Pbam Phase Research Articles

Effect of La doping on the structure and cycloidal spin ordering in multiferroic BiFeO3.

A series of Bi1-xLaxFeO3 samples with 0.00 ≤ x ≤ 0.30 was synthesized by the sol-gel method. The effects of lanthanum concentration on the phase formation, microstructure and cycloidal spin ordering were studied using X-ray diffraction, scanning electron microscopy and Mössbauer spectroscopy. The crystal structure of the La-doped bismuth ferrite transformed from rhombohedral R3c (x ≤ 0.05) to a mixture of R3c and cubic Pm3m (0.07 ≤ x ≤ 0.15) and finally to a mixture of R3c, Pm3m and orthorhombic Pbam (0.20 ≤ x ≤ 0.30). The Pbam phase, with characteristic porous microstructure shown by microscopy images, was observed in Bi1-xLaxFeO3 compounds for the first time. Based on the Mössbauer spectroscopy, it was found that the cycloidal spin ordering started to disappear at x = 0.07. With increasing La concentration the share of the cycloid decreased from 100% at 0.00 ≤ x ≤ 0.05 to 0% at x = 0.30. At the beginning, for x ≤ 0.02, the anharmonicity parameter, m, of the cycloidal spin ordering was about 0.5, which is typical of a pure BiFeO3 compound. In the range 0.05 ≤ x ≤ 0.25, the m parameter was of the order of 0.1, which indicated the practically harmonic character of the cycloid. The structural transition at x = 0.07 was accompanied by a substantial increase in magnetization.

Unusual mechanical strengths of Ta2O5 stable phases: A first-principles calculation study

Tantalum, in its most stable Ta2O5 oxide form, has long been recognized as a superior coating material in orthopedic applications. In this study, the ideal mechanical strengths of I41/amd, Cmmm, C2/c, Pm, and Pbam phases of Ta2O5 are investigated from first-principles calculation, where the stress–strain curves under various deformation strains are obtained and the structural evolution in terms of atomic bonding is analyzed. The results reveal that these stable Ta2O5 polymorphs show unusual mechanical strengths on their high symmetric crystalline planes. Shear super-plasticity is found on the (001) crystalline plane of the I41/amd phase in any shear direction. Shear strain-stiffening exists on the (100) crystalline plane of the Pm phase in the shear direction within an angle of ±18° along the [001] direction. Both shear super-plasticity and shear strain-stiffening make these crystalline planes be able to sustain excessively large shear deformations. Large and isotropic shear strengths are predicted on the (010) crystalline planes of Pm and Pbam phases with nearly identical maximum and minimum peak shear stresses approaching 15 GPa. Much stronger compression and tensile strengths are obtained normal to the (010) crystalline plane of the C2/c phase due to its special spring-like bonding structure, which can endure the compression deformation up to 30%. These findings provide guidance for selecting suitable phases of Ta2O5 and growth directions with crystalline planes possessing excellent mechanical properties in applications of Ta2O5 as a coating material on Ti alloys for biomedical replacements of damaged human organs, such as hip joints, dental implants, and artificial hearts.

Structure, magnetic, opto-electronic and thermoelectric properties of A3In2As4 and A5In2As6 (A = Sr and Eu) Zintl phase compounds

Structural, electronic, optical, thermoelectric and magnetic properties of A3In2As4 and A5In2As6 (A = Eu and Sr) Zintl phase compounds in orthorhombic (Pnma and Pbam) phases are examined using density functional theory. Structure properties are an agreement with the experiment. Magnetic properties disclose the Non-magnetic nature of Sr3In2As4 and Sr5In2As6 and anti-ferromagnetic nature of Eu3In2As4 and Eu5In2As6 compounds.Electronic properties of Sr3In2As4, Sr5In2As6 and Eu3In2As4, Eu5In2As6 shows that all these compounds have narrow direct bandgap semiconductors at ᴦ and M symmetry. Electrical conductivity also gives the information about the semiconducting characteristic of these compounds. These compounds show that reduction in bandgap occurs through the substitution of Sr by Eu while increasing by going from local density approximation to modified Becke-Johnson potential and further reduce by spin orbit coupling effect. The outcome demonstrates that all compounds are optically dynamic in the infrared region, anisotropic and obstruct ultraviolet waves so these compounds are capable to use as shield for ultraviolet radiation and can be used as paramount applicant for optoelectronic devices. Owing to low thermal conductivity, high Seebeck coefficient, power factor and Figure of merit ranges from 0.39 to 1.2 at 800 K indicating that all compounds are active aspirants for thermoelectric applications.

Polymeric Hydronitrogen N4H: A Promising High-Energy-Density Material and High-Temperature Superconductor.

Solid nitrogen-rich compounds are potential high-energy-density materials (HEDMs). The enormous challenge in this area is to synthesize and stabilize these energetic materials at moderate pressure and better under near-ambient conditions. Here, we perform an extensive theoretical study on hydronitrogens by the reverse design method considering both energies and energy densities. Four hydronitrogens with different stoichiometries, that is, N4H, N3H, N2H, and NH, are found to be stable at pressures of about 80-300 GPa and metastable with pressure releasing to ambient pressure. The energy densities of these hydronitrogens are about 5.6-6.5 kJ/g and 1.3-1.5 times larger than that of trinitrotoluene (TNT). Most importantly, the Pbam phase of the N4H compound is an excellent high-temperature superconductor with a Tc of 37.7 K at 72 GPa. The present findings enrich new phases of hydronitrogens under high pressure and characterize their structural and energetic properties and superconductivity, which offer crucial insights for further design and synthesis of exceptional materials with high energy density and high-temperature superconductivity.

Isothermal phase transition across phase boundary in (Pb0.95Ba0.05)ZrO3 ceramics

Paraelectric-ferroelectric-antiferroelectric (PE-FE-AFE) phase transitions induced by external stimulus (temperature, electric field, and hydrostatic pressure) usually produce exotic performances in functional ceramics. This Letter reports an isothermal phase transition in (Pb0.95Ba0.05)ZrO3 ceramics in the proximity of separated PE-FE and FE-AFE phase boundaries as extended waiting time. Through an isothermal process, PE (Pm3¯m)-to-FE (R3cH) and FE-to-AFE (Pbam) phase transformations occur within several minutes (with different transition degrees). This is in contrast to the wide recognition of ultrafast and time-independent ferroelectric transitions. In situ Raman spectra analysis unveils the sluggish growth of FE and AFE phases out of the corresponding parent matrix. Finally, the Landau theory analysis is proposed to explain the isothermal phase transition among PE-FE-AFE phases. This work will refresh the wisdom of phase transitions in functional ceramics.

Dynamical electric and magnetic responses in the Bi0.85Nd0.15FeO3 ceramic with morphotropic phase boundary

A series of experimental and theoretical studies have demonstrated that Bi1-xNdxFeO3 ceramics with morphotropic phase boundaries (MPBs) exhibit a very complex phase-transition process including the displacive ferroelectric phase transition and reorientation of antiferromagnetic alignments [Chem. Mater.23, 2166 (2011); Adv. Funct. Mater.25, 552 (2015)]. In this work, such complex phase-transition process in the Bi0.85Nd0.15FeO3 ceramic is detected by electric and magnetic responses as the function of frequency. First, the phase transition from orthorhombic Pbam phase (O2) to orthorhombic Pbnm phase (O1) is found to be accompanied by the change in the local strain induced by MPBs. Second, the electric response at O2→O1 transition with the disappearance of antiferroelectric order showed the relaxation and reversible rotation of the ferroelectric order coupled with the local strain. The moderate active energy of such coupling (∼2.7 eV) facilitates the possibility of recovering the deformation of Bi1-xNdxFeO3 ceramic by an external electric field. Third, the weak magnetic response is frequency-independent, and the reorientation of the antiferromagnetic order is also reversible. Finally, the rotation of ferroelectric/magnetic order connected to MPBs in the ceramic is discussed.

Ab initio investigation in PbZrO$$_3$$ antiferroelectric: structural and vibrational properties

Using first-principles calculations, we investigate the structural and vibrational properties of PbZrO\(_3\). Starting from the high-symmetry cubic perovskite phase, for which the phonon dispersion curves are reported to have many unstable branches, we identify some key intrinsic characteristics allowing the prediction of materials with the propensity of developing an antiferroelectric behavior. We confirm the key role that R antiferrodistortive modes play in condensing the observed antiferroelectric phase, via a cooperative bilinear coupling, and the nearest with the ferroelectric state. Our work shows that, given all their important potential wells none of the individual modes condensed deletes all other and that it is their coupling which plays a key role in the condensation of the ground state lead zirconate, and these couplings would explain why Pbam and R3c phases are close in energy for promoting the first-order transition.

Negative linear compressibility and unusual dynamic behavior of NaB3

First-principles calculations reveal that sodium boride (NaB3) undergoes a phase transition from a tetragonal P4/mbm phase to an orthorhombic Pbam phase at about 16 GPa, accompanied by counterintuitive lattice expansion along the crystallographic a-axis. This unusual compression behavior is identified as negative linear compressibility (NLC), which is dominantly attributed to the symmetry-breaking of boron framework. Meanwhile, the P4/mbm and Pbam phases form superionic conductors after undergoing a peculiar swap state at high temperature. Specifically, under warm conditions the Na cation pairs exhibit a rare local exchange (or rotation) behavior, which may be originated from the asymmetric energy barriers of different diffusion paths. The study of NaB3 compound sheds new light on a material with the combination of NLC and ion transportation at extreme conditions.

Growth of highly oriented orthorhombic phase of Bi2Fe4O9 thin films by pulsed laser deposition

Phase pure orthorhombic (Pbam) Bi2Fe4O9 thin films are grown on corning glass substrate using pulsed laser deposition technique. Thin films deposited with oxygen partial pressure of 10–150 mTorr are highly oriented along 〈121〉 direction.The indirect bandgap of the deposited thin films of Pbam phase Bi2Fe4O9 is observed to be influenced by the crystallite size, where, the bandgap is found to be blue-shifted with respect to the reported values corresponding to bulk Bi2Fe4O9.

High pressure phase transitions and physical properties of Li2MgH4; implications for hydrogen storage

A systematic calculation has been performed in order to study phase transitions and hydrogen storage properties of ternary hydride Li2MgH4 under pressure. The structural, elastic, electronic and vibrational properties of Li2MgH4 are collected by means of density functional theory. There are three phases identified; Pbam at 0 GPa,Pnma at 5 GPa and Pna21 at 65 GPa Pbam and Pnma phases of Li2MgH4 are found to be mechanically and dynamically stable. Ductility of the phases are determined based on Pugh's criteria. It is found that Li2MgH4 becomes ductile at 5 GPa, otherwise it is a brittle material. Electronic band structures and corresponding partial density of states of phases are also obtained. All phases at 0 GPa, 5 GPa and 65 GPa have wide band gaps, indicating that Li2MgH4 is an insulator at all pressures. The phonon dispersion curves of Pbam and Pnma phases have no imaginary frequency indicating that both phases of Li2MgH4 are dynamically stable. The gravimetric hydrogen density of Li2MgH4 is calculated as 10.52 wt %, which is a great rate along with the hydrogen desorption temperature of 670 K.

Phase coexistence and evolution in sol-gel derived BY-PT-PZ ceramics with significantly enhanced piezoelectricity and high temperature stability

In this study, high Curie Temperature (Tc) perovskite ceramics of optimized composition 0.55(0.1BiYbO3-0.9PbTiO3)-0.45PbZrO3 with unique double orthorhombic main phases were prepared by a modified sol-gel method. Compared to the usual solid-state prepared sample, the sol-gel derived sample has a 1.6 times higher d33 of 325 pC/N, a 2.4 times higher remnant polarization, and a much better high temperature stability with similar depolarization temperature (Td) and Tc. Comprehensive analysis of the xerogel prepared over a wide calcination temperature (Tcal) range of 300–1000 °C revealed that perovskite structure appeared at only 400 °C and it became the main phase above 500 °C. Comparison of XRD refinement results showed that calcination and sintering induced subtle and continuous phase transition, namely, the 400–900 °C calcined powders with coexisted tetragonal (P4mm) and orthorhombic (Pbam) phase changed to a rather stable double orthorhombic (Pmmm and Pbam) main phase in all the differently sintered ceramics, as similar to the 1000 °C calcined powders. The stable phase coexistence well explains the enhanced performance. The results also demonstrate that optimized sol-gel processing can provide high Tc ceramics with desirable multi-phase structure and significantly enhanced performance at a lower temperature.

Structure, magnetic and ferroelectric properties of Sm and Sc doped BiFeO3 polycrystalline ceramics

The polycrystalline Bi1-xSmxFe0.97Sc0.03O3 (x = 0.08, 0.12) ceramics have been synthesized using a modified solid-state reaction method. The Bi0.92Sm0.08Fe0.97Sc0.03O3 ceramic possesses single polar rhombohedral R3c phase, while a polar rhombohedral R3c and an anti-polar orthorhombic Pbam phases coexist in the Bi0.88Sm0.12Fe0.97Sc0.03O3 ceramic. The density and the grain uniformity of all the doped ceramics get improved. Further, the addition of Sm and Sc has resulted in significant improvement in ferroelectric characteristics. In addition to this, it also releases the latent magnetization by suppressing the spiral spin structure. The well-defined polarization along the field direction is confirmed by the saturated hysteresis phase loops from 180° domains by piezoresponse force microscopy. The anti-polar Pbam phase experiences a visually electric field induced phase transition to polar R3c phase in the Bi0.88Sm0.12Fe0.97Sc0.03O3 ceramic.

Magnetic and dielectric response in yttrium (Y)-manganese (Mn) substituted multiferroic Bi1−xYxFe1−yMnyO3 (x=y=;x=0.03,0.06,0.12,y=0.05) ceramics

The effect of Y and Mn substitution on the structural, magnetic, and dielectric properties of Bi1−xYxFe1−yMnyO3 (x=y=0;x=0.03,0.06,0.12,y=0.05) is investigated. X-ray diffraction and Raman spectroscopy analysis revealed that all the compounds were stabilized in the Rhombohedral structure (Space group: R3c) below Y(6%)-Mn co-doping. A minimal contribution from the Pbam phase was also observed at higher Y (12%). Co-doped compounds exhibit canted ferromagnetic behavior with a significant increase in the saturation magnetization (Ms) of 1.28 emu/g when (Y, Mn)=(0.12,0.05). Dielectric measurements at different temperatures ranging between 30°C and 210°C in a wide frequency range of 1 Hz–1 MHz were investigated. Impedance study is useful to understand the correlation of grain and grain boundary with electrical properties. The grain and grain boundary contribution to electrical parameters was presented with equivalent R-C circuit. Furthermore, electrical properties such as complex electrical modulus and electrical conductivity analysis clearly emphasize the substitution of Y and Mn, leading to a significant change in BiFeO3 ceramic.

Strain-Driven Nanoscale Phase Competition near the Antipolar-Nonpolar Phase Boundary in Bi0.7La0.3FeO3 Thin Films.

Complex-oxide materials tuned to be near phase boundaries via chemistry/composition, temperature, pressure, etc. are known to exhibit large susceptibilities. Here, we observe a strain-driven nanoscale phase competition in epitaxially constrained Bi0.7La0.3FeO3 thin films near the antipolar-nonpolar phase boundary and explore the evolution of the structural, dielectric, (anti)ferroelectric, and magnetic properties with strain. We find that compressive and tensile strains can stabilize an antipolar PbZrO3-like Pbam phase and a nonpolar Pnma orthorhombic phase, respectively. Heterostructures grown with little to no strain exhibit a self-assembled nanoscale mixture of the two orthorhombic phases, wherein the relative fraction of each phase can be modified with film thickness. Subsequent investigation of the dielectric and (anti)ferroelectric properties reveals an electric-field-driven phase transformation from the nonpolar phase to the antipolar phase. X-ray linear dichroism reveals that the antiferromagnetic-spin axes can be effectively modified by the strain-induced phase transition. This evolution of antiferromagnetic-spin axes can be leveraged in exchange coupling between the antiferromagnetic Bi0.7La0.3FeO3 and a ferromagnetic Co0.9Fe0.1 layer to tune the ferromagnetic easy axis of the Co0.9Fe0.1. These results demonstrate that besides chemical alloying, epitaxial strain is an alternative and effective way to modify subtle phase relations and tune physical properties in rare earth-alloyed BiFeO3. Furthermore, the observation of antiferroelectric-antiferromagnetic properties in the Pbam Bi0.7La0.3FeO3 phase could be of significant scientific interest and great potential in magnetoelectric devices because of its dual antiferroic nature.

The effect of Cr substitution for Fe on the structure and magnetic properties of BiFeO3 multiferroic

ABSTRACTX-ray diffraction and Мössbauer studies of perovskite BiFe1-xCrxO3 solid solution ceramics fabricated under high pressure (4–6 GPa) have been carried out. Two morphotropic phase boundaries were revealed: one between rhombohedral (R3c) and orthorhombic (Pbam) phases at x ≈ 0.5–0.6 and the other between the orthorhombic (Pbam) and monoclinic (C2/c) phases at x ≈ 0.9. It was found out that substitution of Cr3+ for Fe3+ does not lead to the appearance of a low-spin state of Fe3+ ions, as was predicted by some first-principles calculations. At x ≈ 0.1 a destruction of the space spin-modilated cycloid structure seems to occur.

Crystal structure and magnetic properties of Ti-doped Bi0.84La0.16FeO3 at morphotropic phase boundary

We have studied the crystal structure and magnetic properties of Bi0.84La0.16Fe1-xTixO3 (x = 0.02–0.1) at the morphotropic phase boundary of R3c, Imma, and Pbam phases. The results of Rietveld-refined X-ray diffraction patterns reveal the existence of multi-phase structures, including R3c + Imma for x = 0.02–0.06 and Imma + Pbam for x = 0.08–0.1. The studies of Raman scattering spectra indicate a structural transformation through the disappearance of typical R3c modes. This strongly influences the magnetic properties that are dependent on the crystalline phase ratio. In particular, the magnetic coercivity of x = 0.04–0.06 drastically changes when the Imma phase is dominant. A mechanism of magnetic interactions occurring at the phase boundary is proposed to explain the features of magnetic hysteresis loops of the compounds.

Ferroelectric domains and phase transition of sol-gel processed epitaxial Sm-doped BiFeO3 (001) thin films

BiFeO3, a room-temperature multiferroic material, has recently been increasingly applied as a potential lead-free piezoelectric material due to its large piezoelectricity achieved by doping. In this work, 12% Sm-doped BiFeO3 epitaxial thin films were fabricated on Nb-doped SrTiO3 (001) single crystal substrates via sol-gel method. The epitaxy was verified by reciprocal space mapping (RSM) and transmission electron microscope (TEM). The TEM results indicated the coexistence of R3c and Pbam phases in the film. The domains and piezoelectric properties from room temperature to 200 °C were characterized by piezoresponse force microscopy (PFM). Domains became active from 110 °C to 170 °C, and domain configurations changed obviously. A partially fading piezoresponse indicated the emergence of antiferroelectric Pbam. The in-situ domain analysis suggested that the phase transition was accompanied by domain wall motion. Switching spectroscopy PFM (SS-PFM) was further conducted to investigate the piezoresponse during the phase transition. Anomalous responses were found in both ON and OFF states at 170 °C, and the film exhibits typical antiferroelectric behavior at 200 °C, implying that the completion of phase transition and structure turned to the Pbam phase. This work revealed the origin of the high piezoresponse of Sm-doped BiFeO3 thin films at the morphotropic phase boundary (MPB).

Crystal structure transformation and improved dielectric and magnetic properties of La-substituted BiFeO3 multiferroics

Abstract Samples of Bi 1−x La x FeO 3 with x = 0.1, 0.3, 0.5, and 0.7 have been synthesized by two stage solid state reaction method. Structural characterization was performed using powder x-ray diffraction at room temperature. The crystal structure of perovskite phases are further characterized via Rietveld analysis which revealed a structural transition from R 3 c symmetry of the parent phase of BiFeO 3 to orthorhombic Pnma symmetry of LaFeO 3 . However the intermediate samples with x = 0.3 and 0.5 are bi – phasic (i.e. a combination of rhombohedral R 3 c and orthorhombic Pbam phases co-exist). Rietveld Refinement presents a good agreement between measured and simulated patterns. The transition from a rhombohedral to orthorhombic unit cell is suggested to be driven by the dilution of the stereochemistry of the lone pair of Bi 3+ at A- site. M-H hysteresis loops are recorded at room temperature up to a field of 15 kOe. The G-type antiferromagnetic spin structure and the magnetic moment are very sensitive to increasing La concentration at A-site. La substitution transformed antiferromagnetic BiFeO 3 into ferromagnetic which is closely related to the structural phase transition and modification of antiparallel spin structure. Dielectric constant (e′) and dissipation factor (tan δ) measured in frequency range 1 kHz to 5 MHz showed dispersion behaviour at low frequencies.

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.

Relaxations in Doped PZT and Epoxy-glue/Bi-Mn-O Composite

Effects of doping on ABO3 perovskite PZT compounds were studied. We determined effects of doping with Na ions (0.5–2.5 at.%) in A sublattice of Pb(Zr,Ti)O3 and (Pb,La)(Zr,Ti)O3 ceramics. The phase transition between ferro- and paraelectric phase was shifted, the dielectric losses decreased, and the piezoelectric coefficients, Kp and d33, were improved. The properties of bismuth manganite doped to PZT ceramics, (1-x)PZT-(x)BiMn2O5 (x = 0.055, 0.11), were studied. The effects of Bi doping in A sublattice and Mn doping in B sublattice were determined. The PZT-BiMn2O5 formed a composite of two solid solutions, one the ferroelectric rhombohedral R3m phase related to PZT, and the orthorhombic Pbam phase originated from bismuth manganite. Composition was estimated by using the scanning electron microscopy. Electrical impedance was measured in 100 Hz–1 MHz and 85–705 K ranges. Two types of the relaxations, manifested in the impedance spectra, were determined. One was attributed to charge transfer in B sublattice and/or polaronic mechanism. Second was fitted by using Vogel-Fulcher law that verified relaxor-like state for 0.89PZT-0.11BiMn2O5 compound. The epoxy-glue/bismuth-manganite composite was obtained. Vogel-Fulcher law confirmed glassy features for this material. Effective role of 0-3 connectivity in suppression of dielectric losses and transient currents was verified.