Design and characterization of novel manganite perovskites Ba1-xBixTi1-xMnxO3 (0≤x≤0.2)

Polycrystalline manganite powders of Ba_1-xBi_xTi_1-xMn_xO₃ (x = 0, x = 0.1 and x = 0.2) were synthesized by the conventional solid-state reaction process. Their crystal structure, morphological, optical, dielectric and electrical properties were investigated. X-ray diffraction of the prepared samples was made at room temperature and confirmed the formation of a perovskite phase. Structural refinement, using the Rietveld method, revealed a tetragonal P4mm phase of pure BTO and a tetragonal P4/mmm phase with the presence of vacancies for both doped samples (x = 0.1 and x = 0.2). Scanning electron microscopy indicated that the perovskite samples had a grain size smaller than 1 μm. From UV–vis–NIR spectra, we found that the band gap reduces from 3.29 eV to 1.48 eV with the increase of Bi and Mn amounts, resulting in a shift of the absorption wavelength region toward the visible range. Dielectric analysis was conducted in a wide range of temperatures at different frequencies. Phase transitions were identified from thermal dielectric results, showing that the samples exhibited a non-relaxor behavior. The structural transformation from tetragonal to cubic structure corresponding to the transition from ferroelectric phase to paraelectric phase was observed in the dielectric properties investigation. The complex impedance spectroscopy indicated the presence of grain and grain boundary effects in the conduction mechanism. Electrical analysis showed that doping with Bi and Mn enhanced the DC conductivity. Furthermore, the DC conductivity temperature dependence confirmed that the studied samples present a semiconductor behavior. The activation energies of grain and grain boundaries depended on the amount of incorporated Bi and Mn. The activation energy of grain varied between 0.54 and 0.87 eV suggesting that the DC electrical conductivity is governed by ionized oxygen vacancies. The activation energy of grain boundaries varied between 0.85 and 0.58 eV.     

Oxygen vacancy luminescence and band gap narrowing driven by Ce ion doping with variable valence in SnO2 nanocrystals

Although considerable research works have witnessed the important modulations of oxygen vacancies on the optical, electrical, and magnetic properties of SnO₂ nanostructures, it is not easy to control oxygen vacancy defects in such systems.The difficulty stems from that oxygen vacancy is a kind of atomic defect, and its distribution is sensitive to process conditions and external factors, which makes direct characterization and purposeful control difficult. The purpose of this work on Ce-doped SnO₂ nanocrystals is to investigate the tolerance of the host lattice to Ce ions, the population and evolution of Ce³⁺/Ce⁴⁺ ions, and the possibility to adjust oxygen vacancies by Ce³⁺ ions, and then focus on the influence of oxygen vacancy defects on the band gap and luminescence performance. As Ce doping concentration increases from 0 to 12 at.%, the doped system changes from Ce³⁺ dominated at low doping amount (≤3 at.%) to Ce³⁺/Ce⁴⁺ coexistence at medium doping concentration (3 at.% ∼ 9 at.%), to occurrence of CeO₂ impurity phase at over doping (∼12 at.%). The optimum doping occurs at 6 at.%, which corresponds to the saturated critical point of Ce³⁺ content and the maximum oxygen vacancy concentration. Importantly, the oxygen vacancies in the current Ce-doped SnO₂ nanocrystals is directly regulated by the Ce³⁺ ion concentration on the Sn sites, which plays an important role in the band gap tuning and visible light emission. With Ce concentration increasing from 0 to 12 at.%, the band gap monotonicity decreases from 3.36 eV to 3.12 eV, while the intensity of the oxygen vacancy luminescence band first increases and then decreases, with the turning point at 6 at.%. Both band gap narrowing effect and enhanced emission indicate that Ce-doped SnO₂ should be a promising method to design and manufacture visible light responsive SnO₂ based optoelectronic materials by manipulating oxygen vacancy defects.     

Enhancement of energy storage performance in lead-free relaxor ferroelectric ceramics via band structure engineering

Dielectric capacitors with excellent energy storage performance (ESP) are in great demand in the power electronics industry due to their high power density. For the dielectric materials, the dielectric breakdown strength (BDS) is the key factor to improve ESP, which is the focus and bottleneck of current research, especially in the relaxor ferroelectric (RFE) materials with already low residual polarization (P_r). Here, we stimulate the ESP of the BaTiO₃ (BT)-based RFE ceramics by band structure engineering. The Ta element is selected to enhance the band gap of doped ceramics, occupying Ti-site in supercell of BT and optimizing the bonds length of Ti-O bond to increase the energy band of Ti 3d states. In this way, the band gap of the doped ceramics is efficiently enhanced from 1.8 eV to 2.22 eV resulting in the large BDS. Prospectively, combined with the advantage of fine grain size, the highest recoverable energy storage density (W_rec) of 2.85 J/cm³ is obtained at 350 kV/cm and the ultra-high energy efficiency (η) of 95.26% is found at 200 kV/cm. Our work reveals the relationship between elements doping in B-site and band structure, being expected to benefit for designing energy storage materials.     

Low Temperature Ionic Conductivity of an Acceptor‐Doped Perovskite: II. Impedance of Single‐Crystal BaTiO3

Low temperature conductivity mechanisms were identified in acceptor‐doped BaTiO₃ single crystals equilibrated and quenched from high temperature under different oxygen partial pressures. A range of acceptor ionization states were quenched into samples doped with manganese or iron. Using an appropriate equivalent circuit to interpret impedance spectroscopy data, room temperature conductivity mechanisms in the single crystal samples were identified, and the permittivity/temperature dependence was also shown to be self‐consistent with the nature of a first‐order ferroelectric phase transition. The primary, low temperature, conduction mechanism in acceptor‐doped BaTiO₃ was determined to be dominated by the migration of oxygen vacancies. The activation energy for oxygen vacancy migration was experimentally determined to have a value of nearly 0.7 eV. This activation energy represents an intrinsic value for vacancy hopping and confirms our previous work that revealed minimal interaction between acceptor dopants and oxygen vacancies in BaTiO₃ in contrast to the well‐documented evidence of defect association in SrTiO₃.     

Ionic Transport Properties in Nanocrystalline Ce0.8A0.2O2-δ (with A = Eu,Gd, Dy,and Ho) Materials

The ionic transport properties of nanocrystalline 20 mol% Eu, Gd, Dy, and Ho doped cerias, with average grain size of around 14 nm were studied by correlating electrical, dielectric properties, and various dynamic parameters. Gd-doped nanocrystalline ceria shows higher value of conductivity (i.e., 1.8 × 10⁻⁴ S cm⁻¹ at 550°C) and a lower value of association energy of oxygen vacancies with trivalent dopants Gd³⁺ (i.e., 0.1 eV), compared to others. Mainly the lattice parameters and dielectric constants (ε_∞) are found to control the association energy of oxygen vacancies in these nanomaterials, which in turn resulted in the presence of grain and grain boundary conductivity in Gd- and Eu-doped cerias and only significant grain interior conductivity in Dy- and Ho-doped cerias.     

B-site acceptor doped AgNbO3 lead-free antiferroelectric ceramics: The role of dopant on microstructure and breakdown strength

B-site aliovalent modification of AgNbO₃ with a nominal composition of Ag(Nb_1-xM_x)O_3-x/2 (x = 0.01, M = Ti, Zr and Hf) was prepared. The effects of dopants on microstructure, dielectric, ferroelectric and conduction properties were investigated. The results indicate that the introduction of acceptor dopant does not lead to grain coarsening. Zr⁴⁺ and Hf⁴⁺ doping are beneficial to stabilize the antiferroelectric phase of AgNbO₃. Among all the samples, Ti⁴⁺ doped AgNbO₃ has the minimum resistivity while Hf⁴⁺ doped AgNbO₃ has the maximum resistivity, therefore, Hf⁴⁺ doped AgNbO₃ has high BDS. The XPS results indicate that the conduction behaviour is associated with the concentration of oxygen vacancies. This work hints that acceptor dopant is also effective on the microstructure control and chemical modification of AgNbO₃-based ceramics.     

Defect structure evolution and electrical properties of BaTiO3-based ferroelectric ceramics

Relaxor perovskite ferroelectric 0.1Bi(Zn_1/2Zr_1/2)O₃-0.9BaTiO₃(0.1BZZ-0.9BT) ceramics were successfully prepared, whose powders synthesized by the sol-gel process, with average grain size about 1.29 μm. 1.75 J/cm³ discharge energy density and good dielectric stability were obtained over a wide temperature range from 25°C to 140°C. The pulse discharge capability of 0.1BZZ-0.9BT ceramics was tested under different electric fields. The discharge time was 2.13 μs, which proved its ability to charge and discharge quickly. Complex impedance analysis and thermally stimulated depolarization current tests were applied to investigate the defect types and activation of 0.1BZZ-0.9BT ceramics. The evolution process of composite defects and oxygen vacancies profoundly affects the dielectric temperature stability of 0.1BZZ-0.9BT ceramics’ energy storage property.     

Al3+ doping induced changes of structural,morphology, photoluminescence,optical and electrical properties of SnO2 thin films as alternative TCO for optoelectronic applications

Highly transparent and conductive pure (SnO₂) and aluminum doped tin oxide (Al:SnO₂) thin films were deposited on glass substrates by the sol-gel spin-coating method. The structural, morphological, optical and electrical properties of the prepared thin films at different doping rates have been studied. X-ray diffraction results revealed that all the films were polycrystalline in nature with a tetragonal rutile structure. SEM images of the analyzed films showed a homogeneous surface morphology, composed of nanocrystalline grains. The EDS results confirmed the presence of Sn and O elements in pure SnO₂ and Sn, O, Al in doped SnO₂ thin films. The optical results revealed a high transmittance greater than 85% in the visible and near infrared and a band gap varying between 3.82 and 3.89 eV. PL spectra at room temperature showed that the most dominant defects correspond to oxygen vacancies. A low resistivity of order varying between 10^?3 and 10^?4 Ω cm and a high figure of merits ranging between 10^?3 and 10^?2 Ω^?1 in the visible range were obtained. The best performances were obtained for samples containing 2 at. % Al, which could be used as an alternative TCO layer for future optoelectronic devices.     

Bandgap modulation and magnetic switching in PbTiO3 ferroelectrics by transition elements doping

Transition metal (TM=Fe, Ni and Mn) ions doped PbTiO₃ perovskite ferroelectric ceramics prepared by a solid state reaction method have been studied by means of structural characterizations, optical and magnetic measurements. All the samples have pure tetragonal perovskite structure, but exhibit different grain shapes and sizes with the introduction of TM ions and oxygen vacancies. The observed structural changes arise from internal lattice strain, which is estimated by Williamson–Hall (W–H) analysis model. Moreover, TM ions doping plays simultaneously an important role on the energy band structure and magnetic orderings. The energy gap of PbTi_0.95TM_0.05O_3−δ shows a drastic decrease compared to that of PbTiO₃. Furthermore, PbTi_0.95TM_0.05O_3-δ materials possess multiple magnetism switching, in which diamagnetic–ferromagnetic transition and ferromagnetic–paramagnetic transition occur. In particular, the Fe-doped PbTiO₃ ceramic presents a typical ferromagnetic hysteresis, originating from the effective exchange coupling interaction between oxygen vacancies and Fe 3d spins.     

Synthesis,characterization, and visible-light photocatalytic activity of transition metals doped ZTO nanoparticles

Transition metals doping has been proved to be a feasible way for tuning the physical properties on the surface and bulk of nanomaterials and also for the good performance in decontamination of emerging pollutants. In this context, doped samples of zinc tin oxide or zinc stannate nanoparticles (ZTO NPs) by several transition metals were synthesized in order to enhance the optical absorbance with the aims of reducing the band gap and therefore ameliorated their photocatalytic activity. They were characterized by the X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy, Raman spectroscopy and photoluminescence. The XRD patterns and the microscopic observations showed the formation of spherical nanoparticles with an average size of about 30 nm and highly pure ZTO phase with an inverse spinel structure. The Raman spectra were dominated by bands relatives to the F2g (2) and A1g symmetries modes of inverse spinel structure. The band gap Eg is estimated to be 3.75 eV for the undoped sample, and 3.67, 3.64, 3.78 and 3.21 eV, for 2% Fe, 2% Mg, 2% Gd, and 2% Mn doped ZTO samples, respectively.Furthermore, the undoped ZTO NPs have the intrinsic problem of recombination of photogenerated charge carriers. We have shown that the reduction of the band gap and oxygen vacancies resulting from the doping effect could be a useful tool for trapping and avoid the recombination of electrons coming from photosensitized rhodamine B (RhB) under visible light irradiation. Owing to the structural advantages and low band gap, 2% Mn doped ZTO NPs, with the kinetic rate constants k of 0.024 min⁻¹, show enhanced performance for the elimination of RhB in aqueous solution compared to undoped and other doped ZTO NPs.     

Microstructure and Electrical Properties of Nonstoichiometric 0.94(Na0.5Bi0.5+x)TiO3–0.06BaTiO3 Lead‐Free Ceramics

0.94(Na_0.5Bi_0.5+x)TiO₃–0.06BaTiO₃ (x = ?0.04, 0, 0.02; named NB_0.46T‐6BT, NB_0.50T‐6BT, NB_0.52T‐6BT, respectively) lead‐free piezoelectric ceramics were prepared via the solid‐state reaction method. Effects of Bi³⁺ nonstoichiometry on microstructure, dielectric, ferroelectric, and piezoelectric properties were studied. All ceramics show typical X‐ray diffraction peaks of ABO₃ perovskite structure. The lattice parameters increase with the increase in the Bi³⁺ content. The electron probe microanalysis demonstrates that the excess Bi₂O₃ in the starting composition can compensate the Bi₂O₃ loss induced during sample processing. The size and shape of grains are closely related to the Bi³⁺ content. For the unpoled NB_0.50T‐6BT and NB_0.52T‐6BT, there are two dielectric anomalies in the dielectric constant–temperature curves. The unpoled NB_0.46T‐6BT shows one dielectric anomaly accompanied by high dielectric constant and dielectric loss at low frequencies. After poling, a new dielectric anomaly appears around depolarization temperature (T_d) for all ceramics and the T_d values increase with the Bi³⁺ amount decreasing from excess to deficiency. The diffuse phase transition character was studied via the Curie–Weiss law and modified Curie–Weiss law. The activation energy values obtained via the impedance analysis are 0.69, 1.05, and 1.16 eV for NB_0.46T‐6BT, NB_0.50T‐6BT and NB_0.52T‐6BT, respectively, implying the change in oxygen vacancy concentration in the ceramics. The piezoelectric constant, polarization, and coercive field of the ceramics change with the variation in the Bi³⁺ content. The Rayleigh analysis suggests that the change in electrical properties of the ceramics with the variation in the Bi³⁺ amount is related to the effect of oxygen vacancies.     

Effect of dysprosium doping on structural and vibrational properties of lead-free (Na0.7K0.3)0.5Bi0.5TiO3 ferroelectric ceramics

This study describes the effect of Dy³⁺ doping of lead-free ferroelectric sodium potassium bismuth titanate ((Na_0.7K_0.3)_0.5Bi_0.5TiO₃; NKBT) ceramics on their structural, optical, and vibrational properties. X-ray diffraction and Rietveld refinement analyses revealed that NKBT samples exhibited a rhombohedral perovskite structure belonging to the R3c space group, with the incorporation of Dy³⁺ resulting in structural parameter change and local atomic structure shift to tetragonal P4mm, as supported by the appearance of new Raman modes. Increased Dy³⁺ content decreased the NKBT band gap from 3.4 to 3.3 eV, which was ascribed to the introduction of Dy 4f levels between valence and conduction bands. Ferroelectric behavior studies combined with transport measurements explained the unexpected ferroelectric response weakening of Dy³⁺-modified NKBT by increased leakage current densities.     

The effect of grain boundary on the visible light absorption of BaTi1-x[Ni1/2Nb1/2]xO3-δ ferroelectric ceramics

(Ni²⁺, Nb⁵⁺) co-doped BaTiO₃ ceramics BaTi_1-x(Ni_1/2Nb_1/2)_xO_3-δ (BTNN-100x) are prepared by a solid-state reaction method, and their electrical and light absorption properties are investigated. The results show that BTNN-100x can generate oxygen vacancies which pin the domain walls. BTNN-100x bulk ceramics show strong visible light absorption. However, the phenomenon of visible light absorption disappear for BTNN-100x ceramic powders, and the band gap of doped ceramic powders are nearly unchanged. The experiments demonstrate that stress and density have little effect on the band gap. And the grain boundary shows stronger cathodoluminescence (CL) emission. Actually, oxygen vacancies can be enriched at grain boundaries, and defect [V_o-Ni_Ti-V_o] complex structures can be form and give rise to the visible light absorption as demonstrated by First-principles calculations. Thus, the engineering design of ferroelectric grain boundaries may pave the way for the application of coupled ferroelectric-photovoltaic processes.     

Band gap narrowing and magnetic properties of transition-metal-doped Ba0.85Ca0.15Ti0.9Zr0.1O3 lead-free ceramics

Transition metal (TM = Mn, Fe, Co, and Ni)-doped Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ (BCZT) lead-free ceramics with excellent optical and magnetic properties are synthesized via a solid-state reaction method. The effects of TM elements on the sintering, structure, optical, and magnetic properties of BCZT ceramics are investigated in detail. Structural phase transition from the coexistence of rhombohedral and tetragonal phases to a single rhombohedral phase is observed owing to grain refinement. A narrow band gap of 1.68 eV is achieved in the Co-doped BCZT. The optical absorption of TM-doped BCZT is enhanced, which is ascribed to the molecular orbital rearrangement caused by lattice distortion. Moreover the magnetic behaviors of TM-doped BCZT are observed. The Fe-doped BCZT presents the most evident ferromagnetism, resulting from the exchange coupling interaction between the Fe³⁺ ions and oxygen vacancies. These results provide additional insight into the use of TM-doped BCZT lead-free ferroelectric ceramics for various applications.     

Development and characterization of (1-x)BiYO3-xBiMnO3 ceramics for Ferro-photovoltaic applications

The rare combination of ferroelectricity, low band gap and high carrier mobility in a material can facilitate exceptional photovoltaic efficiency. In this paper, we attempted to reduce the band gap and improve the conductivity of ferroelectric solid solution BiY_(1-x)Mn_xO₃ by varying composition. The BiY_(1-x)Mn_xO₃ (x = 0.0, 0.10, 0.25, 0.50, 0.75) samples were prepared by high energy ball milling method and structural, dielectric, optical, electrical and ferroelectric properties were analyzed systematically. Herein, we show that band gap of the material is drastically reduced from 3.0 eV (BiYO₃) to 1.76 eV (x = 0.50) and the samples exhibit ferroelectric behavior with significant polarization. Compound resistance of grain and grain boundaries were also found to reduce with increase in Mn concentration in the BiY_(1-x)Mn_xO₃ solid solution, demonstrating improvement in semiconducting behaviour of the material. Thus, solid solution formation of BiMnO₃ with BiYO₃, tunes band gap in useful region, improves electrical conduction and enhances ferroelectric polarization showing good potential for high performance solar cell and thermoelectric applications.     

Ferroelectric,dielectric, and impedance properties of Sm-modified Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 ceramics

To investigate the effect of Sm doping on the electrical properties of Ba(Zr_0.2Ti_0.8)O₃-x(Ba_0.7Ca_0.3)TiO₃ (BZT-xBCT) (x = 40, 50, 60) ceramics, three Sm-modified ceramics were prepared using the conventional solid-state reaction method. Related electrical measurements, including ferroelectric and dielectric investigations and impedance spectroscopy, were recorded for these ceramics. It was found that a tilted morphotropic phase boundary resulted from the addition of Sm, which induced the best piezoelectric properties and insulating behaviour in the Sm-BZT-60BCT sample. An abnormal P-E loop shrinkage appeared in the Sm-BZT-50BCT sample but not in the other two samples. This could be attributable to the different electronegativities between Ca²⁺ and Ba²⁺ and between Zr⁴⁺ and Ti⁴⁺, whose contents are different in varied samples and have an effect on defect-dipole alignment as well as spontaneous polarization. The activation energies for the bulk conductivity in the three composites were calculated to be 0.28 ± 0.01, 0.08 ± 0.01, and 0.36 ± 0.01 eV, confirming the existence of oxygen vacancies in our samples. The Sm dopant is responsible for the oxygen vacancies. This also leads to an increased Curie temperature in the three composites.     

Magnetic characteristics and optical band alignments of rare earth (Sm+3, Nd+3) doped garnet ferrite nanoparticles (NPs)

Yttrium iron garnet (YIG) nanoparticles (NPs) doped with rare earth (RE) metal ions (Y_2.5Sm_0.5Fe₅O₁₂, Y_2.5Nd_0.5Fe₅O₁₂) were successfully synthesized by sol-gel auto combustion approach. The cubic crystalline structure and morphology of the prepared garnet ferrite NPs were analyzed by X-ray diffractometer (XRD) and field emission scanning electron microscopy (FESEM). The cubic crystalline garnet phase of the synthesized YIG, Sm-YIG and Nd-YIG samples was successfully achieved at 950 °C sintering temperature. The force constant and absorption bands were estimated by using Fourier transform infrared spectroscopy (FTIR). The doping effect of RE metal ions on the chemical states of YIG were examined by x-ray photoelectron microscopy (XPS). The valence band (from 12.63 eV to 13.22 eV), conduction band (from 10.89 eV to 11.34 eV) edges and optical bandgap values of RE doped YIG samples were calculated using UV–Vis spectroscopy and ultraviolet photo electron spectroscopy (UPS). The magnetic analysis of the prepared NPs was studied using vibrating sample magnetometer (VSM). The XPS analysis of RE doped YIG samples exhibit the existence of RE (Sm⁺³, Nd⁺³) contents on the surface of YIG ferrite by decreasing the oxygen lattice in garnet structure. The optical bandgap (from 1.74 eV to 1.88 eV) explains the semiconducting nature of the synthesized NPs. The UPS results confirm the valence band position of YIG doped samples. The saturation magnetization and remanence of RE doped garnet ferrite samples increased from 13.45 to 18.83 emu/g and 4.06–6.53 emu/g, respectively.     

Role of A-site (Sr), B-site (Y), and A,B sites (Sr,Y) substitution in lead-free BaTiO3 ceramic compounds: Structural,optical, microstructure,mechanical, and thermal conductivity properties

Strontium and Yttrium-doped and co-doped BaTiO₃ (BT) ceramics with the stoichiometric formulas BaTiO₃, B_1-xSr_xTiO₃, Ba_1-xY_xTiO₃, BaTi_1-xY_xO₃, Ba_1-xY_xTi_1-xY_xO₃, and Ba_1-xSr_xTi_1-xY_xO₃ (x = 0.075) noted as BT, BSrT, BYT, BTY, BYTY, and BSrTY have been synthesized through sol-gel method. X-ray diffraction (XRD) patterns of the prepared ceramics, calcined at a slightly low temperature (950 °C/3h), displayed that BT, BSrT, and BYT ceramics possess tetragonal structures and BTY, BYTY, and BSrTY have a cubic structure. The incorporation of the Ba and/or Ti sites by Sr²⁺ and Y³⁺ ions in the lattice of BaTiO₃ ceramic and the behaviors of the crystalline characteristics in terms of the Y and Sr dopant were described in detail. The scanning electron microscopy (SEM) images demonstrated that the densification and grain size were strongly related to Sr and Y elements. UV–visible spectroscopy was used to study the optical behavior of the as-prepared ceramic samples and revealed that Sr and Y dopants reduce the optical band gap energy to 2.74 eV for the BSrTY compound. The outcomes also demonstrated that the levels of Urbach energy are indicative of the created disorder following the inclusion of Yttrium. The measurements of the thermal conductivity indicated the influence of the doping mechanism on the thermal conductivity results of the synthesized samples. Indeed, the thermal conductivity of BaTiO₃ is decreased with Sr and Y dopants and found to be in the range of 085–2.23 W.m^-1. K^?1 at room temperature and decreases slightly with increasing temperature from 2.02 to 0.73-W.m^-1. K^?1. Moreover, the microstructure and grains distribution of the BT, BSrT, BYT, BTY, BYTY, and BSrTY samples impacted the compressive strength, hence; the compressive strength was minimized as the grain size decreased.     

Structural,Raman, and Dielectric Studies on Multiferroic Mn‐doped Bi1−xLaxFeO3 Ceramics

Multiferroic Bi_1?xLa_xFeO₃ [BLFO (x)] ceramics with x = 0.10–0.50 and Mn‐doped BLFO (x = 0.30) ceramics with different doping contents (0.1–1.0 mol%) were prepared by solid‐state reaction method. They were crystallized in a perovskite phase with rhombohedral symmetry. In the BLFO (x) system, a composition (x)‐driven structural transformation (R3c→C222) was observed at x = 0.30. The formation of Bi₂Fe₄O₉ impure phase was effectively suppressed with increasing the x value, and the rhombohedral distortion in the BLFO ceramics was decreased, leading to some Raman active modes disappeared. A significant red frequency shift (~13 cm^?1) of the Raman mode of 232 cm^?1 in the BLFO ceramics was observed, which strongly perceived a significant destabilization in the octahedral oxygen chains, and in turn affected the local FeO₆ octahedral environment. In the Mn‐doped BLFO (x = 0.30) ceramics, the intensity of the Raman mode near 628 cm^?1 was increased with increasing the Mn‐doping content, which was resulted from an enhanced local Jahn–Teller distortions of the (Mn,Fe)O₆ octahedra. Electron microscopy images revealed some changes in the ceramic grain sizes and their morphologies in the Mn‐doped samples at different contents. Wedge‐shaped 71° ferroelectric domains with domain walls lying on the {110} planes were observed in the BLFO (x = 0.30) ceramics, whereas in the 1.0 mol% Mn‐doped BLFO (x = 0.30) samples, 71° ferroelectric domains exhibited a parallel band‐shaped morphology with average domain width of 95 nm. Dielectric studies revealed that high dielectric loss of the BLFO (x = 0.30) ceramics was drastically reduced from 0.8 to 0.01 (measured @ 10⁴ Hz) via 1.0 mol% Mn‐doping. The underlying mechanisms can be understood by a charge disproportion between the Mn⁴⁺ and Fe²⁺ in the Mn‐doped samples, where a reaction of Mn⁴⁺ + Fe²⁺→Mn³⁺ + Fe³⁺ is taken place, resulting in the reduction in the oxygen vacancies and a suppression of the electron hopping from Fe³⁺ to Fe²⁺ ions effectively.     

Low‐Temperature Ionic Conductivity of an Acceptor‐Doped Perovskite: I. Impedance of Single‐Crystal SrTiO3

Low‐temperature conductivity mechanisms were identified in acceptor‐doped SrTiO₃ single crystals quenched from high temperatures under reducing conditions. Impedance spectroscopy measurements made on samples of the prototypical perovskite structure doped with iron provided a framework for creating a complete conductivity model for a well‐defined point defect system. The dominant conductivity mechanism in the room‐temperature range was identified as being controlled by oxygen vacancy hopping. The activation energy for oxygen vacancy migration, an often debated value in the perovskite community, is determined to lie within the range of 0.59–0.78 eV for the iron‐doped system with the bottom of this range approaching the intrinsic value for oxygen vacancy hopping in an undoped single crystal. At low temperatures, oxygen vacancies form defect complexes with iron impurities, and the observed range of activation energies is explained and modeled in terms of an oxygen vacancy trapping mechanism.