R3c Symmetry Research Articles

Structural, dielectric, vibrational and magnetic properties of Sm doped BiFeO3 multiferroic ceramics prepared by a rapid liquid phase sintering method

Rare earth Sm substituted Bi1−xSmxFeO3 with x=0, 0.025, 0.05, 0.075 and 0.10 polycrystalline ceramics were synthesized by a rapid liquid phase sintering method. The effect of varying composition of Sm substitution on the structural, dielectric, vibrational, optical and magnetic properties of doped BiFeO3 (BFO) ceramics have been investigated. X-ray diffraction patterns of the synthesized rare earth substituted multiferroic ceramics showed the pure phase formation with distorted rhombohedral structure with space group R3c. Good agreement between the observed and calculated diffraction patterns of Sm doped BFO ceramics in Rietveld refinement analysis of the X-ray diffraction patterns and Raman spectroscopy also confirmed the distorted rhombohedral perovskite structure with R3c symmetry. Dielectric measurements showed improved dielectric properties and magnetoelectric coupling around Néel temperature in all the doped samples. FTIR analysis establishes O–Fe–O and Fe–O stretching vibrations in BiFeO3 and Sm-doped BiFeO3. Photoluminescence (PL) spectra showed visible range emissions in modified BiFeO3 ceramics. The magnetic hysteresis measurements at room temperature and 5K showed the increase in the magnetization with the increase in doping concentration of Sm which is due to the structural distortion and partial destruction of spin cycloid caused by Sm doping in BFO ceramics.

Enhanced magnetization with unusual low temperature magnetic ordering behaviour and spin reorientation in holmium-modified multiferroic BiFeO3 perovskite ceramics

Holmium-doped Bi1−xHoxFeO3 (x = 0, 0.025, 0.05, 0.075 and 0.10) perovskite ceramics were synthesized by a rapid liquid phase quenching process. The structural analysis performed using x-ray diffraction suggested phase formation with distorted rhombohedral structure in all the synthesized ceramic samples. Rietveld analysis of x-ray diffraction patterns and Raman spectroscopy also confirmed the distorted perovskite structure with R3c symmetry. Optical studies showed characteristic bending vibrations of O − Fe − O, Fe − O stretching and visible range PL emissions in modified BiFeO3 ceramics. Ferromagnetic characteristics were shown by all the holmium-doped samples at room temperature and 5 K. Very high saturation magnetization (at 7 T), four to six times higher at 5 K than at 300 K, is observed for holmium-doped ceramic samples. A complex temperature dependence of magnetization behaviour is observed for holmium-doped samples, which is indicative of a spin reorientation in doped ceramics.

The microstructure and ferroelectric property of Nd-doped multiferroic ceramics Bi0.85Nd0.15FeO3

The microstructures of Bi0.85Nd0.15FeO3 ceramics were investigated by using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), convergent-beam electron diffraction (CBED) and X-ray energy dispersive spectrometry (EDS). The superstructure phase related to 1/4(hh0)p and 1/4(00l)p diffraction spots were observed in the samples. It is found that the superstructure phase can co-exist with R3c phase in a single grain. The bright and dark field images accompanied with SAED patterns provided evidences of the superstructure phase dispersed into the matrix with R3c symmetry and evolution of the domain. The ferroelectric domain wall was observed according to the displacement of Fe3+ ions respect to Bi3+ sub-lattice in the ferroelectric R3c phase based on HRTEM observations. The space group Pnam of the superstructure was identified by combining SAED and CBED techniques. EDS measurement revealed that the concentration of Nd in the Pnam phase is higher than that in R3c phase. This might mean that the transition from R3c to Pnam structure is due to the inhomogeneous distribution of the Nd concentration and the weakened stereochemical activity of Bi3+ lone electron pair, arisen from the increase in Nd content.

Fast synthesis of rare-earth (Pr3+, Sm3+, Eu3+ and Gd3+) doped bismuth ferrite powders with enhanced magnetic properties

Rare-earth (Pr3+, Sm3+, Eu3+ and Gd3+) doped bismuth ferrite powders were synthesized for the first time by solution combustion method, which is a fast soft chemistry route for obtaining oxide powders. The materials were investigated by X-ray diffraction, Raman spectroscopy, as well as scanning and transmission electron microscopy. A distortion from rhombohedral R3c symmetry, specific to pure bismuth ferrite, to orthorhombic symmetry was observed for all doped samples. The SEM analysis of pure and doped bismuth ferrite powders showed the formation of sintered grains, with faceted cuboids-shaped particles with different size and lower average dimension in the case of doped samples. Magnetic properties were analyzed using SQUID magnetometry, M–H hysteresis loops being measured at 10K and 300K. All studied pure and doped bismuth ferrite samples presented high susceptibility values for high magnetic fields indicating strong antiferromagnetic interactions, whereas the behavior at low magnetic field demonstrates the existence of ferromagnetic coupling.Compared to BiFeO3, Bi0.9RE0.1FeO3 (RE=Pr, Sm, Eu and Gd) powders exhibit higher susceptibility, remanence and coercivity values, Bi0.9Eu0.1FeO3 sample displaying the highest remanence and coercivity at room temperature.

Pr and Gd co-doped bismuth ferrite thin films with enhanced multiferroic properties

Pr and Gd co-modified Bi 0⋅95−xPr x Gd 0⋅05FeO 3 (x = 0⋅00, 0⋅05, 0 ⋅10) (BPGFO) thin films on Pt(111) / Ti /SiO 2 /Si(100) substrates were prepared by a sol-gel together with spin coating technique. A detailed study of electrical and magnetic properties of these thin films is reported. X-ray diffraction analysis shows that, with an increase in Pr content, the crystal structures of BPGFO thin films retain rhombohedral (R3c) symmetry accompanied by structure distortion. Polarization-electric field hysteresis loops of these thin films demonstrate that the incorporation of Pr and Gd into the Bi site of BiFeO 3 thin film could enhance the ferroelectric performance. Compared to other thin films, the optimal ferroelectric behaviours in Bi 0⋅85Pr 0⋅1Gd 0⋅05FeO 3 thin film are ascribed to its large dielectric constant, low dissipation factor and low leakage current density. Room temperature magnetization-magnetic field curves of these thin films indicate that all the samples are of paramagnetic behaviours and the enhanced saturation magnetic properties can be found.

Room-temperature polar ferromagnet ScFeO3 transformed from a high-pressure orthorhombic perovskite phase.

Multiferroic materials have been the subject of intense study, but it remains a great challenge to synthesize those presenting both magnetic and ferroelectric polarizations at room temperature. In this work, we have successfully obtained LiNbO3-type ScFeO3, a metastable phase converted from the orthorhombic perovskite formed under 15 GPa at elevated temperatures. A combined structure analysis by synchrotron X-ray and neutron powder diffraction and high-angle annular dark-field scanning transmission electron microscopy imaging reveals that this compound adopts the polar R3c symmetry with a fully ordered arrangement of trivalent Sc and Fe ions, forming highly distorted ScO6 and FeO6 octahedra. The calculated spontaneous polarization along the hexagonal c-axis is as large as 100 μC/cm(2). The magnetic studies show that LiNbO3-type ScFeO3 is a weak ferromagnet with TN = 545 K due to a canted G-type antiferromagnetic ordering of Fe(3+) spins, representing the first example of LiNbO3-type oxides with magnetic ordering far above room temperature. A comparison of the present compound and rare-earth orthorhombic perovskites RFeO3 (R = La-Lu and Y), all of which possess the corner-shared FeO6 octahedral network, allows us to find a correlation between TN and the Fe-O-Fe bond angle, indicating that the A-site cation-size-dependent octahedral tilting dominates the magnetic transition through the Fe-O-Fe superexchange interaction. This work provides a general and versatile strategy to create materials in which ferroelectricity and ferromagnetism coexist at high temperatures.

XRD and Raman spectroscopy studies of (Bi1–xLaxFeO3)0.5(PbTiO3)0.5 solid solution

(Bi1–xLaxFeO3)0.5(PbTiO3)0.5 solid solution obtained by mechanochemical synthesis was characterized by X-ray diffraction (XRD), atomic force microscopy and Fourier transform near-infrared Raman spectroscopy. The single-phase solid solution with La content between x = 0 and 0.5 was prepared. The solid solution exhibits a tetragonal symmetry P4mm for 0 ≤ x ≤ 0.2. It was found that Raman spectra undergo a significant change for the solid solution with La content between x = 0.16 and 0.20. It may indicate the existence of a morphotropic boundary in this range of the La content. Detailed XRD study on (Bi0.5La0.5FeO3)0.5(PbTiO3)0.5 pointed to a rhombohedral symmetry R3c.

Electron spin resonance studies and improved magnetic properties of Gd substituted BiFeO3 ceramics

Polycrystalline Bi1−xGdxFeO3 (x=0.00–0.15) ceramics were prepared by solid-state reaction method. The samples preserve rhombohedral symmetry for x≤0.10 and coexistence of rhombohedral R3c and orthorhombic Pn21a symmetries for x>0.10. A significant change in the phonon frequencies of A1 modes and line widths in Raman spectra also confirms the structural transition from rhombohedral to orthorhombic for x=0.12 and 0.15. The improved magnetic behaviour indicates weak ferromagnetism in the samples with remnant magnetization (2Mr) of 0.145emu/g for x=0.15. An asymmetric shift in the coercive field indicates the presence of exchange bias effect in samples. Electron spin resonance spectra are of the ferromagnetic resonance type at higher dopant concentration and indicate the presence of ferromagnetically coupled clusters in the samples. Optical studies in the spectral range from 1.6–5.2eV were dominated by one doubly degenerate d–d transition and three charge transfer transitions. The optical band gap decreases from 2.25 to 2.16eV with increasing Gd substitution due to increased lattice distortion.

Electron spin resonance study and improved magnetic and dielectric properties of Gd–Ti co-substituted BiFeO3 ceramics

Polycrystalline BiFeO3 (BFO), Bi0.90Gd0.10FeO3 (BGF), Bi0.90Gd0.10Fe1−xTixO3 (x = 0.03–0.10; BGFTx) ceramics were prepared via solid state reaction method. X-ray diffraction studies reveal R3c symmetry for BFO and BGF samples and coexistence of R3c + Pn21a symmetries for BGFTx samples. The change in line width of Raman modes indicates the structural distortion and substitution of dopants ions in the BFO lattice. Magnetic studies show weak ferromagnetism in BGF and BGFTx samples as a result of Gd3+–Fe3+, Gd3+–Gd3+ interactions and imbalance created between two antiparallel Fe3+ spin sublattices by Ti substitution. The maximum remnant magnetization of 0.141 emu/g is observed for BGFTx=0.10 sample. Further, electron spin resonance study confirms the weak ferromagnetism of BGFTx samples, associated with small grains and increase in anisotropy of particles distribution as found during SEM studies. UV–Visible absorption spectra in the spectral range from 1.6 to 3.5 eV showed one d–d crystal field transition and two charge-transfer transitions with optical band gap variation in visible region. Improved dielectric properties with very low values of dielectric loss have been observed for BGF and BGFTx samples.

Symmetry and defects in rhombohedral single-crystallineNa0.5Bi0.5TiO3

Recent work has indicated that the symmetry of the lead-free piezoelectric perovskite Na0.5Bi0.5TiO3 can be changed from monoclinic to rhombohedral through the application of an electric field, which may have implications for the study and design of piezoelectric materials close to a morphotropic phase boundary. We have examined high-quality, single-crystal Na0.5Bi0.5TiO3 using transmission electron microscopy and have used digital electron diffraction to observe the symmetry of defect-free regions of material on length scales of a few nanometers. This unequivocally demonstrates that the material is rhombohedral with space group R3c on this length scale. We find that a model that allows disordered displacements of Bi atoms from their nominal sites in the R3c symmetry, while retaining this symmetry on average, gives a very significant improvement in fit to simulations. We use conventional transmission electron microscopy to enumerate the different types of defects that are observed in other regions of the crystal and find a complex microstructure of antiphase boundaries, domain walls, and tetragonal platelets. Their interaction leads to the formation of very high densities of nanotwins. We show that these are expected to have a variable monoclinic Cc symmetry that is driven by the constraint of continuity of the crystal across a domain wall.

Structural, dielectric and magnetic properties of La and Sc codoped BiFeO3

The La and Sc codoped ceramic Bi0·85La0·15Fe0·9Sc0·1O3 (BLFS) was prepared by a rapid liquid phase sintering method. For comparison, Bi0·85La0·15FeO3 (BLF) and BiFe0·9Sc0·1O3 (BFS) were also prepared using the same method. X-ray diffraction (XRD) patterns and Rietveld refinements indicate that samples crystallise in the rhombohedral structure with R3c symmetry, except for a small amount of secondary phase. The dielectric properties of the samples were affected by the codoping of La and Sc as well as the ferromagnetic property. The remarkable improved ferromagnetism of BLFS could be ascribed to the doping induced structural distortion and the destruction of the antimagnetic ordering.

Local structure of NaNbO3: A neutron scattering study

We report the results of a neutron diffraction study of structural evolution in sodium niobate, NaNbO${}_{3}$, which is the parent compound for lead-free ferroelectric materials, as a function of temperature from 15 to 930 K over six phases. The Rietveld analysis of the high-resolution powder neutron diffraction data shows the variation in the structure from cubic to rhombohedral ferroelectric structures. However, the refinements on local structure by the pair distribution function (PDF) method indicates that there are only three basic patterns of the local structure, and the ground states of NaNbO${}_{3}$ in the low-temperature antiferroelectric and ferroelectric phases have the R3c symmetry, even though in the long range the system shows the Pbcm symmetry or the coexistence of two phases. The origin of the complex phase behavior and its implications on the performance as lead-free ferroelectrics are discussed.

Fast aging treatment for the synthesis of hydrocalumites using microwaves

Friedel's salt [Ca2Al(OH)6]Cl·2H2O, commonly known as hydrocalumite, was synthesized by co-precipitation following different aging treatments, using microwave irradiation or conventional heating and by refluxing or in autoclave, with the aim of decreasing the aging times (24h) employed when aging by conventional heating or at room temperature. Hydrocalumites were characterized by XRPD, FT-IR, N2 physisorption, ICP-OES, TEM, TEM-electron diffraction and TGA techniques. The use of microwaves favors a faster crystalline growth in the stacking direction whereas the use of autoclave improves the lamellar crystallinity. The highest crystallinity was found for the sample aged with microwave irradiation by autoclaving at 453K for 1h. This sample presented the highest crystallite size in the stacking direction (76nm), the largest lamellar crystals (4000nm) with R3c symmetry, and the highest dehydration- and dehydroxylation temperatures.

A New Phase Boundary in (Bi1/2Na1/2)TiO3−BaTiO3 Revealed via a Novel Method of Electron Diffraction Analysis

AbstractA new phase boundary is revealed in (1–x)(Bi1/2Na1/2)TiO3−xBaTiO3, the most extensively studied lead‐free piezoelectric solid solution. This discovery results from a novel method of electron diffraction analysis, which allows the precise determination of oxygen octahedra tilting in multi‐domain perovskite ferroelectrics. The study using this method supports the recently proposed Cc symmetry for pure (Bi1/2Na1/2)TiO3, and, more importantly, indicates the crystal structure evolves into the R3c symmetry with the addition of BaTiO3, forming a Cc/R3c phase boundary at x = 3–4%. In the poling field Epol versus composition x phase diagram for polycrystalline ceramics, this phase boundary exists with Epol below 5.5 kV mm−1; the Cc phase is transformed to the R3c phase during poling at higher fields. The results reported here provide the microstructural origin for the previously unexplained strain behavior and clarify the low‐BaTiO3‐content phase relationship in this popular lead‐free piezoelectric system.

Structural, optical, and multiferroic properties of single phased BiFeO3

A soft chemical coprecipitation method has been proposed for synthesis of nano-sized multiferroic BiFeO3 (BFO) powders. The X-ray diffraction pattern confirms the perovskite structure of BFO and Rietveld refinement reveals the existence of rhombohedral R3c symmetry. Crystallite size and strain value are studied from Williamson–Hall (W–H) analysis. The transmission electron microscope (TEM) image shows that the particle size of BFO powders lies between 50–100 nm. 4A1 and 7E Raman modes have been observed in the range 100–650 cm−1 and a prominent band centered around 1150–1450 cm−1 have also been observed corresponding to the two-phonon scattering. Differential Thermal Analysis (DTA) shows the existence of two prominent peaks at 330 ∘C and 837 ∘C corresponding to the magnetic and ferroelectric ordering, respectively. From the temperature dependent dielectric studies, an anomaly in the dielectric constant is observed at the vicinity of Neel temperature (T N ) indicating a magnetic ordering. Also, BFO shows antiferromagnetic behavior measured from the magnetic studies.

Structural evolution and magnetization enhancement of Bi1−xTbxFeO3

The structural, vibrational, and magnetic properties of well prepared Bi1−xTbxFeO3 (x=0–0.2) powders are investigated by combining the X-ray diffraction, Raman scattering, and magnetometry measurements. It is found that partial substitution of Tb (10–12.5%) in Bi site results in a structural transformation from the rhombohedral R3c symmetry to the orthorhombic Pnma phase, accompanied by a ferroelectric–paraelectric phase transition. The magnetization of Bi1−xTbxFeO3 first gradually increases before reaching maximum at the polar–nonpolar phase boundary and then decreases for x≥0.125 until x=0.20. The enhancement of the remnant magnetization is attributed to the suppression of the space-modulated spin structure in the rhombohedral R3c phase. Meanwhile, the reduction of the remnant magnetization in the orthorhombic Pnma phase is suggested to result from the establishment of well parallel antiferromagnetic ordering.

Structural, magnetic, and optical properties of Pr and Zr codoped BiFeO3 multiferroic ceramics

Single-phase (Bi1-xPrx)(Fe1-xZrx)O3 ceramics with x = 0.03, 0.06, 0.10 (named BPFZ-3, BPFZ-6, and BPFZ-10, respectively) were synthesized by solid state reaction method. X-ray diffraction suggested the pure phase formation with distorted rhombohedral structure in all samples. Raman spectroscopy also confirmed the rhombohedrally distorted perovskite structure with R3c symmetry. The x-ray photoelectron spectroscopy measurements showed the absence of Fe2+ ions indicating the suppression of oxygen vacancies due to Pr and Zr codoping. Room temperature magnetic hysteresis loops revealed that the spontaneous magnetization increases due to structural distortion and partial destruction of spiral spin structure caused by the codoping in BiFeO3 ceramics. The maximum remnant magnetization of 0.1234 emu/g was observed for BPFZ-6 sample. Optical studies showed absorption of light in the visible region, with a red shift in energy band gap with increasing concentration of Pr and Zr in BiFeO3 ceramics. The broad absorption bands due to the overlapping of Fe-O and Bi-O groups’ vibrations were found in FTIR. Frequency independent dielectric constant and low dielectric loss at room temperature were observed for all samples.

BiGaO3-Based Perovskites: A Large Family of Polar Materials

Solid solutions of BiGaxM1–xO3 (M = Cr, Mn, and Fe) were prepared using the high-pressure high-temperature method at 6 GPa and 1700 K (M = Cr and Fe) and 1300 K (M = Mn). The formation of a large family of polar materials with R3c and Cm symmetries was found. Crystal structures were studied with laboratory X-ray powder diffraction: space group Cm, a = 5.3150(1) A, b = 5.2960(1) A, c = 4.6965(1) A, and β = 92.556(2)° for BiGa0.4Fe0.6O3; space group Cm, a = 5.2798(1) A, b = 5.2577(1) A, c = 4.6465(1) A, and β = 91.974(2)° for BiGa0.7Mn0.3O3; space group R3c, a = 5.51623(8) A and c = 13.61391(17) A for BiGa0.4Cr0.6O3. Samples with the Cm symmetry have square-pyramidal coordination of (Ga,M)3+ ions, and their structure is very similar with the structure of supertetragonal PbVO3, BiCoO3, and Bi2ZnTiO6 materials. Samples with the R3c symmetry are isostructural with BiFeO3 and have comparable calculated (based on the point-charge model) spontaneous polarization (58 μC/cm2 for BiGa0.4Cr0.6O3). The calculated pola...

Evidence of a medium-range ordered phase and mechanical instabilities in strontium under high pressure

We provided the first theoretical evidence for a medium-range ordered phase in high pressure strontium from the first-principles calculations. At the absolute zero temperature, the enthalpy–pressure relation shows that the bcc and hcp are energetically more favorable than the other experimentally observed phases between 24 and 27GPa. In the present work, we concentrate on the bcc phase because we found a link to a medium-range ordered phase. Our results reveal that the bcc phonon dispersion at the N and H points starts softening at around 24.1GPa. The ab initio molecular dynamics at 300K and 27GPa showed that the bcc is quickly transformed into a lower energy structure with R3c symmetry and distorted basis. The simulated diffraction patterns showed that the R3c structure has only a single major peak at low angle. The R3c peak locates near the first peak of the bcc structure. This is the evidence of the so-called medium-range ordered phase. This structure is a strong candidate for the unsolved S-phase reported by experiments.

On the phase identity and its thermal evolution of lead free (Bi1/2Na1/2)TiO3-6 mol% BaTiO3

Temperature-dependent dielectric permittivity of 0.94(Bi1/2Na1/2)TiO3-0.06BaTiO3 (BNT-6BT) lead-free piezoceramics was studied to disentangle the existing unclear issues over the crystallographic aspects and phase stability of the system. Application of existing phenomenological relaxor models enabled the relaxor contribution to the entire dielectric permittivity spectra to be deconvoluted. The deconvoluted data in comparison with the temperature-dependent dielectric permittivity of a classical perovskite relaxor, La-modified lead zirconate titanate, clearly suggest that BNT-6BT belongs to the same relaxor category, which was also confirmed by a comparative study on the temperature- dependent polarization hysteresis loops of both materials. Based on these results, we propose that the low-temperature dielectric anomaly does not involve any phase transition such as ferroelectric- to-antiferroelectric. Supported by transmission electron microscopy and X-ray diffraction experiments at ambient temperature, we propose that the commonly observed two dielectric anomalies are attributed to thermal evolution of ferroelectric polar nanoregions of R3c and P4bm symmetry, which coexist nearly throughout the entire temperature range and reversibly transform into each other with temperature.