Morphotropic Phase Transition Research Articles

Dielectric properties of Na1−xKxNbO3, near x = 0.5 morphotropic phase region

Sodium-potassium niobate (Na1−xKxNbO3) ceramic pellets, with x = 0.490, 0.495, 0.500, 0.505 and 0.510, were prepared by solid state reaction method with double sintering. With varying potassium content (x), dielectric properties and X-ray diffraction (XRD) peaks shifting pattern exhibit strong compositional dependence. Among the prepared compositions, a break in the XRD peaks shifting tendency, and minimum dielectric constant, loss tangent and Electric conductivity indicate a morphotropic phase transition type behavior, at the composition with x = 0.500.

Large electrocaloric effect in lead-free Ba1-xCaxTi1-yZryO3 ceramics under strong electric field at room-temperature

In this study, lead-free Ba1-xCaxZryTi1-yO3 (BCTZ(x, y)) ceramics were prepared by means of the classic solid-state reaction method. The morphotropic phase transition (MPB) from the orthorhombic to the tetragonal phase (O-T) was identified in this composition. Besides, the identification of those two structures at room temperature (RT) was made possible thanks to an X-ray diffraction (XRD) study. In order to determine the phase transitions dielectric measurements were conducted. Based on Maxwell equation, the electrocaloric (EC) effect in the studied ceramics was performed via the indirect method. The compositions gave maximum EC temperature changes (ΔT) at above their TC on application of a 3 kV/mm electric field. These temperature changes are ΔT = 0.565 K at TEC= 392 K, ΔT = 0.548 K at TEC= 365 K and ΔT = 0.235 K at TEC= 307 K for BCZT(10%,5%), BCZT(13%,10%) and BCZT(20%,15%), respectively. At RT, these compositions provided a very interesting EC coefficient (ξ = ΔT/ΔE) compared to the pure BaTiO3 (BT). These values, lying between 0.105 Kmm/kV and 0.188Kmm/kV for ΔE = 3 kV/mm, are also greater than those related to some lead-based ferroelectric.

Structure–luminescence relationship in Eu3+-doped Sr3La2(Ge3O9)2 phosphors

A series of Sr3La2-xEux(Ge3O9)2 (x = 0.1–1.8) cyclogermanates has been prepared by the citrate method. The powder XRD study has shown that the compounds crystallize in the monoclinic space group C2/c, Z = 4. An increase in the dopant content causes a morphotropic phase transition: at x = 0.1–0.7 the samples are isostructural to Sr3La2(Ge3O9)2, while at x = 0.9–1.8 the germanates have a crystal structure of Sr3Eu2(Ge3O9)2. In all compounds, strontium and rare earth ions occupy three symmetry independent cation sites and form layers alternating along the [101¯] direction. The relationships between the crystal structure and the luminescence characteristics: the energy of the charge transfer transition, the values of 4f–4f/CTB and 5D0 → 7F2/5D0 → 7F1 intensity ratios have been revealed. CIE chromaticity coordinates for Sr3La1.4Eu0.6(Ge3O9)2 and Sr3La0.8Eu1.2(Ge3O9)2 phosphors under 393 nm excitation have been measured.

Dependence of the Thermal Conductivity of BiFeO3 Thin Films on Polarization and Structure

Manipulating the thermal-transport properties of solids is of great technological interest for heat management. This study investigates the influence of polarization and crystal structure on the room-temperature thermal conductivity $\ensuremath{\kappa}$ of epitaxial thin films of ferroelectric BiFeO${}_{3}$. $\ensuremath{\kappa}$ shows only slight dependence on ferroelectric domain-wall density, but depends significantly on crystal structure. This may provide a route to reversible control of thermal properties through the morphotropic phase transition between the two structures of BiFeO${}_{3}$.

Kinetics of nucleation of thermodynamically ordered ferroelectric phases in PbMg1/3Nb2/3O3–xPbTiO3 crystals with different compositions

The kinetics of electric field-induced nucleation of ordered ferroelectric phases from a mixed glassy relaxor state has been studied in a number of single-crystal (1–x)PbMg1/3Nb2/3O3–xPbTiO3 (PMN–xPT) solid solutions (x = 29, 33, 35%) lying in a morphotropic phase region. It is shown that the formation of these phases and fast establishment of a macroscopic polarization are preceded by some delay time, depending on the electric field strength and temperature. It is found that the monoclinic phase is thermodynamically stable at room temperature in all the compounds in the time (~3000 s) and electric field (~1 kV/cm) ranges under study, whereas the monoclinic phase of the compound with x = 35% transforms, at temperatures near the temperature of the morphotropic phase transition after insignificant time interval of ~100 s, to another stable ferroelectric tetragonal phase.

Concentration- and temperature-induced phase transitions in NdAlO3−TbAlO3 and NdAlO3−DyAlO3 systems

Phase and structural behaviour in the NdAlO3–RAlO3 (R = Tb, Dy) systems has been studied in a wide concentration and temperature ranges. Two kinds of solid solutions Nd1−xTbxAlO3 and Nd1−xDyxAlO3 with rhombohedral (space group No 167 R3¯c) and orthorhombic (space group No 62 Pbnm) perovskite structure are formed in these systems. Morphotropic phase transition between two perovskite-like structures occurs at x = 0.1 for Nd1−xTbxAlO3 and at x = 0.05–0.1 for Nd1−xDyxAlO3. First-order structural phase transition Pbnm↔R3¯c has been detected in Nd0.8Tb0.2AlO3, N0.6Tb0.4AlO3, Nd0.8Dy0.2AlO3 and Nd0.7Dy0.3AlO3 samples at 540 K, 1174 K, 687 K and 900 K, respectively. Phase diagrams of the pseudo-binary systems NdAlO3−TbAlO3 and NdAlO3−DyAlO3 have been constructed.

Role of the electronic structure in the morphotropic phase boundary of TbxDy1−xCo2 studied by first-principle calculation

Physically parallel to ferroelectric morphotropic phase boundary, a phase boundary separating two ferromagnetic phases of different crystallographic symmetries was experimentally found in TbxDy1−xCo2 via high-resolution synchrotron X-ray diffraction. However, lack of the theoretical support makes the morphotropic phase boundary in ferromagnetic system debatable. Here, a first-principle calculation was employed to investigate the electronic structure variation during the morphotropic phase transition in TbxDy1−xCo2. It offers a theoretical basis for the ferromagnetic phase of different crystallographic symmetries in TbxDy1−xCo2. It also provides an explanation for why morphotropic phase boundary occurs in TbxDy1−xCo2 alloys and offers a serviceable method to search for the morphotropic phase boundary phenomena in other alloys via computational rather than experimental method.

Crystal structure and phase transitions in R2TeO6 (R = La, Pr, Nd, Tb, Ho, Er, Tm, Lu) oxides: A neutron diffraction study

Eight members of the R2TeO6 family for the neutron non-absorbing rare-earths (R=La, Pr, Nd, Tb, Ho, Er, Tm, Lu) have been synthesized by solid-state reactions as pure specimens and structurally characterized by neutron powder diffraction (NPD), allowing for the precise determination of the oxygen positions and thermal displacements in the presence of heavy rare-earth and tellurium atoms. For the large rare-earth cations (from La3+ to Tm3+) the crystal structure adopts the Nd2WO6 type, defined in the orthorhombic P212121 space group, containing two types of crystallographically independent rare earth atoms, R1 and R2, sevenfold coordinated with oxygen atoms; the tellurium atoms form distorted TeO6 octahedra, sharing corners and edges with both kinds of R1O7 and R2O7 polyhedra. For Lu2TeO6 a morphotropic phase transition is observed, and this compound crystallizes in the trigonal P321 space group, with two RO6 and two independent TeO6 coordination polyhedra, also sharing vertices. For Yb2TeO6 a mixture of orthorhombic and trigonal phases is identified. A bond valence study, confirming that R is trivalent and Te is hexavalent in the whole series, helps to understand the driving force for the morphotropic transition to a more stable crystal structure where Lu3+ adopts a lower coordination index.

Multiferroic and magnetoelectric properties of BiFeO3–YCrO3 ceramics at the rhombohedral–orthorhombic phase boundary

Solid solutions of perovskites (1−x)BiFeO3−xYCrO3 (BFO–YCO) with x ranging from 0 to 0.3 have been synthesized under high pressure conditions in an attempt to find magnetoelectric materials. A morphotropic phase transition from rhombohedral (R3c) to orthorhombic (Pbnm) phase is found at x around 0.2. The magnetic and leakage properties of BFO are improved by the introduction of YCO. The 0.8BFO–0.2YCO ceramics, which are located at the structural boundary, exhibit weak ferromagnetism and a typical ferroelectric hysteresis loop with a large remnant polarization of 14.90μC/cm2 at room temperature. The anomaly noticed from the temperature dependent dielectric analysis and magnetodielectric effect observed in the 0.8BFO–0.2YCO ceramics indicate the coupling behavior between the magnetic and electric orders. These properties entitle the BFO–YCO solid solution one of few multiferroic materials with enhanced properties.

Revealing Morphotropic Phase Boundary and Phase Transition Behavior in Strained BiFeO3 Thin Films

As the advance of technology, devices with multi-function have dictated the material design. Complex oxide materials, having the spatial variation of electron charge and lattice degree of freedom at interface, have attracted wide attention because of their various properties. Recently, strained bismuth ferrite (BiFeO3, BFO) thin film is the most promising multifunctional material due to its spontaneous magnetic, ferroelectric and unique electromechanical properties at room temperature. In this study, we have investigated the temperature-dependent structural and strain-driven phase transition in epitaxial BFO thin films deposited on LaAlO3 substrates. Atomic force microscopy (AFM) and X-ray diffraction spectroscopy (XRD) demonstrated that the tetragonal-like phase (T-phase) and rhombohedral-like phase (R-phase) were involved in the strain BFO thin films. High-resolution transmission electron microscopy (HRTEM) images show that the T-phase BFO has a large c-axis parameter of ~4.65 Å, whereas R-phase BFO has a relatively small c-axis parameter of ~4.05 Å. We further find that the tilt angle between R/T phases is about 4.65° along the a-axis. Using in-situ TEM, we observed that the BFO thin film exhibited R→T phase transition with increasing temperature. Upon increasing the temperature from room temperature to 200℃, we found a significant increase in the width of T-phase region with the broader morphology phase boundary (MPB). Interestingly, upon further increasing the temperature to 250~300℃, the MPB disappears gradually with the change of tilt angle. Eventually, only T-phase BFO would be preserved at high temperature. Our study directly demonstrates the detail of structural changes with temperature effect, which may provide important information for material design and promising multifunctional applications in future technology.

Lattice crossover and phase transitions in NdAlO3–GdAlO3 system

Phase and structural behaviour in the (1−x)NdAlO3–xGdAlO3 system in a whole concentration range has been studied by means of in situ high-resolution X-ray synchrotron powder diffraction technique and differential thermal analysis. Two kinds of solid solutions Nd1−xGdxAlO3 have been found at room temperature: one with rhombohedral (x<0.15) and one with orthorhombic (x≥0.20) symmetry. A morphotropic phase transition occurs at x≈0.15, where the co-existence of both phases was observed. Peculiarity of the orthorhombic solid solution is the lattice parameter crossover at the compositions with x=0.33, 0.49 and 0.62. First-order structural transition Pbnm↔R3¯с has been detected both from in situ powder diffraction and thermal analysis data. Continuous phase transformation R3¯с↔Pm3¯m above 2140K has been predicted for Nd-rich sample Nd0.85Gd0.15AlO3 from the extrapolation of high-temperature behaviour of the lattice parameter ratio of the rhombohedral phase. Based on the experimental data, the phase diagram of the pseudo-binary system NdAlO3–GdAlO3 has been constructed.

Crystal structure, morphotropic phase transition and luminescence in the new cyclosilicates Sr3R2(Si3O9)2, R=Y, Eu–Lu

A new series of promising luminescent materials, cyclosilicates Sr3R2(Si3O9)2, R=Y, Eu–Lu, has been synthesized via a solid-state reaction. X-ray and neutron powder diffraction studies show that these oxides crystallize in the monoclinic crystal system (S.G. C2/c, Z=4) and have a morphotropic phase transition between Er and Tm compounds followed by a step-like change of the unit cell constants. Isolated [Si3O9] rings located in layers are basic building units and stack with Sr/R layers along the [101¯] direction. The rare earth atoms are distributed among three independent Sr/R sites coordinated by 8, 7 and 6 oxygen atoms, and the Sr-R populations change from mixed to 0.5/0.5 over site (1) and full occupation of sites (2) and (3) by Sr and R, respectively, at the transition. Changes of the conformation and mutual arrangement of [Si3O9] rings, as well as exchange of oxygen atoms from the first and the second coordination sphere of two Sr/R sites also feature the phase transition. Luminescence in Sr3Y2(Si3O9)2:Eu3+ under ultraviolet (UV) excitation has been discussed.

Morphotropic phase transition in a lead-free system (1−x)NaNbO3-xCaTiO3

Morphotropic phase transition in a lead-free system (1−<i>x</i>)NaNbO₃-<i>x</i>CaTiO₃

Synthesis and thermochemistry of relaxor ferroelectrics in the lead magnesium niobate–lead titanate (PMN–PT) solid solutions series

The energetics of the (1 − x)PMN–xPT solid solutions have been investigated using high temperature oxide melt solution calorimetry in 3Na2O·4MoO3solvent at 702 °C. The solid solutions show positive heats of mixing, reflecting the changes in structure from cubic to tetragonal to monoclinic and the morphotropic phase transition. The synthesis of a perovskite phase without a secondary pyrochlore phase depends on the annealing temperature and time. The enthalpies of reactions involved in synthesis and decomposition of the PMN perovskite were measured for the first time by oxide melt solution calorimetry. The decomposition of perovskite to pyrochlore plus MgO and PbO is energetically favorable, as is the formation of perovskite from columbite and lead oxide.

Structural features of the formation of La1−x Ca x FeO3−δ (0 ≤x ≤ 0.7) hetero valent solid solutions

A synthesis method with the use of polymer-salt compositions (calcination temperature 800°C) provides the preparation of various solid solutions of a La1−x Ca x FeO3−δ series in the 0≤ x≤ 0.7 range, which belong to the perovskite structure type. A morphotropic phase transition occurs from the orthorhombic perovskite modification (0≤ x ≤ 0.4) to the cubic one (0.5 ≤ x≤ 0.7). A growing number of microdistortions in the perovskite structure and the formation of a microblock structure in the morphotropic phase transition region are observed with increasing degree of calcium substitution for lanthanum. Calcination of solid solutions with x = 0.6 and 0.7 at temperatures above 1000°C in the air or under conditions of reduced oxygen partial pressure (laboratory vacuum of 10−3 Torr) results in the formation of a nanostructured state with coherently grown blocks of perovskite and Grenier phase, which is due to irreversible oxygen loss.

Composition Dependent Dielectric Anomaly in Na1−xKxNbO3, at x = 0.475

Sodium-potassium niobate (Na1−xKxNbO3) pellet samples, with x = 0.40, 0.47, 0.475, 0.48 and 0.50, were prepared by solid state reaction method. With varying composition (x), X-ray diffraction patterns show a break in the peak shifting tendency, at x = 0.475. Lattice parameters calculated from X-ray diffraction patterns show a slope change anomaly, in the a/b vs. x plot, from positive to negative, at x = 0.475. Dielectric constant, loss tangent and dielectric conductivity were found minimum for the composition with x = 0.475, among the prepared compositions, at all measured frequencies and temperatures. The observed results show a strong compositional dependence of dielectric properties, in this system. Observed minimum values of dielectric parameters may be associated with increased Na site vacancies and structural anomaly- a morphotropic type phase transition, at the composition with x = 0.475.

Morphotropic phase boundary in [formula omitted], near [formula omitted

Pellet samples of Na 1 − x K x NbO 3 with x = 0.28 , 0.30, 0.32, 0.325 and 0.40 were prepared by a solid state reaction method. X-ray diffraction and dielectric measurements were carried out on the prepared samples. With varying composition ( x ) , a break was observed in the X-ray diffraction peaks shifting tendency; a slope change anomaly in the a / b vs. x plot; and minimum values of dielectric constant, loss tangent and dielectric conductivity, at all the measured frequencies and temperatures, showing a strong compositional dependence of structural and dielectric properties — a morphotropic phase transition, at x = 0.32 . The observed minimum values of dielectric parameters may be associated with the increased Na-site vacancies and structural anomaly, at x = 0.32 .

Phase transitions in PbTixHf1-xO3 determined by thermal analysis and impedance spectroscopy

The objective of this study is to determine the influence of partial substitutions of Ti4+ by isovalent Hf4+ in the perovskite-type crystalline structure of PbTiO3. Different samples over the whole composition range (0 ≤ x≤1) in the PbTi1-xHfxO3 family have been prepared. Phase transitions have been determined by thermal analysis (differential scanning calorimeter: DSC) and complex impedance spectroscopy (IS) over a wide temperature range. As a consequence of the cation replacement the changes that take place in the different phase transition temperature are reported. By both techniques, thermal analysis and electrical characterization, it is shown that for all compositions prepared there is only one phase transition in a temperature range between 230 and 460 °C. With these results and the previously known crystalline structure of pure PbTiO3 and PbHfO3 perovskites, the phase diagram of the PbTi1-xHfxO3 family is presented including a morphotropic phase transition at x ~ 0.5.

Enhanced magnetization and polarization in chemically modified multiferroic (1−x)BiFeO3–xDyFeO3 solid solution

In order to develop multiferroics with large magnetization and polarization, we have prepared the (1−x)BiFeO3–xDyFeO3 (BF-DF) solid solution and systematically studied its properties. A morphotropic phase transition from rhombohedral perovskite (R3c) to orthorhombic orthoferrite (Pbnm) was found at x around 0.1. The magnetic properties of BiFeO3 were improved by the introduction of Dy3+ on the perovskite A site. Chemically modified Bi0.9Dy0.1Fe1−xTixO3+x/2 ceramics by aliovalent ionic substitution of Ti4+ for Fe3+ with excess Bi2O3 exhibit weak ferromagnetism and a typical ferroelectric hysteresis loop with a large remnant polarization of 23 μC/cm2 at room temperature and a large saturated magnetization (Ms=0.5 μB/f.u.) at 2 K. These properties entitle the BF-DF solid solution one of few multiferroic materials that exhibit both decent magnetization and electric polarization.

Phases and morphotropic regions in the PbNb2/3 Mg1/3O3-PbTiO3 system

Ceramic samples of solid solutions (1 − x) PbNb2/3Mg1/3O3 · xPbTiO3 (0 ≤ x ≤ 1.0, Δx = 0.0025–0.05) are prepared by the columbite method. A detailed x−T phase diagram of the system is constructed (isothermal join at 25°C), and dielectric, piezoelectric, and elastic properties are investigated. It is established that the region of the morphotropic phase transition is positioned in the range 0.28 < x ≤ 0.43 and consists of a series of narrower regions. Inside one phase (cubic, rhombohedral, tetragonal), regions are found in which a qualitative and quantitative difference in structural and electrical parameters is observed. An interpretation of the observed effects in the context of the defect structure of the objects is suggested.