Rhombohedral Phase Research Articles

Structural evaluation in vicinity of composition induced non-ergodic to ergodic crossover in niobium doped (Na0.41K0.09Bi0.5)TiO3

A structural crossover from non-ergodic to ergodic phase in niobium doped (Na0.41K0.09Bi0.5)TiO3, i.e., (Na0.41K0.09Bi0.50)1−x/2(Ti1−xNbx)O3 was investigated using high resolution x-ray diffraction and Raman spectroscopy. The diffraction studies reveal the co-existence of cubic (Pm3m), rhombohedral (R3c), and tetragonal (P4bm) phases for lower concentrations of niobium, i.e., for x ≤ 0.005 and the emergence of the pseudo-cubic (Pm3m) structure for higher niobium contents (for x ≥ 0.0075). On the other hand, Raman spectroscopy reveals the augmentation of the tetragonal phase with niobium content. These observations advocate the crossover from the non-ergodic to ergodic phase with niobium content. The presence of the pseudo-cubic phase (Pm3m), especially for x ≥ 0.0075, does not indicate the existence of the long-range cubic phase; rather, it models the contribution of nano-regions with the tetragonal symmetry as indicated in the Raman measurements. It further suggests that, with the incorporation of niobium, the size polar nano-regions diminish that leads to the crossover from the non-ergodic to ergodic phase.

DFT studies of BaTiO3

The structure of stable phases is investigated using first-principle calculations based on the functionals: LDA, GGA and newly developed general-purpose heavily constrained and appropriately normalized meta GGAfunctional (SCAN). The purpose of this study is to theoretically study the atomic displacements and band gap of the cubic, tetragonal, orthorhombic, rhombohedral perovskite phases for the comparison LDA, GGA-PBE and meta-GGA functionals using the density functional theory method. The obtained data of the density of states (DOS) showed that the values of the band gap energies are underestimated, and the DOS values show that the upper part (valence band) mainly consists of O 2p orbitals, the lower part (conduction band) consists of Ti 3d orbitals. The rhombohedral phase has a mixed composition of Ti states in the conduction band with a greater degree of 3dz2 than 3dxy. The values of the energies of the band gap (Egap) and the density of states show reasonable agreement with experimental and theoretical data. The LDA functional and, to a lesser extent, the GGA - PBE functional can also provide fairly accurate information about atomic displacements in these crystals. The values calculated by the SCAN functional do not differ much from the GGA and LDA functionals.

Fabrication of mesoporous Cr2O3 with highly distributed α-Fe2O3 and Pt nanoparticles for ultrafast H2 evolution rate

In the present contribution, p-n type heterojunction α-Fe2O3/Cr2O3 S-scheme system photocatalyst has been fabricated utilizing a sol-gel approach with assisted nonionic surfactant for a highly effective H2 evolution rate under visible illumination. Pt NPs have been reduced by photodeposition during the photocatalytic reaction to collect Pt@α-Fe2O3/Cr2O3 finally. XRD analysis of Fe2O3/Cr2O3 nanocomposites verified the construction of Fe2O3 and Cr2O3 with rhombohedral phases. TEM images of Cr2O3 NPs were almost spherical and uniform in shape and size (20 ± 5 nm), and very small Fe2O3 NPs (3-5 nm) were distributed on the mesoporous Cr2O3 networks. The obtained α-Fe2O3/Cr2O3 photocatalyst exhibited noteworthy photocatalytic H2 evolution with high efficiency and stability for 45 h. Interestingly, the photocatalytic H2 evolution rate gradually boosted with the extent of Fe2O3 percentage up to 15% and its rate of 2215.4 μmol g-1h-1, which was fostered 7.25 folds larger than that of Cr2O3 NPs (305.7 μmol g-1h-1). The enhancement H2 evolution rate of Fe2O3/Cr2O3 photocatalyst in comparison with bare Cr2O3 NPs was ascribed to facilitate the separation of photocarriers and existing considerable reactive sites. In addition, constructing n-type Fe2O3 and p-type Cr2O3 with close contact is essential in improving the H2 evolution rate. The possible photocatalytic mechanism over Fe2O3/Cr2O3 nanocomposite was addressed based on electrochemical measurements. The construction of the S-scheme system of Fe2O3/Cr2O3 nanocomposite could be suggested to improve the separation of photocarriers through optimal transfer channels owing to the formation of synergistic characteristics. Our results provide avenues for constructing stable photocatalysts with high efficiency for H2 evolution through visible exposure.

Decoding the Atomic-Scale Structural Origin of Large Strain Performance in BNT-6BT Based Relaxor Ferroelectrics.

The relationship between matter properties and their atomic-scale structures is a challenging investigation. For relaxor ferroelectrics, correlating the relaxor mechanisms on the atomic scale to properties is still ambiguous. Here, the correlation between the atomic-scale structure and strain performance of 0.94 Bi0.5Na0.5TiO3-0.06BaTiO3 (94BNT-6BT) and 0.93 Bi0.5Na0.5TiO3-0.06BaTiO3-0.01BaZrO3 (93BNT-6BT-1BZ) is reported. The δTi-Bi/Na displacement vector map based on the annular dark field (ADF) scanning transmission electron microscopy (STEM) image demonstrates the coexistence of tetragonal (T) and rhombohedral (R) phases of the resulting ceramics, and BZ doping increases the proportion of the T phase. Furthermore, the enhanced annular bright field (eABF) STEM image demonstrates that BZ doped ceramics exhibit obvious oxygen octahedral tilt. The oxygen octahedral tilt increased gradually from the domain wall to the inner place of the nanodomain, indicating a regional consistency, which results in enhancement of the relaxor performance and stain property. This study opens exciting opportunities to the design of relaxor ferroelectrics with large strain for high-displacement actuator applications.

The impact of oxygen partial pressure in modifying energy storage property of lanthanum doped multiferroic bismuth ferrite thin films deposited via pulsed laser deposition

Lead-free dielectric thin films' potential as pulse capacitors in power electronic devices has garnered attention to their energy storage usage. The current study aims at the growth of multiferroic thin films using the pulsed laser deposition technique for energy storage capacitor applications. Under various OPPs, single-phase La-doped bismuth ferrite (Bi0.993La0.007FeO3) BLFO thin film of 200 nm thickness is deposited on quartz and n-type Si substrates (15 to 30 SCCM). The crystal planes (110) and (006) of the rhombohedral phase with space group R3c are confirmed by high-resolution HRTEM and SAED patterns that augmented the XRD analysis. The optimized films have a smooth, uniform microstructure with 3 nm root-mean-square (RMS) roughness. The film shows an enhanced dielectric constant of 447 at room temperature with an improved ferroelectric property. At 488 kV/cm, the 20 SCCM BLFO films achieved 78.42 % energy efficiency and 2.09 J/cm3 energy density. The same films also exhibited a lower value of bandgap energy, confirming the strong absorption at 452 nm wavelength. The relatively high refractive index of 2.78 is determined for the same film suggesting the higher crystallinity of the film. Further, the low magnitude of leakage current density of the grown film is ∼10−7 A/cm2 at ±100 kV/cm. Though achieving the pure phase and fabricating BLFO thin films is challenging, these results are appropriate for the thin films to realize as energy storage capacitor applications.

Enhanced electro-strain with high electrostrictive performances of Bi0.5Na0.4K0.1TiO3 based lead-free piezoelectric ceramics by Ba(Fe0.5Nb0.5)O3

Complex piezoelectric ceramics of (1-x)[0.97(Bi0.5Na0.4K0.1)TiO3–0.03(Ba0.7Sr0.3)TiO3]-xBa(Fe0.5Nb0.5)O3 or (1-x)[0.97BNKT-0.03BST]-xBFN were investigated, where the x value was between 0 and 0.03. All compositions were synthesized via a conventional mixed oxide method. X-ray diffraction and Raman spectroscopy techniques indicated that all ceramics exhibited a coexistence of rhombohedral (R) and tetragonal (T) phases without any impurity phase. However, the T phase became dominant at higher BFN contents. The maximum temperature (Tm), the ferroelectric to ergodic relaxor phase transition temperature (TF-R), and Burn’s temperature (TB) decreased with increasing BFN content, while the dielectric constant at Tm was enhanced for the x = 0.01 ceramic. The BFN additive interrupted ferroelectric ordering, which led to constriction in the hysteresis loops as well as large electric field-induced strains (Smax = 0.42%), a normalized strain coefficient (d*33 = Smax/Emax = 846 pm/V), and an electrostrictive coefficient (Q33 = 0.0432 m4/C2) for the x = 0.01 ceramic under a moderate applied electric field of 50 kV/cm. These results indicated that this ceramic had the potential for electromechanical applications.

Probing the incoherent admixture of low and high-spin states of Co in (LaPr)CoO3 perovskite: focus on structural phase transitions

We report the mixed valence and intermediate spin-state (IS) transitions in Pr substituted LaCoO3 perovskites in the form of bulk and nanostructures. Various compositions (x) of La1−x Pr x CoO3 (0 ⩽ x⩽ 0.9) were synthesized using the sol–gel process under moderate heat treatment conditions (600 °C). The structural analysis of these compounds reveals a phase crossover from the monoclinic phase (space group, s.g.: I2/a) to an orthorhombic one (s.g.: Pbnm), and a rhombohedral phase (s.g.: R-3c) to an orthorhombic one (s.g.: Pnma) in the bulk and nanostructures, respectively, for the composition range 0 ⩽ x⩽ 0.6. Such a structural transformation remarkably reduces the Jahn–Teller distortion factor ΔJT: 0.374 → 0.0016 signifying the dominant role of the IS state (S Avg = 1) of trivalent Co ions in the investigated system. Magnetization measurements reveal the ferromagnetic (FM) nature of bulk LaCoO3 along with a weak antiferromagnetic (AFM) component coexisting with an FM component. This coexistence results in a weak loop-asymmetry (zero-field exchange-bias effect ∼134 Oe) at low temperatures. Here the FM ordering occurs due to the double-exchange interaction (J EX /k B∼ 11.25 K) between the tetravalent and trivalent Co ions. Significant decrease in the ordering temperatures was noticed in the nanostructures (T C ∼ 50 K) as compared to the bulk counterpart (∼90 K) due to the finite size/surface effects in the pristine compound. However, the incorporation of Pr leads to the development of a strong AFM component (J EX/k B ∼ 18.2 K) and enhances the ordering temperatures (∼145 K for x = 0.9) with negligible FM correlations in both bulk and nanostructures of LaPrCoO3 due to the dominant super-exchange interaction: Co3+/4+‒O‒Co3+/4+. Further evidence of the incoherent mixture of low-spin (LS) and high-spin (HS) states comes from the M–H measurements which yields a saturation magnetization of M S ∼ 275 emu mol−1 (under the limit of 1/H → 0) consistent with the theoretical value of 279 emu mol−1 corresponding to the spin admixture: 65% LS + 10% IS of trivalent Co along with 25% of LS Co4+ in the bulk pristine compound. A similar analysis yields: Co3+ [30% LS + 20% IS] + Co4+ [50% of LS] for the nanostructures of LaCoO3, yet the Pr substitution decreases the spin admixture configuration. The Kubelka–Munk analysis of the optical absorbance results in a significant decrease in the optical energy band gap (E g:1.86 → 1.80 eV) with the incorporation of Pr in LaCoO3 which corroborates the above results.

Large remnant polarization and improved dielectric, magnetic properties of Te-modified 0.7BiFeO3–0.3BaTiO3 ceramics

BiFeO3–BaTiO3 (BF–BT) ceramics, as a kind of binary ferroelectric material with excellent properties, have a wide range of applications. A novel Te-doped 0.7BiFe1−xTexO3–0.3BaTiO3 (x = 0–0.07) ceramics with enhanced ferroelectric properties were designed and synthesized via a solid-phase reaction. Results of X-ray diffraction and Rietveld refinement revealed a phase transition from the rhombohedra and tetragonal phases to a single tetragonal phase with increasing Te content, indicating the formation of a morphotropic phase boundary (MPB) in samples with x = 0–0.03. The fitting results of the X-ray photoelectron spectra showed that the Te introduction reduced the Fe2+ concentration and improved the insulating performance. Due to the Te-induced structure distortion of the rhombohedral phase and tetragonal phase, a large remnant polarization of 51 μC/cm2 is obtained at x = 0.01, which is superior to those reported in many studies on BF–BT bulk materials. The dielectric properties revealed that Te addition effectively adjusted the relaxation degree of the ceramics, showing an ideal relaxor ferroelectric behavior in samples with x = 0.05 and 0.07. Additionally, Te substitution reduced the Fe–O–Fe bond angle and the grain size, achieving enhanced ferromagnetism and affording the largest remnant magnetization in the case of x = 0.03, which is around six times that of the pure BF–BT ceramic.

Dielectric, ferroelectric, and piezoelectric properties of rare earth Sm-doped 0.94 Bi0.5Na0.5TiO3-0.06 BaTiO3 lead-free ceramics

Bi0.47-xSmxNa0.47Ba0.06TiO3 (BNT-6BT-xSm) ferroelectric ceramics with different doping contents of samarium (Sm) were prepared by solid-state reaction method. All ceramics lie in the coexistence region of the rhombohedral perovskite phase (R3c) and tetragonal perovskite phase (P4bm). When the Sm content increases, the grain growth is limited, and the average grain size is reduced from 1.69 µm (at the composition of x = 0.005) to 0.86 µm (at the composition of x = 0.05). When Sm is introduced into Bi0.5Na0.5TiO3-BaTiO3 binary system, the composition-induced local structure transition occurs, namely the transition from order to disorder. And after the introduction of Sm3+ ions, the depolarization temperature (Td) increases considerably. With the increase of the Sm content, the remnant polarization (Pr) decreases from 52 μC/cm2 to 8 μC/cm2, and the coercive field (Ec) gradually decreases from 37 kV/cm to 14 kV/cm, respectively. Under an electric field of 80 kV/cm, the maximum electric field-induced strain and corresponding normalized strain (d33*) reached 0.35% and 318 pm/V (at the composition of x = 0.03), respectively. The maximum piezoelectric coefficient (d33) is 158 pC/N (at the composition of x = 0.015), resulting from the structure transition between ferroelectric phase and anti-ferroelectric phase. Therefore, BNT-6BT-0.03Sm lead-free ferroelectric ceramic is a promising candidate of lead-based ferroelectric ceramics for piezoelectric actuators.

Fabrication of titanium in-diffusion waveguide in the PIN-PMN-PT single crystal

Fabrication of the optical waveguide is essential for the application of high-performance relaxor ferroelectric single crystals in broadband integrated electro-optic devices. Here, a multimode titanium in-diffusion planar waveguide is demonstrated in a rhombohedral phase Pb(In1/2Nb2/3)O3–Pb(In1/2Nb2/3)O3–PbTiO3 (PIN-PMN-PT) single crystal. Influences of titanium in-diffusion on the crystalline phase and composition were carefully studied. Guided modes in the planar waveguide were characterized by prism couple measurements. Using the inverse Wentzel-Krames-Brillouin method, the refractive index profile in the waveguide was constructed. The results show that the PIN-PMN-PT crystal retains its single crystal structure and excellent electro-optic response after titanium in-diffusion. The mode refractive index change between the diffused layer and substrate crystal is approximately 3%, which is sufficient to constrain the energy of transmitted light. Moreover, the titanium-diffused PIN-PMN-PT waveguide is confirmed to be a graded optical waveguide with a Gaussian index profile. Our work indicates that the titanium in-diffusion is a feasible way to fabricate waveguides in PIN-PMN-PT crystals, which is of great significance for their application in integrated photonic devices.

Composition-driven structural phase transition in ferroelectric PbZrxTi1-xO3 across the morphotropic phase boundary

Materials with coexisting ferroelectric, piezoelectric, and pyroelectric properties have taken a great scientific attraction owing to their potential technological applications. In this work, employing X-ray diffraction and Raman scattering measurements combined with a photoluminescence investigation at room temperature, we report a composition-driven structural phase transition in the ferroelectric PbZrxTi1-xO3 (PZT) system by tuning the Zr content close to the morphotropic phase boundary (MPB) in the range of 0.44≤x≤0.58. The Ti-rich compound (x = 0.44) crystallizes in a tetragonal structure while the Zr-rich compound (x = 0.58) belongs to the rhombohedral symmetry. The system exhibits a phase coexistence region composed of these tetragonal and rhombohedral phases in the vicinity of MPB for compositions with x = 0.50, 0.52, and 0.53. We reveal strong and systematic evolution of the Raman shift, Raman strength, and damping constants for the Raman active phonon modes associated with the aforementioned structural transition and the increased local disorder of Ti4+ or Zr4+ cations as a function of x. We further uncover a resonance Raman phenomenon with 633 nm (1.96 eV) excitation, enhancing the Raman strength and damping constants of certain low-lying transverse optical (TO) phonon modes in the Raman spectra. The photoluminescence emission spectrum shows evidence of in-gap energy levels that could be ascribed to self-trapped excitons, explaining the origin of the resonance Raman effect in PZT ceramics.

Investigation of Self-Assembled Dual-Phase Cathodes with Triple Conductivity for Protonic Ceramic Fuel Cells

Efficient cathodes with high catalytic activity and stability are crucial for the development of protonic ceramic fuel cells (PCFCs). Single-phase cathode is difficult to meet multiple requirements, such as high electrochemical performance, oxygen reducing activity, stability, and CO2-resistance. In this regard, we report self-assembled dual-phase perovskite cathodes Ba(Co0.7Fe0.3)x(Ce0.8Y0.2)1-xO3-δ (BCFCY, x = 0.6, 0.7, 0.8) prepared by a one-pot method for PCFCs. The cathode displays a triple O2–/e–/H+ conducting behavior, which consists of a cubic phase (C-BCFCY, Co-rich) and a rhombohedral phase (R–BCFCY, Ce-rich) in this perovskite. The C-BCFCY has high O2–/e– conductivity and low basicity, while the latter R–BCFCY exhibited excellent hydration performance, which synergistically results in cathode with high electrochemical activity and CO2-resistance. Owing to its appropriate dual phase composition, the Ba(Co0.7Fe0.3)0.7(Ce0.8Y0.2)0.3O3-δ (BCFCY73) electrode demonstrates low polarization resistance (0.065 Ω cm2) at 700 °C in 5 vol % water steam. The peak power density of anode-supported single cells BZCYYb | BCFCY73 | BZCYYb is 545.3 mW cm–2 at 700 °C. This result proves that regulating the content of two phases (proton conductors and ionic conductors, respectively) of perovskite oxide by the one-pot method is an effective strategy to design PCFC cathode materials with high activity, and this method can also be applied in other energy transfer materials fields.

Electric field-driven modulation in structural and luminescent properties of Eu3+-doped (1 − x)(Na0.5Bi0.5)TiO3 − xBaTiO3 relaxor ferroelectrics

We investigated the structural, electrical, and luminescent properties of Eu3+-doped (1 − x)(Na0.5Bi0.5)TiO3 − xBaTiO3 (NBT-BT:Eu) relaxor ferroelectrics for x = 0.00–0.08. The introduction of Eu3+ generated good emission properties in the samples without weakening their piezoelectric properties. Rietveld refinement structural analysis revealed that the structural phase of NBT-BT:Eu was a mixture of rhombohedral and monoclinic phases with a significant x-dependence. We observed that NBT-BT:Eu underwent a structural phase transformation upon the application of an electric field. Interestingly, the structural changes caused by poling led to a reduction in the emission of Eu3+. The poling-driven emission quenching behavior resulted from the increase in the local structural symmetry around Eu3+ in the poled samples. We also identified light-induced modulation in the emission of our samples, probably attributed to the photochromic behavior. The multifunctionality of NBT-BT:Eu paves the way for the development of a new type of optoelectronic material.

B-site ion driven morphotropic phase transformation in NN-BT-ABO3 lead-free ceramics through chemical pressure

Forming morphotropic phase boundary (MPB) is crucial to improve the piezoelectric properties of lead-free NaNbO3-BaTiO3-xABO3 (NN-BT-xABO3) ceramics. In this work, four NN-BT-xABO3 systems (ABO3 = K0.5Bi0.5TiO3 (KBT), Bi0.5Na0.5TiO3 (BNT), K0.5Bi0.5ZrO3 (KBZ), and Bi0.5Na0.5ZrO3 (BNZ)) were constructed to analyze the relationship between ABO3 characteristic and phase transformation behavior. The results showed that the NN-BT-xKBT/xBNT systems underwent ferroelectric tetragonal (T)-relaxor ferroelectric T phase transformation with d33 ∼ 150 pC/N and kp ∼ 0.2 in NN-BT-0.11KBT and NN-BT-0.09BNT compositions, while the NN-BT-xKBZ/xBNZ systems underwent ferroelectric T- ferroelectric rhombohedral (R) morphotropic phase transformation with improved d33 ∼ 256 pC/N and kp ∼ 0.31 in NN-BT-0.04BNZ composition. The B-site ion radius was found to be a key factor in inducing phase transformation. The incorporation of small Ti4+ ions tends to break the long ordering of ferroelectrics, inducing the normal-relaxor ferroelectric phase transformation, however, the appearance of the R phase in NN-BT-xAZrO3 compositions was attributed to the chemical pressure caused by buckled Zr-O-Zr bonds as larger Zr4+ ions enter the NN-BT matrix lattices. This study will deepen the understanding of the ferroelectric phase transformation mechanism in the lead-free NaNbO3 material system and provide a guidance for designing morphotropic NN-based lead-free piezoelectric ceramics.

Evaluation of Photocatalytic Activity and Electrochemical Properties of Hematite Nanoparticles

The symmetric nano morphologies, asymmetric electronic structures, and as well as the heterojunctions of the developed photocatalytic systems perform a vital role in promoting light absorption, separation of electron and hole pairs and charge carrier transport to the surface when exposed to near-infrared (NIR) light. In this present work, we synthesized hematite (α-Fe2O3) nanoparticles (NPs) by a facile hydrothermal method and studied their structural, optical, and photocatalytic properties. Powder X-ray diffraction (XRD) confirmed the rhombohedral phase of the α-Fe2O3 NPs, and Fourier transform infrared spectroscopy (FT-IR) was used to investigate symmetric and asymmetric stretching vibrations of the functional groups on the surface of the catalysts. The optical bandgap energy was estimated to be 2.25 eV using UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and scanning electron microscopy (SEM) images indicated sphere like morphology. The oxidation and reduction properties of α-Fe2O3 NPs were analyzed by cyclic voltammetry (CV). The α-Fe2O3 NPs were utilized for the degradation of methylene blue (MB) dye under natural sunlight. The experimental results demonstrate that the degradation efficiency was achieved at 33% in 2 h, and the pseudo-first-order rate constant was calculated to be 0.0033 min−1.

Effects of oxygen vacancies on electric properties of high valence ions Mo/Zr/Ti doped Bi0.85Nd0.15FeO3 ceramics with morphotropic phase boundary

The effects of oxygen vacancies on electric properties of high valence ions (Mo/Zr/Ti) doped Bi0.85Nd0.15FeO3 (marked as BNFM, BNFZ, BNFT) ceramics with morphotropic phase boundary (MPB) are investigated. MPB structures consisting of rhombohedral, intermediate and orthorhombic phases exist in all samples, which is confirmed by the XRD Rietveld refinements and TEM analyses. The BNFT has the maximum remnant polarization of 34.5 μC/cm2 at 180 kV/cm, owing to its lowest paraelectric orthorhombic phase content. XPS and impedance results reveal that the BNFT ceramic has the relatively low oxygen vacancy contents and the largest impedance value, which is beneficial for the ferroelectric test. The best comprehensive properties of BNFT open wide prospects for improving BiFeO3-based ceramics.

Three-phase material mapping with incomplete X-ray diffraction spectral information.

An equiatomic nickel-titanium shape-memory alloy specimen subjected to a uniaxial tensile load undergoes a two-step phase transformation under stress, from austenite (A) to a rhombohedral phase (R) and further to martensite (M) variants. The pseudo-elasticity that goes accompanies the phase transformation induces spatial inhomogeneity. To unravel the spatial distribution of the phases, in situ X-ray diffraction analyses are performed while the sample is under tensile load. However, the diffraction spectra of the R phase, as well as the extent of potential martensite detwinning, are not known. A novel algorithm, based on a proper orthogonal decomposition and incorporating inequality constraints, is proposed in order to map out the different phases and simultaneously yield the missing diffraction spectral information. An experimental case study illustrates the methodology.

Pressure-induced topological crystalline insulating phase in TlBiSe2 : Experiments and theory

We report in situ high-pressure studies on three-dimensional topological insulator ${\mathrm{TlBiSe}}_{2}$ using Raman scattering and synchrotron x-ray-diffraction experiments corroborated with the first-principles theoretical calculations. The phonon modes of a rhombohedral phase of ${\mathrm{TlBiSe}}_{2}$ show a systematic increase in frequencies under hydrostatic pressure up to $\ensuremath{\sim}7.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Interestingly, the linewidth of the ${A}_{1g}, N$, and ${E}_{g}$ phonon modes show clear anomalies at $\ensuremath{\sim}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ which is indicating the isostructural electronic transition. With the help of calculated electron-phonon coupling constant $\ensuremath{\lambda}$, anomalies in the Raman linewidth of ${E}_{g}$ mode are attributed to electron-phonon coupling changes. Moreover, our theoretical results reveal that the observed phonon anomalies are due to pressure-induced band inversion at the $F$ points of the Brillouin zone which leads to the changes in electronic topology reflected in the mirror Chern number ${n}_{M}$ and ${\mathbb{Z}}_{2}$ topological invariant. Therefore, the phonon anomalies and change in mirror Chern number ${n}_{M}$ confirm the pressure-induced topological crystalline insulator phase in ${\mathrm{TlBiSe}}_{2}$ at $\ensuremath{\sim}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Further, a reversible structural phase transition has been observed above $\ensuremath{\sim}7.0\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ from both synchrotron x-ray-diffraction and Raman-scattering measurements. Finally, our studies suggest the use of hydrostatic pressure as a potential pathway for exploring the topological crystalline insulating phase in strong spin-orbit coupling compounds, such as the thallium-based III-V-$\mathrm{V}{\mathrm{I}}_{2}$ ternary chalcogenide $\mathrm{TlBi}{X}_{2}$ ($X=\mathrm{S}$, Se, Te) family.

Sintering temperature induced structural phase transitions in Ba(Fe0.7Ta0.3)O3-δ ceramic: A detailed rietveld study

We report a detailed Rietveld analysis of sintering temperature (Ts) induced structural phase transitions in Ba(Fe0.7Ta0.3)O3-δ; a high-temperature oxygen sensing material. The compound was synthesized through the conventional solid-state reaction method, and sintered in the temperature range of 1200 °C ≤ Ts ≤ 1350 °C at an interval of 50 °C. The misfit regions found in the characteristic Bragg reflections (200), and (211) in refined patterns of single-phase structural models discount the stabilization of the compound in single-phase. Refinement using two-phase plausible structural models confirms the compound sintered at 1200 °C stabilizes in orthorhombic and rhombohedral mixed phase, whereas the compound sintered at and beyond 1250 °C stabilizes in tetragonal and rhombohedral. The phase fraction of underlined phases is quantified and reported. The sensing performance of material depends on structural and microstructural features, hence the derived understanding could be useful to develop Ba(Fe0.7Ta0.3)O3-δ materials in appropriate conditions.

Defect-Healing Induced Monoclinic Iron-Based Prussian Blue Analogs as High-Performance Cathode Materials for Sodium-Ion Batteries.

Prussian blue analogs (PBAs) have attracted wide interest as a class of ideal cathodes for rechargeable sodium-ion batteries due to their low cost, high theoretical capacity, and facile synthesis. Herein, a series of highly crystalline Fe-based PBAs (FeHCF) cubes, where HCF stands for the hexacyanoferrate, is synthesized via a one-step pyrophosphate-assisted co-precipitation method. By applying this proposed facile crystallization-controlled method to slow down the crystallization process and suppress the defect content of the crystal framework of the PBAs, the as-prepared materials demonstrate high crystallization and a sodium-rich induced rhombohedral phase. As a result, the as prepared FeHCF can deliver a high specific capacity of up to 152.0mAhg-1 (achieving ≈90% of its theoretical value) and an excellent rate capability with a high-capacity retention ratio of 88% at 10C, which makes it one of the most competitive candidates among the cathodes reported regarding both capacity and rate performance. A highly reversible three-phase-transition sodium-ion storage mechanism has been revealed via multiple in situ techniques. Furthermore, the full cells fabricated with as-prepared cathode and commercial hard carbon anode exhibit excellent compatibility which shows great prospects for application in the large-scale energy storage systems.