Tolerance factor mismatch-induced phase coexistence in LaFe1−xNixO3: Mössbauer spectroscopy insights

Tuning ferroelectric domains and polarization properties of flexible BiFeO3 thin films by phase transition engineering

Sodium bismuth titanate (NBT) reveals a rhombohedral (R3c) phase at room temperature. Ferroelectricity reduces with the advent of a tetragonal (P4bm) phase at the depolarization temperature, Td ∼ 456 K. AC conductivity (σac) studies exposed a small-to-large polaron transition at Td. Barrier energy (WH) was ∼1.60 eV at T < Td for the small polarons in the R3c phase, which drastically reduced to ∼0.043 eV with the advent of the P4bm phase for the large polarons for T > Td. This is associated with the sharp rise in conductivity for T > Td. Ab initio calculations consider the electronic distortion due to oxygen vacancies, which creates trap states in the band structure. The energy gap (ΔE) between the trap states and the conduction band was ∼1.4 eV (R3c) and ∼0.2 eV (P4bm). These values are comparable to the experimental WH. The P4bm phase is more distorted than the R3c phase from charge density and structural distortion calculations. This indicates the formation of large polarons in the P4bm phase, compared to that of small polarons in R3c. The formation energy of the polaron (Epolaron) was calculated from the structural distortion and electron localization energies. The P4bm phase shows lower Epolaron (−0.26 eV) than R3c (−0.36 eV), indicating higher conductivity for the P4bm phase. NBT was chemically modified by adding BCZT to validate the small to large polaronic crossover at Td. This is discussed in light of σac measurements. WH decreased with BCZT incorporation, thereby increasing the conductivity. This is a consequence of the increased lattice distortion due to BCZT incorporation.

A combined powder x-ray and neutron diffraction studies on (1-x) NaNbO3-x Na0.50Bi0.50TiO3 solid solution

Pressure induced phase transition of La-substituted BiFeO3

Elucidating the Mn-Doping Induced Phase Transition from Orthorhombic Pnma to Rhombohedral R3c in LaFeO3: Insights from Mössbauer Spectroscopy

Control over relaxor, piezo-photocatalytic and energy storage properties in Na0.5Bi0.5TiO3 via processing methodologies

The present study was dedicated to the classical piezoelectric, lead-zirconate-titanate ceramic with composition Pb(Zr0.54Ti0.46)O3 at the Zr-rich side of the morphotropic phase boundary at which two phases co-exists. The pressure-induced changes in the phase fractions were studied by high-pressure neutron powder diffraction technique up to 3 GPa and 773 K. The two co-existing phases were rhombohedral R3c and monoclinic Cm at room temperature and R3c and P4mm above 1 GPa and 400 K. The experiments show that pressure favors the R3c phase over the Cm and P4mm phases, whereas at elevated temperatures entropy favours the P4mm phase. At 1 GPa pressure, the transition to the cubic Pm3¯m phase occurred at around 600 K. Pressure lowers the Cm→P4mm transition temperature. The Cm phase was found to continuously transform to the P4mm phase with increasing pressure, which is inline with the usual notion that the hydrostatic pressure favours higher symmetry structures. At the same time, the phase fraction of the R3c phase was increasing, implying discontinuous Cm→R3c phase transition. This is in clear contrast to the polarization rotation model according to which the Cm would link the tetragonal and rhombohedral phases by being a phase in which the polarization would, more or less continuously, rotate from the tetragonal polarization direction to the rhombohedral direction. Pressure induces large changes in phase fractions contributing to the extrinsic piezoelectricity. The changes are not entirely reversible, as was revealed by noting that after high-pressure experiments the amount of rhombohedral phase was larger than initially, suggesting that on the Zr-rich side of the phase boundary the monoclinic phase is metastable. An important contribution to the intrinsic piezoelectricity was revealed: a large displacement of the B cations (Zr and Ti) with respect to the oxygen anions is induced by pressure.

Despite wide studies of Na0.5Bi0.5TiO3, structure of this material and its connection with the observed physical properties still raise numerous questions due to mutually contradicting results obtained. Here, structure and dielectric properties of poled and unpoled Na0.5Bi0.5TiO3-CaTiO3 solid solutions are studied, projecting the obtained concentration dependence of structure and dielectric properties on pure Na0.5Bi0.5TiO3 as the end member of this material group. X-ray diffraction patterns for Na0.5Bi0.5TiO3-CaTiO3 solid solutions reveal dominating of an orthorhombic Pnma phase, even for the compositions approaching the end composition (Na0.5Bi0.5TiO3), whereas structure of pure Na0.5Bi0.5TiO3 can be considered, assuming coexistence of rhombohedral and orthorhombic phases. This allows one to avoid appearance of a large difference of rhombohedral distortions between the unpoled and poled Na0.5Bi0.5TiO3, if the rhombohedral distortion is calculated as for single R3c phase. Features of dielectric permittivity, corresponding to the observed structural phase transition, are identified. It is discussed that the rhombohedral R3c phase is responsible for appearance of the frequency-dependent shoulder of dielectric permittivity temperature dependence, characteristic for unpoled Na0.5Bi0.5TiO3.

Bi1−xSmxFe0.98Mn0.02O3 (x = 0, 0.02, 0.04, 0.06; named BSFMx) (BSFM) films were prepared by the sol-gel method on indium tin oxide (ITO)/glass substrate. The effects of different Sm content on the crystal structure, phase composition, oxygen vacancy content, ferroelectric property, dielectric property, leakage property, leakage mechanism, and aging property of the BSFM films were systematically analyzed. X-ray diffraction (XRD) and Raman spectral analyses revealed that the sample had both R3c and Pnma phases. Through additional XRD fitting of the films, the content of the two phases of the sample was analyzed in detail, and it was found that the Pnma phase in the BSFMx = 0 film had the lowest abundance. X-ray photoelectron spectroscopy (XPS) analysis showed that the BSFMx = 0.04 film had the lowest oxygen vacancy content, which was conducive to a decrease in leakage current density and an improvement in dielectric properties. The diffraction peak of (110) exhibited the maximum intensity when the doping amount was 4 mol%, and the minimum leakage current density and a large remanent polarization intensity were also observed at room temperature (2Pr = 91.859 μC/cm2). By doping Sm at an appropriate amount, the leakage property of the BSFM films was reduced, the dielectric property was improved, and the aging process was delayed. The performance changes in the BSFM films were further explained from different perspectives, such as phase composition and oxygen vacancy content.

The structural and vibrational properties of 5% Y-substituted BiFeO3 under pressure have been investigated using synchrotron x-ray diffraction (SXRD) and Raman scattering measurements. At a pressure below 30.3 GPa, distinct changes in the Raman spectra and SRXD pattern show evidence for one pressure-induced structural transition from the polar rhombohedral R3c phase to the nonpolar orthorhombic Pnma phase commencing at 3.6 and completed at 7.2 GPa, where there is a region of phase coexistence between the R3c and Pnma phases. At a higher pressure of 40.8 GPa, another phase transition from orthorhombic to cubic is observed accompanied by the insulator–metal transition. Our data do not suggest the pressure-induced re-entrance of ferroelectricity in the model multiferroic Bi0.95Fe0.05O3 in the pressure range studied.

Effects of Hf4+ substitute on the enhanced electrostrain properties of 0.7BiFeO3-0.3BaTiO3-based lead-free piezoelectric ceramics

A detailed study was carried out to investigate the effects of poling on structure, vibrational, dielectric, and ferroelectric properties of donor-doped (V5+ at Ti4+-site) lead-free Na0.47Bi0.47Ba0.06Ti(1-x)VxO3, (x = 0, 0.01, and 0.03) ceramics fabricated via a modified sol-gel method. Rietveld refinement of synchrotron radiation source powder x-ray diffraction data showed that unpoled samples are in rhombohedral R3c phase whereas poled samples showed a mix rhombohedral R3c and tetragonal P4mm phases at ambient temperature, due to a long-range order established in lattice system after poling. V+5 doping increases the rhombohedral distortion in unpoled and poled samples while it reduces the tetragonality in poled samples. Vibrational study revealed that unpoled samples have more lattice disorder compared to poled samples. X-ray absorption near edge spectroscopy measurement confirmed that Ti and V are in 4+ and 5+ oxidation states, respectively, for all poled and unpoled samples. The average grain size was found to decrease from 5.6 ± 0.5 μm for x = 0 to 1.0 ± 0.2 μm for x = 0.03. Depolarization temperature was found to increase significantly in poled samples from ∼104 °C for undoped sample to 150 °C for the sample with 1% vanadium substitution. Drastic improvements in ferroelectric and dielectric properties are explained in terms of structural changes. High remnant polarization Pr ∼ 31.4 μC/cm2 and moderately low coercive field Ec ∼ 20 kV/cm have been observed at an applied electric field of ∼35 kV/cm for the sample with 1% vanadium substitution which makes it an attractive candidate for ferroelectric applications.

Evolution of structure and electrical properties with lanthanum content in [(Bi1/2Na1/2)0.95Ba0.05]1−xLaxTiO3 ceramics

Enhanced energy storage properties in A-site substituted Na0.5Bi0.5TiO3 ceramics

Bismuth ferrite (BiFeO3) is a semiconductor with multiferroic properties, synthesized by the sol-gel method. While static high-pressure studies have advanced our understanding of the phase behavior of BiFeO3, the effects of dynamic pressure via acoustic shock waves remain unexplored. In this study, BiFeO3 was subjected to 100 shock pulses with 0.59 MPa pressure, 520 K temperature, and a Mach number of 1.5 to investigate its structural, optical, and morphological responses. X-Ray diffraction (XRD) analysis revealed a shock wave-induced phase transition from the rhombohedral distortive R3c phase to the rhombohedral non-distortive R3m phase. UV-Vis diffuse reflectance spectroscopy showed a significant reduction in the band gap from 2.58 eV to 2.05 eV, indicating enhanced optical absorption, which is crucial for optoelectronic applications. Scanning electron microscopy (SEM) demonstrated a morphological evolution from densely agglomerated to porous morphology due to dynamic recrystallization, which significantly enhances catalytic and sensor applications. The combination of phase transition, bandgap tunability, and morphological changes illustrates the dynamic pressure versatility response in BiFeO3, suggesting new avenues for its use in advanced materials and devices, including energy storage, sensors, and optoelectronics.