thermal conductivity that are important in industrial application. Structural transition is reflected mainly in Mn-O bond lengths and Mn-O-Mn angles between MnO6 octahedrons for Jahn-Teller effect during this progress and traditional born–mayer (BM) model has difficulty in description of structural distortion and thermal properties. In this paper, a new classic interatomic potential model for LaMnO3 was developed within the framework of bond valence (BV) theory. First-principles and intelligent optimization algorithm were used to fit the parameters, enabling the study of temperature-dependent structures by accurate large-scale molecular dynamics (MD) simulations. Calculated structural distortions by our model and thermal properties calculated with equilibrium molecular dynamics (EMD) method accorded with the experimental values within a reasonable error, and had higher accuracy than traditional born-mayer model. These results provided further insight about temperature-depended phase transition and further performance mining of LaMnO3. Most importantly, our potential model showed better accuracy than traditional potential model and is applicablefor all crystal materials with perovskite structures.

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.

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.

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.

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.

Structural and functional characterisation of KNNS–BNKZ lead-free piezoceramics

ABSTRACTPromising piezoelectric properties have been reported recently for lead-free 0.96(K0.48Na0.52Nb0.95Sb0.05)–0.04Bi0.5(Na0.82K0.18)0.5ZrO3 (KNNS–BNKZ) ceramics. The presence of coexisting rhombohedral and tetragonal phases is thought to play a key role in their functional properties, but a thorough understanding is currently lacking. In this experiment, (1−x)KNNS–(x)BNKZ ceramics with x = 0–0.05 were prepared by the mixed-oxide method. High-resolution synchrotron X-ray powder diffraction (SXPD) measurements reveal that the addition of BNKZ into KNNS ceramics leads to an increase in the rhombohedral–orthorhombic phase transition temperature (TR−O) and a decrease in the orthorhombic–tetragonal phase transition temperature (TO−T) leading to orthorhombic–tetragonal and rhombohedral–tetragonal phase coexistence at room temperature for compositions with x = 0.02 and 0.04, respectively. By combining the SXPD results with microstructure, evidence is also found for the occurrence of chemical heterogeneity, which could provide an additional means to control the functional properties. The structural observations are correlated with changes in the dielectric and ferroelectric properties.

Composition induced rhombohedral–tetragonal phase boundary and high piezoelectric activity in (K0.48,Na0.52) (Nb(1-x)Sbx)O3 - 0.05Ca0.2(Bi0.5,Na0.5) 0.8ZrO3 lead-free piezoelectric ceramics

New lead-free piezoelectric ceramics (K0.48,Na0.52) (Nb(1-x)Sbx)O3 −0.05Ca0.2(Bi0.5,Na0.5) 0.8ZrO3 (KNNSx-CBNZ) were fabricated by traditional solid-state sintering process. The effects of Sb content on phase structures and electrical properties of the KNNSx-CBNZ ceramics were studied. Because Sb5+ can increase the rhombohedral-orthorhombic phase transition temperature and decrease orthorhombic-tetragonal phase transition temperature simultaneously, it is found that the ceramics in the composition range of 0.02<x<0.04 possess a rhombohedral-tetragonal phase boundary. The best piezoelectric properties are obtained in the ceramics with x=0.03: d33=470 pC/N, kp=52.4%, εr=1563, tan δ=3.30% and Tc=243°C. The results show that KNNSx-CBNZ ceramics have potential applications for substituting the lead based piezoelectric ceramics.

Effect of rhombohedral-orthorhombic phase transition on structure and electrical properties of KNN-based lead-free piezoceramics

ABSTRACTLead-free ceramics (1-x)(Na0.7K0.3)0.92Li0.08NbO3-xCaZrO3 (KNNL-xCZ) have been synthesized by the conventional solid-state reaction. X-ray diffraction shows that all ceramics are in pure perovskite phase. The study of their phase transition observed by the temperature-dependent dielectric measurements confirms that the addition of CZ leads to alternation of crystal structure. The KNNL-0.03CZ ceramics exhibit optimum electrical properties and produce large strain response (dS/dE = 193 pm/V). The enhanced properties of the KNNL-xCZ are attributed to the effect of rhombohedral-orthorhombic phase transition in the vicinity of room temperature.

Microstructure, phase structure and electrical properties of 0.954K1−x Na x NbO3–0.04Bi0.5Na0.5ZrO3–0.006BiFeO3 lead-free ceramics

In this work, 0.954K1−x Na x NbO3–0.04Bi0.5Na0.5ZrO3–0.006BiFeO3 (KN x N–BNZ–BF) lead-free ceramics were fabricated by conventional ceramic technique. The effect of K/Na ratio on microstructure, phase structure and electrical properties was systematically investigated. The orthorhombic-tetragonal phase transition temperature (T O–T) increases and rhombohedral–orthorhombic phase transition temperature (T R–O) drops simultaneously with increasing the Na content, leading to an R–O–T phase boundaries in the ceramics with 0.44 ≤ x ≤ 0.60. By tailoring their K/Na ratio and optimizing the sintering temperature, an enhanced electrical properties (e.g. d 33 ~ 438 pC/N, k p ~ 0.51, T c ~ 320 °C, e r ~ 2304 and tan δ ~ 0.029) was obtained at the ceramics with x = 0.56 sintered at 1090 °C, which could be attributed to the preferably density as well as the R–O–T phase boundary nearer the room temperature. Therefore, we think that the KN x N–BNZ–BF ceramic is a promising candidate for piezoelectric devices.

Dielectric and piezoelectric properties of (1-x)(K0.474Na0.474Li0.052)(Nb0.948Sb0.052)O3-xBaMO3 (M = Ti or Zr) ceramics

ABSTRACTThe effects of Ba and M (M = Ti or Zr) substitutions for K, Na, Li, Nb and Sb on the ferroelectric and piezoelectric properties of (1-x)(K0.474Na0.474Li0.052)(Nb0.948Sb0.052)O3 (KNLNS)-xBaMO3 ceramics were investigated. X-ray powder diffraction profiles showed the coexistence of rhombohedral-orthorhombic-tetragonal phases, representing a morphotropic phase boundary (MPB), in both (1-x)KNLNS-xBaTiO3 (BT) and (1-x)KNLNS-xBaZrO3 (BZ) ceramics. DSC measurement of (1-x)KNLNS-xBZ ceramics indicated that the Ba and Zr substitutions for A- and B-sites in KNLNS led to the further increase in the dielectric constant in comparison with that of (1-x)KNLNS-xBT ceramics. Moreover, the rhombohedral-orthorhombic phase transition temperature was decreased to room temperature by Ba and Zr substitutions; as a result, a d33 value of 337 pC/N was obtained at 0.94KNLNS-0.06BZ (x = 0.06) ceramic.

Studying the roles of Cu and Sb in K0.48Na0.52NbO3 lead-free piezoelectric ceramics

The co-effects of Sb and Cu on crystal structure and electrical performance of K0.48Na0.52NbO3 were studied by respective doping in the composition. The introduction of Cu2+ showed occupying B-site at low concentration and substituting on A-site at high concentration without inducing phase transformation. The mechanical quality factor (Qm) was enhanced with adding 1.5mol% CuO content resulted from the formation of oxygen vacancies, and the degradation of piezoelectric constant (d33) and the increase of internal bias field (Ei) were attributed to the “hard doping effect”. Antimony was selected to successfully weaken the Ei and improve the piezoelectricity by “soft doping effect”. Meanwhile, the addition of Sb resulted in shifting rhombohedral–orthorhombic phase transition around to room temperature and the appearance of ferroelectric relaxor behavior. In this work, the “hard doping effect” and “soft doping effect” can be modulated and interacted by doping relevant elements.

Phase transition and piezoelectricity of sol–gel-processed Sm-doped BiFeO3 thin films on Pt(111)/Ti/SiO2/Si substrates

We prepared high-quality Bi1−xSmxFeO3 films on Pt(111)/Ti/SiO2/Si substrates by sol–gel processing and found rhombohedral–orthorhombic phase transition with enhanced piezoelectricity.

Effect of SrZrO3 on phase structure and electrical properties of 0.974(K0.5Na0.5)NbO3–0.026Bi0.5K0.5TiO3 lead-free ceramics

(0.974−x)(K0.5Na0.5)NbO3–0.026Bi0.5K0.5TiO3–xSrZrO3 lead-free piezoelectric ceramics have been prepared by the conventional solid state sintering method. Systematic investigation on the microstructure, crystalline structures as well as electrical properties of the ceramics was carried out. With the addition of SrZrO3, the rhombohedral–orthorhombic phase transition temperature of the ceramics increases. Both the rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions of the ceramics were modified to be around room temperature when x~0.05, and as a result remarkably strong piezoelectricity has been obtained in 0.924(K0.5Na0.5)NbO3–0.026Bi0.5K0.5TiO3–0.05SrZrO3 ternary system, whose piezoelectric parameters were d33=324pC/N and kp=41%.

STRUCTURAL AND MAGNETIC PHASE TRANSITIONS OF BiFeO3 INDUCED BY PRESSURE

First-principles calculations on BiFeO3 under low-pressure region show that the rhombohedral-monoclinic and rhombohedral-orthorhombic phase transitions can be found around the critical pressure value, ~9 GPa.We suggest that the structure involve the combination of these three phases and change to the pure orthorhombic phase gradually as pressure is beyond the critical value. Moreover, it is a first-order structural phase transition, accompanied with antiferromagnetic–ferromagnetic transition.

Synthesis and Characterization of Nano-LaFeO3 Powders by a Softchemistry Method and Corresponding Ceramics

The preparation of a nano-sized LaFeO3 powder by a soft-chemistry method using starch as complexing agent is described herein. Phase evolution and development of the specific surface area during the decomposition process of (LaFe)-gels were monitored up to 1000 °C. A phase-pure nano-sized LaFeO3 powder with a high specific surface area of 25.7 m2/g and a crystallite size of 37 nm was obtained after calcining at 570 °C. TEM investigations reveal a porous powder with particles in the range of 20 to 60 nm. Calcinations to 1000 °C result in crystallite sizes up to 166 nm. Dilatometric measurements of the sintering behaviour show that the beginning of shrinkage of pellets from the nano-sized powder is downshifted by more than 300 °C compared to coarse-grained mixed-oxide powder. The orthorhombic-rhombohedral phase transition was observed at 980 °C in DTA measurements for coarse-grained ceramic bodies. The enthalpy change (DH) during the phase transition and the thermal expansion coefficient (adil) for ceramics was determined as 410 J/mol and 11.8×10-6 K-1, respectively. Whereas the enthalpy changes during the phase transition of the nano-sized LaFeO3 powders are £ 200 J/mol.

Hysteresis in the rhombohedral–orthorhombic phase transition of KNbO3 under inhomogeneous strain

The influence of the heating rate on the low temperature phase transition (PT) of the piezoelectric crystal potassium niobate (KNO) was studied by micro-Raman spectroscopy. It is found that crystallographic defects are more important than the heating rate for the onset of the PT. If the strain shifts the transition temperature (TT) from the rhombohedral to the orthorhombic phase to lower values, then it also shifts the reverse PT to higher temperatures. The PT on heating is more sensitive to the rate than the PT on cooling.

Perovskite La1−x Sr x Ga1−y Mn y O3 solid solution crystals: Raman spectroscopy characterization

The first-order orthorhombic–rhombohedral phase transition of La1−xSrxGa1−yMnyO3 (LSGMn) crystals is studied by Raman spectroscopy. An increase in Mn content causes significant changes in Raman spectra, especially in the high-wavenumber spectral range. The Raman spectrum of La1−xSrxGa1−yMnyO3 compound with x = y = 0.02 is dominated by MnO6 internal vibrations. Moreover, a change in the spectrum of this crystal is observed when the 785-nm excitation line is used. Essential reduction in the number of Raman modes during the phase transition is experimentally observed. The temperature of the phase transition decreases with increasing Sr and Mn contents, at the rate of about 20 K/at%.