R3c Phase Research Articles

Structural, optical, dielectric and magnetic properties of Bi1−xLaxFeO3 nanoparticles

In this work, the structural, optical, dielectric and magnetic properties of Bi1−xLaxFeO3 nanoparticles (x = 0.50 and 0.75) have been investigated at room temperature. Structural analysis indicated the coexistence of orthorhombic (Pbnm) and rhombohedral (R3c) phases. UV–visible absorption spectra of the samples exhibit two doubly degenerate d-d transitions and three charge transfer transitions. Using diffused reflectance UV–Vis spectra, the direct and indirect band gaps of the samples decreases with increasing La content. The dielectric constant (ε) of the samples in the microwave region exhibited an anomalous phenomenon at room temperature. Magnetic hysteresis loops (M−H) of the samples exhibit weak ferromagnetism and an antiferromagnetic order. Additionally, the exchange bias effect was observed in these samples at room temperature. These results demonstrate that the method presented may be considered an effective way to improve the magnetic and optical properties of Bi1−xLaxFeO3.

Increase in depolarization temperature and improvement in ferroelectric properties by V5+ doping in lead-free 0.94(Na0.50Bi0.50)TiO3-0.06BaTiO3 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.

Structural transition and its effect on magnetoelectric coupling in the BiFe1−xMnxO3 ceramics prepared by sol–gel method

Significant perturbation on the local magnetic structure and related behavior in BiFeO3 is achieved through a substitution of Fe by Mn. The BiFe1−xMnxO3 ceramics were synthesized by sol–gel method and self-sustaining combustion. The crystal structure of BiFe1−xMnxO3 (0 ≤ x ≤ 0.3) changes from rhombohedral R3c to orthorhombic Pbnm forming a biphasic region at 0.05 ≤ x ≤ 0.15. It is found that the phase fraction change can effectively regulate the macroscopic magnetization and magnetoelectric coupling. The amount of the R3c phase reduces significantly with the Mn content increasing. The ceramic consisting of two phases with almost equal fractions shows a remnant magnetization and magnetoelectric coupling coefficient of 0.40 emu/g and 7.65 mV/(cm·Oe), respectively.

Structural, magnetic and ferroelectric properties of Sm and Mn co-substituted BiFeO3 ceramics with composition near the morphotropic phase boundary

The polycrystalline Sm and Mn co-substituted bismuth ferrites Bi1-xSmxFe0.95Mn0.05O3 (x = 0.08 and 0.16) with composition near the morphotropic phase boundary have been prepared using a modified solid-state reaction method. The Bi0.92Sm0.08Fe0.95Mn0.05O3 ceramic possesses single rhombohedral phase, whereas the Bi0.84Sm0.16Fe0.95Mn0.05O3 ceramic consists of both the R3c and Pnma phases. The addition of Sm and Mn not only enhances density and grain uniformity but also suppresses the spiral spin modulation which improves the polarization and magnetization. Excellent ferroelectric properties of microdomains are attained in the Bi0.92Sm0.08Fe0.95Mn0.05O3 ceramic. The transition of the nonpolar/antipolar phase to the polar phase and the polarization switching induced by the electric field occur in the Bi0.84Sm0.16Fe0.95Mn0.05O3 ceramic.

Stabilization of Ferromagnetism in BiFeO3:Ho at Hydrostatic Pressure

The isothermal magnetization of the Bi1 – xHo x FeO3 (x = 0−0.2) multiferroic has been studied at a hydrostatic pressure up to 9 GPa in the range of room temperatures. A new anomaly at PC ≈ 3.81 GPa related to intermediate phases between the structural transition R3c → Pnma has been found against the background of the pressure-induced antiferromagnetic ordering in BiFeO3 (BFO) at P ≈ 2.59 GPa. It is established that the ferromagnetic behavior under pressure depends on the Ho impurity concentration: PC decreases at 0.05 ≤ x ≤ 0.1 because of the decrease in R3c bond lengths in the structure, and the stabilization of ferromagnetism is implemented at 0.1 ≤ x ≤ 0.2 probably because of the coexistence of the R3c and Pnma phases. The results of studies indicate that, in Bi1 – xHo x FeO3 with x = 0.2, the transition pressure PC = 3.7 GPa exceeds the values for BFO doped with other 4f elements (Eu, Y, Sm) in the region R3c → Pnma of the transition.

Structural Transformation Pathways of Multiferroic BiFeO3 under High Pressures

High-pressure phase transitions in multiferroic BiFeO3 have been investigated by synchrotron radiation X-ray diffraction up to ∼54 GPa at room temperature. BiFeO3 exhibits a complex structural behavior under high pressures. Four phase transitions with the phase sequence R3c → OI and OIII → OI → Pnma → Pnm21 were observed at ∼3, ∼6, ∼12, and ∼45 GPa, respectively. Because of octahedral tilts and cation displacements under high pressures, BiFeO3 presents large unit cells and complex superstructures or domain structures between the stable R3c and Pnma phases. The OI and OIII phases have superstructure lattices 2apc×32bpc×cpcand2apc×22bpc×2cpc, respectively, and the OIII phase is isostructural with antiferroelectric PbZrO3 (space group: Pbam). The Pnma → Pnm21 transition at ∼45 GPa indicates the occurrence of a large unit cell and an increase of the structural distortion. The Pnm21 phase with a superstructure lattice 22apc×2bpc×2cpc has a group–subgroup relation with the Pnma phase.

Improved dielectric, magnetic and optical properties of Pr and Ti co-substituted BFO ceramics

Bi1-xPrxFe1-xTixO3 ceramics with x ≤ 0.20 were synthesized by solid state reaction method. XRD patterns confirm single phase formation for all the samples. Rietveld refinement of diffraction data reveals the structural transition from rhombohedral (R3c) phase for x ≤ 0.10 to orthorhombic (Pnma) phase for x ≥ 0.15. Temperature dependence dielectric plots exhibit two dielectric anomalies one at low temperature (∼250 °C) and other at high temperature (∼370 °C) corresponding to the magnetic transition temperature TN. Dielectric properties of samples were improved by controlling the amount of co doping to help ferroelectric measurements. Room temperature M − H measurements have shown an increase in magnetization with x which may be due to the partial destruction of spin cycloid of FeOFe network caused by Ti doping. FT-IR spectra exhibit a broad absorption band at 567 cm−1, whose broadness decreases with x. Absorption of light in the spectral range from 520 nm to 546 nm indicates an optical band gap in the visible range for these materials.

Temperature-Induced Reversible and Irreversible Transitions between Metastable Perovskite Phases in the BiFe1−yScyO3 Solid Solutions

The antipolar orthorhombic Pnma phase with the 2 a p × 4 a p × 2 2 a p superstructure (ap ~4 Å is the pseudocubic perovskite unit-cell parameter) is observed in many perovskite compositions derived from BiFeO3. Temperature-induced structural transformations in metastable perovskite solid solutions with the Pnma structure corresponding to the range of 0.30 ≤ y ≤ 0.60 of the (1−y)BiFeO3-yBiScO3 quasi binary system were studied using temperature X-ray and neutron powder diffraction. These compositions cannot be prepared in bulk form at ambient pressure but can be stabilized in the Pnma phase by means of quenching after synthesis under high pressure. The compositions were investigated in situ between 1.5 K and the temperature of the stability limit of their metastable phases (about 870–920 K). It has been found that heating the as-prepared compositions with the Pnma phase leads to formation of the rhombohedral R3c phase ( 2 a p × 2 a p × 2 3 a p ), which, on cooling down to room temperature, either remains or transforms into a polar orthorhombic Ima2 phase ( 2 a p × 2 a p × 2 a p ). The observed phase transformations in the BiFe1−yScyO3 perovskite series on heating and on cooling are considered in terms of geometrical factors.

Effect of the lanthanum concentration on the physical properties of the (Bi0.5Na0.5)0.92Ba0.08-3x/2LaxTiO3 ceramic system

The influence of lanthanum concentration on the structural, ferroelectric and dielectric properties of BNT–BT ceramic compounds with a composition close to the morphotropic phase boundary (Bi0.5Na0.5)0.92Ba0.08-3x/2LaxTiO3 (x = 0, 1, 2 and 3 at.%) has been studied. The structural properties reveal the coexistence of both ferroelectric (P4mm) and antiferroelectric (P4bm) tetragonal phases for the lanthanum-free system. Nonetheless, for the La3+ modified samples, both antiferroelectric tetragonal (P4bm) and ferroelectric rhombohedral (R3c) phases coexist. On the other hand, slim-like hysteresis loops for the x = 1 at.% La3+ concentration have been observed, suggesting a higher stability of the antiferroelectric phase. However, for the other compositions (including the lanthanum-free sample) the P–E hysteresis loops suggested a higher stability of the ferroelectric phase. The dielectric response has shown the presence of three anomalies in the entire analyzed temperature range, which have been associated to different phase transitions with an important influence of the lanthanum content.

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.

The giant strain response mechanism in textured Mn-modified 0.925(Bi0.5Na0.5)TiO3-0.075BaTiO3 relaxor ferroelectric ceramics

This work highlights structural and domain evolution, orbital hybridization, and giant strain in the textured [(Bi1/2Na1/2)0.925Ba0.075](Ti0.998Mn0.002)O3 (BNT-7.5BT-0.2%Mn) ceramics. The as-sintered textured specimens reveal a predominant tetragonal P4bm (fraction∼94.4%) phase with a minor rhombohedral R3c (fraction∼1.4%) phase. X-ray absorption spectra indicate reduced hybridization between the O 2p and the Na 3sp/Bi 6sp/Ti 3d orbitals, implying decreased atomic bonding due to Mn doping. A mixture of Mn2+, Mn3+, and Mn4+ valence states was identified in the ceramic matrix. In-situ electric (E)-field dependent XRD results show a reversible E-field-induced phase transition, suggesting an ergodic relaxor ferroelectric. An E-field-induced giant strain of ∼0.495% at room temperature is attributed to the oriented relaxor ferroelectric P4bm phase and decreased bonding strength, which facilitate a transition from relaxor to ferroelectric phase. The poled specimen at E = 60 kV/cm shows a structural transition sequence of tetragonal P4bm + rhombohedral R3c→∼220°C tetragonal P4bm→∼483°C cubic.

Phase evolution and electrical behaviour of samarium-substituted bismuth ferrite ceramics

Bi1-xSmxFeO3 (x = 0.15–0.18) ceramics with high density were produced using spark plasma sintering. The effects of composition, synthesis conditions and temperature on the phase evolution were studied, using XRD, TEM and dielectric spectroscopy. The coexistence of the ferroelectric R3c, antiferroelectric Pnam and paraelectric Pnma phases was revealed, with relative phase fractions affected by both calcination conditions and Sm concentration. Experiments on powdered samples calcined at different temperatures up to 950 °C suggest higher calcination temperatures promote Sm diffusion, allowing samples to reach compositional homogeneity. The structural transitions from the Pnam and R3c phases to the Pnma phase were comprehensively investigated, with phase transition temperatures clearly identified. The dielectric permittivity, electrical resistivity and breakdown strength were increased upon Sm-substitution, while ferroelectric switching was suppressed. The polarization-electric field loop became increasingly narrow with increasing Sm-content, but double hysteresis loops, which may reflect a reversible antiferroelectric to ferroelectric transformation, were not observed.

Low-voltage-driven Pt/BiFeO3/DyScO3/p-Si-based metal–ferroelectric–insulator–semiconductor device for non-volatile memory

In recent years, ferroelectric random access memory has drawn considerable attention as promising replacement to both dynamic random access memory and flash memory. Specifically in metal–ferroelectric–insulator–semiconductor (MFIS)-based structures, bi-stable polarization of ferroelectric gate even in absence of power holds the resistance state of semiconductor-drain channel between two logic states and offers additional features of non-destructive readout and non-volatile storage capability. However, insulating layer in such structure leads to high depolarizing field across FE layer and in turn high-voltage operation. In the present work, comprehensive performance of low-voltage-driven MFIS device, i.e., Pt (40 nm)/BiFeO3 (265 nm)/DyScO3 (6 nm)/Si is evaluated for gate voltage stress (± 2 to ± 9 V) at different thermal agitation (200–400 K). Fat capacitance–voltage (C–V) hysteresis centered at zero bias with large memory window (ΔV FB) of 1.9 V at low operating voltage of ± 5 V, and stable data retention with distinguishable ON/OFF state values specifies strong charge storage potential of MFIS device in extreme conditions of ± 100 K. X-ray diffraction revealed polycrystalline and rhombohedral R3c phase of BiFeO3 film and out-of-plane piezoresponse force microscopy analysis showed the ultrafast domain switching with sharp contrast. Complete 180° phase reversal in hysteresis loop and bufferfly-shaped piezo-actuation amplitude loop further confirmed the enhanced ferroelectric properties of BiFeO3 thin films. Nonlinear J–V curves of MFIS structure were investigated to understand the device reliability and charge transport mechanism. These encouraging results are crucial for designing more reliable integrated MFIS-based non-destructive readout non-volatile memory devices.

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).

Optical and magnetic properties of (1-x)BiFeO3-xCaTiO3 nanoparticles

We report the structural, Raman, optical and magnetic properties of (1−x)BiFeO3 -xCaTiO3 (BF-xCT) nanoparticles. BF-xCT nanoparticles were synthesized through the sol-gel route in the large compositional range of 0.1 ≤ x ≤ 0.6. X-ray diffraction investigation shows the rhombohedral (R3c) phase structure for 0.1 ≤ x ≤ 0.2 and the orthorhombic (Pbnm) phase structure for 0.3 ≤ x ≤ 0.6. The transmission electron microscope (TEM) investigation confirms the well crystalline structure with the average particle size of 42 and 40 nm for x = 0.2 and 0.4, respectively. The changes in phonons frequencies and linewidths in Raman spectra reveals CT induced rhombohedral (R3c) to the orthorhombic (Pbnm) phase transition for x ≥ 0.3. The optical results shows strong absorption of visible light with small optical band gap values from 1.96 eV to 2.55 eV for BF-xCT with 0.1 ≤ x ≤ 0.6, indicating the possibility of its application in ultrafast optoelectronic devices. For all BF-xCT nanoparticles, magnetization studies at room-temperature exhibit hysteretic behavior with remnant magnetization, Mr ≤ 0.112 emu/g, which indicates that the CT substitution in the BF matrix released the latent magnetization.

The impact of Yb and Co on structural, magnetic, electrical and photocatalytic behavior of nanocrystalline multiferroic BiFeO3 particles

The nanocrystalline particles of BiFO3,co-doped with ytterbium and cobalt Bi1−xYbxCoyFe1−yO3 (BYCFO) were fabricated via micro-emulsion method using cetyltrimethyl ammonium bromide (CTAB) as a surfactant. The dopant contents were kept x = 0.00, 0.025, 0.05, 0.075, 0.1, 0.125 and y = 0.00, 0.05, 0.1, 0.15, 0.2, 0.25 for six different compositions. The BYCFO-nanoparticles were investigated for structure, electric, dielectric, magnetic and photocatalytic properties. The X-ray diffraction (XRD) analysis confirmed the normal growth habitat Rhombohedral (R3c) phase on inclusion of ytterbium and cobalt ions as it is observed in undoped BiFeO3 multiferroic. However further doping of Yb+3 and Co+3 ions (x = 0.125, y = 0.25) resulted in some extra peaks, that is attributed to the formation of extra phase. The vibrating sample magnetometer recorded saturated magnetization (Ms) 0.385emu/g with remanent magnetization (Mr) 0.13emu/g for a coercivity of 405Oe which is higher than the undoped BiFeO3. The substitution of Yb+3 and Co+3 ions improved the canting angle and optimally aligned the sub lattices in ferromagnetic order. The grain size was found in the range of 20–24nm which is less than spiral spin modulation (≈ 62nm). The entire dielectric parameters showed smooth decreasing trend in low range frequency but at higher frequency (1.98GHz) resonance peaks appeared in the spectra. The insulating character was also enhanced up to 16.42 × 1010Ωcm−1, with increase in doping concentrations. The optical band gap was measured 1.5eV which was lower than the undoped BiFeO3 (2.15eV). The photocatalytic performance of co doped ceramic (Bi0.875Yb0.125Co0.25Fe0.75O3) was monitored using the textile dye Congo red. Approximately 28% of initial concentration (0.5ppm) was degraded by 200mg of ceramics in 42min under visible light irradiation. Being magnetic in nature, the Bi1−xYbxCoyFe1−yO3 particles could be separated by conventional magnetic bar.

Ab initio studies on phonon, dielectric, and piezoelectric responses in perovskite‐like bismuth aluminate

AbstractAb initio method was used to explore the phonon, dielectric, and piezoelectric responses mechanism of the R3c phase BiAlO3 with rhombohedral structure by the generalized gradient approximation (GGA) of density function theory (DFT) and density functional perturbation theory (DFPT) using the plane wave (PAW) method of VASP code. The lattice parameters calculated from the stability of the structure are consistent with the existing experimental data. The micromechanism of dielectric and piezoelectric response was explored. Through the analysis and research, it can be seen that the large values of dielectric element ε11 and piezoelectric element e26 are basically due to low‐frequency infrared (IR) active phonons (ωλ = 1.561 and 1.962 THz) with E irreducible representation.

A correlative study on strain and variation of coercive field in lead-free (Na0.5Bi0.5)TiO3–Bi(Mg0.5Zr0.5)O3–Bi(Mg0.5Ti0.5)O3 ternary system

Sodium bismuth titanate–bismuth magnesium zirconate–bismuth magnesium titanate (NBT–BMZ–BMT) lead-free ternary ceramics are prepared by solid-state reaction method. Consequence on structural, dielectric, ferroelectric properties and piezoelectric coefficient is investigated. Single phase rhombohedral R3c phase is retained in all poled samples. Coercive field decreases due to minimum homogeneous strain (δ) which facilitates domain reorientation and domain switching. Reduced homogeneous strain is the key factor which lowers the coercive field and enhances the dielectric constant. NBT–BMZ–BMT ternary ceramics reduces the coercive field and possesses high remnant polarization, dielectric constant and piezoelectric coefficient—the important requirement in functional devices.

First-principles examination of low tolerance factor perovskites

Perovskites are generally formed in the non-polar Pnma phase. The materials found in the polar subgroup of Pnma are not common. This article reports how perovskites having a low tolerance factor display ferroelectric instabilities in artificially constrained Pnma phase. The bond length distribution and the bond valence sum were analyzed to examine the origin of the ferroelectric behavior of low tolerance factor materials. However, the ground state structure of low tolerance perovskite is not Pnma phase, but either ilmenite (R 3¯) or lithium niobate (R3c) phase. Here, epitaxial strain was used as a tuning parameter to stabilize the polar subgroup of Pnma. The rationalized principles from these studies will be valuable for designing new ferroelectric materials in the future.

Crystal Structures of CaB3N3 at High Pressures.

Using global structure searches, we have explored the structural stability of CaB3N3, a compound analogous to CaC6, under pressure. There are two high-pressure phases with space groups R3c and Amm2 that were found to be stable between 29 and 42 GPa, and above 42 GPa, respectively. The two phases show different structural frameworks, analogous to graphitic CaC6. Phonon calculations confirm that both structures are also dynamically stable at high pressures. The electronic structure calculations show that the R3c phase is a semiconductor with a band gap of 2.21 eV and that the Amm2 phase is a semimetal. These findings help advance our understanding of the Ca-B-N ternary system.