Nonpolar Orthorhombic Phase Research Articles

Crystal Structures and Piezoelectric Properties of Quenched and Slowly-Cooled BiFeO3-BaTiO3 Ceramics.

The BiFeO3-BaTiO3 (BF-BT) ceramics were here prepared through the solid-state reaction of Bi2O3, Fe2O3 and nano-sized BT powders. The crystal structures and piezoelectric properties were investigated in both quenched (AQ) and slowly cooled (SC) 0.7BF-0.3BT ceramics. Prior work has shown that rhombohedral and pseudo-cubic phases coexist in 0.7BF-0.3BT ceramics. In this work, the crystal structure of the pseudo-cubic phase was refined as a non-polar orthorhombic Pbnm phase in the SC sample and as a polar orthorhombic Pmc21 phase in the AQ sample. In addition to a sharp dielectric peak at about 620 °C, corresponding to the Curie temperature of the rhombohedral phase, a broad dielectric peak with strong frequency dispersion and a sharp frequency-independent dielectric peak were observed at around 500 °C in the SC and AQ samples, respectively. We determine that the dielectric anomalies around 500 °C were caused by a relaxor phase transition of the non-polar orthorhombic phase in the SC sample and a ferroelectric-paraelectric phase transition of the polar orthorhombic phase in the AQ sample. The AQ sample showed better ferroelectric and piezoelectric properties than the SC sample. The 0.7BF-0.3BT ceramic slowly cooled in a nitrogen atmosphere showed a well-saturated P-E curve and a similar temperature-dependent dielectric constant as the AQ sample. Our results indicate that large concentrations of oxygen vacancies produce a more distorted polar orthorhombic phase and better piezoelectric properties in the AQ sample than in the SC sample.

Temperature-induced hexagonal–orthorhombic phase transition in lutetium ferrite nanoparticles

The x-ray diffraction, Raman, and infrared spectroscopies and magnetic measurements were used to explore the correlated changes of the structure, lattice dynamics, and magnetic properties of the LuFeO3 nanoparticles, which appear in dependence on their sintering temperature. We revealed a gradual substitution of the hexagonal phase by the orthorhombic phase in the nanoparticles, with sintering temperature increasing from 700 to 1100 °C. The origin and stability of the hexagonal phase in the LuFeO3 nanoparticles are of the special interest, because the nanoparticles in the phase can be a room-temperature multiferroic with a weak ferromagnetic and pronounced structural and ferroelectric long-range ordering. The antiferromagnetic and nonpolar orthorhombic phase is more stable in the bulk LuFeO3. To define the ranges of the hexagonal phase stability, we determine the bulk and interface energy densities of different phases from the comparison of the Gibbs model with experimental results. Using effective parameters of the Gibbs model, we predict the influence of size effects and temperature on the structural and polar properties of the LuFeO3 nanoparticles. Analysis of the obtained results shows that the combination of the x-ray diffraction, Raman and infrared spectroscopies, magnetic measurements, and theoretical modeling of structural and polar properties allows us to establish the interplay between the phase composition, lattice dynamics, and multiferroic properties of the LuFeO3 nanoparticles prepared under different conditions.

Structure and Lattice Dynamics of Bi1−xNdxFeO3 and Bi1−xGdxFeO3 Ceramics near the Morphotropic Phase Boundary

The crystal structures of Bi1−xNdxFeO3 and Bi1−xGdxFeO3 solid solutions (0 ≤ x ≤ 0.2) with chemical compositions across structural transformations from the polar rhombohedral phase to the orthorhombic phase with an antipolar distortion and then to the nonpolar orthorhombic phase have been investigated using X-ray diffraction and infrared reflective spectrometry. The obtained results clarify details of the structural transitions assuming the changes that occurred in the crystal lattice dynamics of the compounds. Increase in the dopant content causes a notable change in the intensity and position of the reflectance lines at 18.2 μm and 22.6 μm (550 cm−1 and 440 cm−1) ascribed to the transverse optical phonon modes associated with Bi (Nd, Gd)–O and Fe–O bonds. In the concentration region attributed to the dominant rhombohedral phase, the chemical substitution leads to an increase in intensity of the modes A1 for solid solutions of both systems. Meanwhile, in the case of Gd doping, the mode A1 shifts towards the red side of the spectrum, but there is an opposite tendency in the case of Nd doping; the intensity of the modes E decrease regardless of both the dopant-ion type and concentration. This behavior is discussed assuming the change in mass for the chain of chemical bonds caused by different dopant ions and the structural transformations occurring in the compounds upon chemical doping.

Pressure induced phase transitions in Sm-doped BiFeO3 in the morphotropic phase boundary

Sm-doped BiFeO3 compacted powders with composition across the morphotropic phase boundary region were prepared by sol-gel method. Crystal structure, morphology and magnetic state of the compounds were analyzed as a function of dopant concentration, temperature and external pressure using synchrotron and laboratory X-ray diffraction, electron microscopy, Raman and Mössbauer spectroscopy. Application of external pressure shifts the phase transition from the rhombohedral structure to the nonpolar orthorhombic structure towards lower concentration of the dopant content, wherein the amount of the anti-polar orthorhombic phase notably decreases. Raman and Mössbauer spectroscopy data provides additional information about the structural distortion on local scale level which testifies faster formation of nonpolar orthorhombic phase and associated modification in the magnetic state in the compounds subjected to high pressure. Temperature increase leads to the structural transition to the nonpolar orthorhombic phase regardless the structural state at room temperature; furthermore, application of external pressure decreases the phase transition temperature.

Crystal and Magnetic Structure Transitions in BiMnO3+δ Ceramics Driven by Cation Vacancies and Temperature.

The crystal structure of BiMnO3+δ ceramics has been studied as a function of nominal oxygen excess and temperature using synchrotron and neutron powder diffraction, magnetometry and differential scanning calorimetry. Increase in oxygen excess leads to the structural transformations from the monoclinic structure (C2/c) to another monoclinic (P21/c), and then to the orthorhombic (Pnma) structure through the two-phase regions. The sequence of the structural transformations is accompanied by a modification of the orbital ordering followed by its disruption. Modification of the orbital order leads to a rearrangement of the magnetic structure of the compounds from the long-range ferromagnetic to a mixed magnetic state with antiferromagnetic clusters coexistent in a ferromagnetic matrix followed by a frustration of the long-range magnetic order. Temperature increase causes the structural transition to the nonpolar orthorhombic phase regardless of the structural state at room temperature; the orbital order is destroyed in compounds BiMnO3+δ (δ ≤ 0.14) at temperatures above 470 °C.

Morphotropic phase boundary in Sm-substituted BiFeO3 ceramics: Local vs microscopic approaches

Samarium substituted bismuth ferrite (BiFeO3) ceramics prepared by sol-gel synthesis method were studied using both local scale and microscopic measurement techniques in order to clarify an evolution of the crystal structure of the compounds across the morphotropic phase boundary region. X-ray diffraction analysis, transmission and scanning electron microscopies, XPS, EDS/EDX experiments and piezoresponse force microscopy were used to study the structural transitions from the polar active rhombohedral phase to the anti-polar orthorhombic phase and then to the non-polar orthorhombic phase, observed in the Bi1−xSmxFeO3 compounds within the concentration range of 0.08 ≤ x ≤ 0.2. The results obtained by microscopic techniques testify that the compounds in the range of 0.12 ≤ x ≤ 0.15 are characterized by two phase structural state formed by a coexistence of the rhombohedral and the anti-polar orthorhombic phases; two phase structural state observed in the compounds with 0.15<x<0.18 is associated with a coexistence of the anti-polar orthorhombic and the non-polar orthorhombic phases. Local scale measurements have revealed a notable difference in the concentration range ascribed to the morphotropic phase boundary estimated by microscopic measurements, the obtained results testify a wider concentration range ascribed to a coexistence of different structural phases, the background of the mentioned difference is discussed.

Evolution of the crystal structure and magnetic properties of Sm-doped BiFeO3 ceramics across the phase boundary region

Samarium doped BiFeO3 compounds having nano-size crystallites were prepared by the ethylene glycol assisted sol-gel synthesis method. X-ray diffraction and SEM measurements as well as Raman spectroscopy and FTIR experiments were used to clarify an evolution of the crystal structure on microscopic and local scale levels in the compounds having formula Bi1-xSmxFeO3, where 0 = x ≤ 1. Magnetization measurements along with the structural data were used to determine a correlation between magnetic properties and crystal structure of the compounds. The compounds with 0.1 = x ≤ 0.2 are characterized by the sequence of the structural transitions driven by the dopant increase, viz. from the polar rhombohedral phase to the non-polar orthorhombic phase via two-phase regions characterized by the presence of the anti-polar orthorhombic phase. The concentration regions ascribed to a coexistence of the rhombohedral and the anti-polar orthorhombic phases as well as the anti-polar orthorhombic and the non-polar orthorhombic phases are observed in the ranges 0.12 = x ≤ 0.15 and 0.15 < x < 0.18 respectively. The compounds having single phase rhombohedral structure are characterized by a release of remnant magnetization as compared to ceramics with similar chemical compositions but synthesized by the solid-state reaction method and having microscopic size crystallites. A correlation between the type of structural distortion, morphology of crystallites and an onset of remnant magnetization is discussed highlighting a difference in the evolution of crystal structure and magnetization observed for the Sm-doped ceramics prepared by modified sol-gel method and conventional solid-state reaction technique.

Crystal Structure of Bi1 – xPrxFeO3 Multiferroics at the Rhombohedral–Orthorhombic Phase Boundary

The crystal structures of Bi1 – xPrxFeO3 solid solutions in the vicinity of the transition from the polar rhombohedral phase to the nonpolar orthorhombic phase have been analyzed by X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. These methods made it possible to reveal the specific features of the change in the crystal structures of the solid solutions with an increase in temperature in dependence of their structural state at room temperature. The analysis of the structural data indicates the presence of antipolar orthorhombic phase in the compounds with dominant rhombohedral phase at the temperature transition to the nonpolar state. The evolution of the crystal structures of Bi1 – xPrxFeO3 solid solutions with a change in the concentration of praseodymium ions and temperature has been considered, and the role of the antipolar orthorhombic phase as an intermediate phase in the structural transition from the rhombohedral phase to the nonpolar orthorhombic phase has been discussed.

Asymmetric covalence bonding between In 5s and O 2p states in ferroelectric InFeO3

The structural evolution, electronic and magnetic properties of the high-pressure nonpolar orthorhombic phase (orthorhombic, space group Pnma) and the LiNbO3-type (rhombohedral, space group R3c) InFeO3 have been explored via first-principles calculations. Furthermore, the ferroelectric properties of the LiNbO3-type phase have been revisited thoroughly. First of all, the nonpolar orthorhombic phase crystallized within Pnma space group was theoretically recognized. Secondly, the results reasonably reproduced the pressure-induced R3c →Pnma structural phase transition, which is in fair agreement with the experimental observation that the LiNbO3-type phase can be achieved from the decompression of the high pressure perovskite phase. In addition, the electric polarization for the non-centrosymmetric LiNbO3-type within the G-type antiferromagnetic ground state is evaluated to be 86.5 μC/cm2 by means of the Berry phase formula. More interestingly, in conjunction with the geometric factor, an additional mechanism behind the ferroelectric displacement is clarified as the strong chemical bonding between the In 5s and O 2p orbitals, i.e., the hidden tendency to create space for the unoccupied antibonding states, independent of whether the cation is an s0 or s2 configuration, and is manifested in terms of the analysis of Born effective charges, potential energy surfaces, charge density isosurfaces.

Intermediate structural state in Bi1−x Pr x FeO3 ceramics at the rhombohedral–orthorhombic phase boundary

Crystal structure of the Bi1−x Pr x FeO3 ceramics of the compositions corresponding to the threshold concentrations separating the polar rhombohedral (R3c) and anti-polar orthorhombic (Pbam) phases has been investigated with X-ray diffraction, transmission electron microscopy and differential scanning calorimetry measurements performed in a broad temperature range. The structural study specifies the peculiarities of the temperature-driven transition into the non-polar orthorhombic (Pnma) phase depending on the structural state of the compounds at room temperature. The crystal structure analysis reveals the revival of the anti-polar orthorhombic phase upon the temperature-induced transition, thus assuming that it can be considered as the bridge phase between the polar rhombohedral and the non-polar orthorhombic phases.

Magnetic properties of the multiferroics Bi1−xCaxFe1−xMnxO3 and Bi1−xCaxFe1−xTixO3

This is a study of the crystalline structure and magnetic properties of the multiferroics Bi1−xCaxFe1−xMexO3 (Me = Mn, Ti). It is shown that Mn and Ti are in a quadrivalent state and the oxygen stoichiometry is conserved. The Me = Mn series undergoes a conversion at x = 0.19 from a polar rhombohedral phase (R3c) to a nonpolar orthorhombic phase (Pnma). A similar transition in the Me = Ti series was observed at x = 0.25. In both series, the polar phase (x ≥ 0.1) is weakly ferromagnetic. The spontaneous magnetization of the titanium series, however, exceeds that of the manganese series by more than a factor of three, despite the fact that Ti4+ is a diamagnetic ion. The residual magnetization of the titanium series decreases with rising temperature, while that of the manganese series increases anomalously within the interval 0.1 ≤ x ≤ 0.22. This behavior of the magnetic properties of the manganese series is explained by the extremely high sensitivity of the magnetic anisotropy to changes in the parameters of the crystal lattice, which leads to spin reversal.

Structural and magnetic phase transitions in Bi1−xCaxFe1−xMnxO3 multiferroics

Crystal and magnetic structure of Bi1−xCaxFe1−xMnxO3 (x ≤ 0.3) multiferroics have been studied by X-ray and neutron powder diffraction as well as magnetometry techniques. It was shown that polar rhombohedral phase specific for the compounds with x ≤ 0.19 transforms to the nonpolar orthorhombic one through very narrow (∼1% on concentration) two phase structural state. The polar phase is characterized by antiferromagnetic structure, wherein the compounds with x ≤ 0.1 show metamagnetic behavior. Compounds with x ≥ 0.1 having polar rhombohedral structure exhibit small remnant magnetization (up to 0.07 emu/g for x = 0.17 compound). Structural transition into the nonpolar orthorhombic phase does not change significantly the magnetic structure of the compounds. However temperature decrease down to 10 K leads to notable change in magnetic anisotropy and spontaneous magnetization. The magnetic properties of the compounds are discussed assuming their inhomogeneous magnetic state affected by external magnetic field and temperature.

Multiferroic properties of Bi1 − xDyxFeO3 (x = 0–0.2) ceramics at various temperatures

Insulated Bi1−xDyxFeO3 (x=0–0.2) were prepared to study their multiferroic properties at various temperatures. Bi1−xDyxFeO3 (x=0–0.08) shows rhombohedral phase, ferroelectric polarization of >18μC/cm2, and piezoelectric d33 value of >35pC/N at room temperature. After the polarized ceramics were annealed at various high temperatures, the piezoelectric d33 changes suggest that ferroelectric component exists below 700°C. There are ferroelectric phase and PZT-like anti-polar orthorhombic phase together in Bi1−xDyxFeO3 (x=0.11–0.17), although only ferroelectric phase contributes polarization and d33 which decreases with Dy concentration at this two-phase coexistence region. Finally, Bi1−xDyxFeO3 (x=0.2) transforms to non-polar orthorhombic phase according to structural and electric measurements.

Structure, ferroelectricity and piezoelectricity evolutions of Bi1−xSmxFeO3 at various temperatures

Insulated Bi1−xSmxFeO3 (x=0–0.25) were prepared to study their multiferroic properties at various temperatures. Bi1−xSmxFeO3 (x=0−0.12) shows a rhombohedral-like phase, ferroelectric polarization of >17.8 μC/cm2, piezoelectric d33 value of >35 pC/N at room temperature. Bi1−xSmxFeO3 (x=0.125–0.2) has a ferroelectric triclinic phase and an anti-polar orthorhombic phase together at room temperature, and Bi0.75Sm0.25FeO3 is mainly composed of a non-polar orthorhombic phase. After the polarized Bi1−xSmxFeO3 (x=0–0.165) was annealed at various high temperatures, the d33 evolution supports ferroelectric component exist above 500 °C. Raman spectra suggests that the main component of Bi0.875Sm0.125FeO3 does not keep a rhombohedral ferroelectric phase above 400 °C, thus its ferroelectric traits at high temperature should be due to ferroelectric clusters embedded in orthorhombic phase.

Terahertz and infrared studies of antiferroelectric phase transition in multiferroic Bi0.85Nd0.15FeO3

High-frequency dielectric studies of Bi0.85Nd0.15FeO3 ceramics performed betweeen 100 and 900 K reveal hardening of most polar phonons on cooling below antiferroelectric phase transition, which occurs near 600 K. Moreover, a strong THz dielectric relaxation is seen in paraelectric phase. Its relaxation frequency softens on cooling towards TC ≈ 600 K, its dielectric strength simultaneously decreases, and finally the relaxation disappears from the spectra below 450 K. Both phonon and dielectric relaxation behavior are responsible for a decrease in the dielectric permittivity at the antiferroelectric phase transition. Origin of unusual strong THz dielectric relaxation in paraelectric phase is discussed. Bi0.85Nd0.15FeO3 structure lies on the phase boundary between polar rhombohedral and non-polar orthorhombic phase and owing to this, the polarization rotation and polarization extension can enhance the piezoelectric response of this system. Similarities and discrepancies with lead-based piezoelectric perovskites, exhibiting morphotrophic phase boundary between two ferroelectric phases, are discussed.

Microstructure-electromechanical property correlations in rare-earth-substituted BiFeO3 epitaxial thin films at morphotropic phase boundaries

Structure-electromechanical property correlations in rare-earth (RE)-substituted (001) BiFeO3 (BFO) epitaxial thin films have been investigated. Quantitative piezoelectric coefficient (d33) and dielectric constant (ε33) measurements, in conjunction with selected area electron diffraction, reveal that the enhancement in d33 and ε33 at the morphotropic phase boundary (MPB) of the RE-substituted films (RE=Dy3+, Gd3+, and Sm3+) is correlated with the presence of a competing intermediate antipolar phase with the rhombohedral ferroelectric and nonpolar orthorhombic phase. This leads to a complex nanoscale phase coexistence at the MPB. Extending the studies to RE=La3+ case, we find the nanoscale phase coexistence to be less pronounced. This explains the lack of increase in d33 for the La3+-substituted BFO films, in contrast to the Dy3+, Gd3+, and Sm3+-substituted films.