Thermodynamic modeling of YO1.5-TaO2.5 system and the effects of elastic strain energy and diffusion on phase transformation of YTaO4

Thermodynamic parameters of the YO_1.5-TaO_2.5 system were obtained, and the effects of elastic strain energy and diffusion on phase transformation of YTaO₄ were analyzed in this work. The YO_1.5-TaO_2.5 system was critically modeled using the CALPHAD technique based on our calculated formation energies by DFT and available experimental data. According to DFT calculations, M′-YTaO₄ was suggested to be the thermodynamically stable phase at low temperature. For a displacive transformation of T→M between the equilibrium tetragonal and monoclinic YTaO₄, our calculations suggested it cannot be hindered by elastic strain energy. For a diffusive transformation of T→M′, it can be divided into T→M and M→M′. Diffusive transition of M→M′ was likely to be impeded due to large diffusion energy barrier, which was calculated to be 3.260 eV. However, the driving force ΔG _M→M′ is about -0.121 kJ/mol. The large diffusion energy barrier and small driving force may be the main reason that T cannot transform to M′ after cooling.     

Structural,magnetic and optical properties of Ce substituted BiFeO3 nanoparticles

Ce substituted Bi_1−xCe_xFeO₃ (x=0.03, 0.05, 0.07 and 0.10) nanoparticles were prepared by a tartaric acid based sol–gel route. X-ray diffraction patterns and Raman spectra revealed a structural phase transition from rhombohedral to orthorhombic phase for x=0.10 sample. Room temperature magnetic measurements showed weak ferromagnetic ordering and enhancement in magnetization with increasing Ce concentration. The improved magnetic properties due to the breaking of spin cycloid with Ce substitution have been observed from electron spin resonance (ESR) study. The measured g-values for all samples are greater than 2 and the ESR lines shift towards higher g-value with increasing Ce concentration, indicating ferromagnetic nature of these samples. UV–visible diffuse reflectance spectra showed a sharp absorption in the visible region with two d–d and three charge transfer (C-T) transitions. Prominent red shift in the band gap indicates a significant change in the band structure of the doped nanoparticles.     

Structural phase transition,impedance spectroscopy and narrow optical band Gap in 1-xKNbO3-xBaSc12Nb12O3

In the progress of exploring lead-free ferroelectric perovskites, a new solid solution of $\left(1-x\right)\mathrm{}\mathrm{K}\mathrm{N}\mathrm{b}{\mathrm{O}}_3-\mathrm{}x\mathrm{}\mathrm{B}\mathrm{a}\left({\mathrm{S}\mathrm{c}}_{\frac{1}{2}}\mathrm{}{\mathrm{N}\mathrm{b}}_{\frac{1}{2}}\right)O_3$ is synthesized using solid state method. The effect of Ba and Sc codoping on structural phase transition, dielectric, ferroelectric, electrical, and optical properties is systematically studied. A narrow band gap of 1.98eV is observed at x?=?0.05. On further increase in x, the optical band gap increases due to increased strain. The crystal symmetry changes from orthorhombic at x?=?0.00 to tetragonal phase at x?≤?0.15, remains in pseudocubic phase for 0.15?<?x?<?0.35, and finally transforms to the cubic symmetry at x?≥?0.35. A new Raman active mode evolves at 180?${\mathrm{c}\mathrm{m}}^{-1}$ at x?=?0.15, which could be the TO or LO phonon of ${\mathrm{A}}_1$ symmetry. The electrical microstructure of the prepared electroceramics at room temperature has been investigated using impedance spectroscopy. This newly synthesized ferroelectric perovskite material has promising potential applications for photocatalysis and photovoltaics, especially under the visible light spectrum.     

Crystal structure,leakage conduction mechanism evolution and enhanced multiferroic properties in Y-doped BiFeO3 ceramics

Ceramics of pure phase Yttrium (Y) doped BiFeO₃ prepared by a solid-state sintering route were characterized by X-ray diffraction, Raman spectroscopy, magnetic and electrical measurements. The results and analysis show that Y substitution greatly reduces the leakage current and enhances the multiferroic properties of BiFeO₃. Leakage conduction mechanism is shown to change from space-charge-limited conduction type in pure BiFeO₃ to a Poole–Frenkel emission behavior in Bi_0.90Y_0.10FeO₃. A Fowler-Nordheim tunneling mechanism in Bi_0.95Y_0.05FeO₃ ceramic is also evidenced under high electric fields. At the same time, enhanced magnetic properties due to Y-doping are confirmed by temperature dependent magnetometry and supported by Raman spectroscopy. An unexpected and sharp switching behavior in the magnetization under low magnetic fields observed in Bi_0.90Y_0.10FeO₃ ceramic, together with its improved ferroelectric property, may trigger such system for promising magneto-electric applications.     

Pressure and temperature dependence of second-order elastic constants from third-order elastic constants in TMC (TM=Nb,Ti, V,Zr)

In this paper, we present an efficient and effective method to predict the pressure dependence and temperature dependence of second-order elastic constants (SOECs) by introducing third-order elastic constants (TOECs) in the monocarbide ultrahigh temperature ceramics. The method is validated by comparing with experiments and previous calculations in four TMCs (TM = Nb, Ti, V, Zr). Using this method, we investigate the derivatives of SOECs against pressure and temperature as well as the anisotropic properties of polycrystalline modulus. In addition, we fit the SOECs with pressure and temperature under the framework of CALPHAD for practice usage.     

Modulation of magnetic,ferroelectric and leakage properties by HoFeO3 substitution in multiferroic 0.7BiFeO3-0.3Ba0.8Ca0.2TiO3 solid solutions

An investigation on the structural, magnetic, electrical properties of HoFeO₃-substituted 0.7BiFeO₃-0.3Ba_0.8Ca_0.2TiO₃ solid solutions synthesized using conventional solid state reaction method was carried out. The structural study confirms that an additional orthorhombic (Pbnm) phase of HoFeO₃ appears in the ceramic matrix and the presence of the aforementioned phase significantly influences the magnetic characteristics in the solid solutions. A detailed high temperature dielectric study suggests that the oxygen vacancies can be effectively controlled by the appropriate amount of substitution, successively regulating the ferroelectric as well as the leakage properties in the ceramics. Furthermore, the foremost remnant polarization and the feeble dielectric loss is achieved when the substitution content is x~0.2. Therefore, an appropriate amount of HoFeO₃ substitution can be an effective way to modulate the multiferroic properties in the present ceramics.     

Evaluation of Ni doping for promoting favorable electronic structures in CuCrO2 and AgCrO2 from a first-principles perspective

Doping transparent conducting oxides in the delafossite form with Ni has been presented in the literature as an exciting candidate to improving the conductivity while maintaining the transparency of these materials. Here, the effects of 6.25% Ni doping on the electronic, structural, and hole effective masses in the 2H phase of XCrO₂ (X = Cu, Ag) is studied using spin polarized ab initio calculations. 6.25% Ni doping is found to produce an asymmetry across spin in both materials, as well as decrease the bandgaps, potentially harming transparent character. Hole effective masses are calculated to be heavier in every lattice vector direction for CuCrO₂, but in AgCrO₂ hole effective masses are lighter overall. Considering that 6.25% Ni doping introduces an increase in hole concentration, AgCr_0.94Ni_0.06O₂ should have a higher conductivity as result of 6.25% Ni doping, while the same doping may reduce the conductivity of CrCr_0.94Ni_0.06O₂.     

First-principles study of structural,mechanical, and thermodynamic properties of cubic Y2O3 under high pressure

The structural, mechanical, and thermodynamic properties of cubic Y₂O₃ crystals at different hydrostatic pressures and temperatures are systematically investigated based on density functional theory within the generalized gradient approximation. The calculated ground state properties, such as equilibrium lattice parameter a₀, the bulk modulus B₀, and its pressure derivative B₀′ are in favorable agreement with the experimental and available theoretical values. The pressure dependence of a/a₀ and V/V₀ are also investigated. Furthermore, the elastic constants C_ij, bulk modulus B, shear modulus G, Young's modulus E, the ductile or brittle (B/G), Vickers hardness H_v, isotropic wave velocities and sound velocities are calculated in detail in a pressure range from 0 to 14 GPa. It was found that the Debye temperature decreases monotonically with an increase in pressure, the calculated elastic anisotropic factors indicate that Y₂O₃ has low anisotropy at zero pressure, and that its elastic anisotropy increases as the pressure increases. Finally, the thermodynamic properties of Y₂O₃, such as the dependence of the heat capacities C_V and C_P, the thermal expansion coefficient α, the isothermal bulk modulus, and the Grüneisen parameter γ on temperature and pressure, are discussed from 0 to 2000 K and from 0 to 14 GPa, respectively, applying the non-empirical Debye model in the quasi-harmonic approximation.     

Polarization switching and rotation in KNN-based lead-free piezoelectric ceramics near the polymorphic phase boundary

(K,Na)NbO₃ (KNN)-based ceramics have attracted considerable attention owing to their excellent piezoelectric performance in the polymorphic phase boundary (PPB); however, many researchers have found that the optimal composition usually appears on the tetragonal side near the PPB zone. In this study, it is found that the maximum piezoelectric performance is achieved in the PPB region for unpoled ceramics due to the more efficient and facile polarization switching. However, the most outstanding piezoelectricity shifts to the tetragonal side after the ceramics are poled. Raman spectra and first-principles calculations reveal the occurrence of a phase transformation from a tetragonal to monoclinic structure under an external electric field. Hence, the unpoled tetragonal ceramics transform to a two-phase coexistence condition after the poling process and exhibit the best electrical properties driven by the combined effects of polarization switching and rotation. This study reveals that the electric-field-induced phase transformation leads to the optimal composition on the tetragonal side, and this can provide useful guidance for the design of high-performance KNN-based materials.     

Enhancing photostriction in KNN-based ceramics by constructing the morphotropic phase boundary and narrowing the energy band gap

Lead-free ferroelectric ceramics exhibiting photostriction effect has attracted a lot of attention in the past decade. Herein, we fabricated a series of (1?x)K_0.5Na_0.5NbO₃-x(La_0.51Na_0.49)(Zr_0.54Ni_0.46)O₃ ((1?x)KNN-xLNNZ) ceramics by traditional solid-state synthesis method aiming to improve photostriction. The addition of LNNZ in KNN has led to significant changes in phase structure and grain size. The ceramics with a composition of 0.97KNN-0.03LNNZ has shown a narrow bandgap ~2.43 eV, large piezoelectric coefficient (~209 pC/N), low dielectric loss (0.021 at 1 kHz) and high remnant polarization (~24 μC/cm²). Further, 0.97KNN-0.03LNNZ also exhibits a considerable photostrictive coefficient (~1.83 × 10^?9 m²W^?1), which is attributed to the combined effect of significantly narrow bandgap along with the morphotropic phase boundary. Further, all the substituted samples show distinct red shift of v1 mode compared to the pure KNN, which suggest a new kind of distortion introduced in (Nb, Zr, Ni)O₆ octahedron for KNN-based ceramics under variable power laser excitation. The obtained results indicate that LNNZ-substituted KNN ceramics can serve as a potential material for the fabrication of optomechanical devices.     

Anomalously large lattice strain contributions from rhombohedral phases in BiFeO3-based high-temperature piezoceramics estimated by means of in-situ synchrotron x-ray diffraction

Thermally-stable (0.75-x)BiFeO₃-0.25PbTiO₃-xBa(Zr_0.25Ti_0.75)O₃ (0.1?≤?x?≤?0.27) piezoelectric ceramics were reported to have excellent dielectric and electromechanical properties of d₃₃～405 pC/N, k_p～46%, ε₃₃^T/ε₀～1810, tanδ～3.1% and T_c～421?°C close to tetragonal (T)-rhombohedral (R) morphotropic phase boundary. The dielectric measurement indicates that R ferroelectric phase is gradually transformed into relaxor ferroelectric across the phase boundary due to the substitution of BZT for BF. The transmission electron microscopy and convergent beam electron diffraction provide clear evidences that both the R-T phase coexistence and polar nanodomains contribute to enhanced piezoelectric properties at x?=?0.19 through cooperatively facilitating polarization orientation. In combination with the macroscopic piezoelectric coefficient measurement, the quantitative analysis of synchrotron diffraction data under electric fields suggests that extremely large lattice strain contribution predominantly from R phases plus little extrinsic domain switching contribution should dominate the piezoelectric response of the x?=?0.19 sample, mainly owing to both irreversible field-induced T to R phase transition and irreversible non-180° domain switching.     

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

Rare earth Sm substituted Bi_1−xSm_xFeO₃ 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 5 K 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 dielectric,thermal stability,and energy storage properties in compositionally engineered lead-free ceramics at morphotropic phase boundary

Structural inhomogeneity at morphotropic phase boundary (MPB) offers a novel paradigm to explore and modulate the physical properties of dielectric materials to design next-generation multifunctional devices. In this work, two lead free materials at MPB; Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ (BCZT) and (Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ (BNTBT), are combined together to synthesize polycrystalline composite samples of (1-x) BCZT-xBNTBT with x = 0.0, 0.25, 0.50, 0.75, and 1.0. Structural investigations using XRD show the coexistence of double phases for pristine BCZT (tetragonal (P4mm) + rhombohedral (R3m)), and for pristine BNTBT (tetragonal (P4bm) + rhombohedral (R3c)). However, all the doped samples with x = 0.25, 0.5, and 0.75 display a coexistence of triple phases with P4mm, P4bm, and R3c symmetries. Detailed dielectric study reveals a normal ferroelectric to macroscopic ergodic relaxor crossover for samples with x = 0.25 and 0.75. Intriguingly, sample x = 0.25 displays a coexistence of high dielectric constant (4050), ultralow dielectric loss (≤0.02), high temperature thermal stability of permittivity (variation ≤ ±15%) in a temperature range 135 °C–450 °C, large recoverable energy density (W_rec = 423 mJ/cm³) with ultrahigh energy storage efficiency (η = 95.4%) at low applied electric field - 23 kV/cm. Nevertheless, at similar applied field strength, the obtained values of W_rec and η exceed most of the selected lead-free energy storage materials. This work may pave a new path to design superior high-temperature dielectrics, through intermixing of MPBs, for energy storage applications.     

Simultaneously achieving high energy storage density and efficiency under low electric field in BiFeO3-based lead-free relaxor ferroelectric ceramics

BiFeO₃-BaTiO₃-based relaxor ferroelectric ceramic has attracted increasing attention for energy storage applications. However, simultaneously achieving high recoverable energy storage density (W_rec) and efficiency (η) under low electric field has been a longstanding drawback for their practical applications. Herein, a novel relaxor ferroelectric material was designed by introducing (Sr_0.7Bi_0.2)TiO₃ (SBT) into the composition 0.67BiFeO₃-0.33BaTiO₃ (BF-BT-xSBT). A large W_rec of ∼2.40 J/cm³ and a high η of ∼90.4 % were simultaneously realized under a low electric field of 180 kV/cm, which is superior to that of most previously reported lead-free ceramics. Moreover, moderate temperature endurance and excellent frequency stability were also obtained. More importantly, this ceramic has a large discharge current density (∼289.18 A/cm²), a discharge power density (∼14.46 MW/cm³) and short discharge time (<0.25 μs). These results not only demonstrate superior potential in BF-BT-SBT ceramics, but also offer a new design to tune the energy storage performance of lead-free relaxor ferroelectric ceramics.     

The effect of the ethylene glycol to metal nitrate molar ratio on the phase evolution,morphology and magnetic properties of single phase BiFeO3 nanoparticles

In this work, single phase BiFeO₃ nanoparticles have been synthesized by thermal decomposition of a glyoxylate complex achieved by the redox reaction between ethylene glycol and nitrate anions. The effects of different molar ratios of ethylene glycol to metal nitrate anions on the phase evolution, morphology and magnetic properties were investigated by infrared spectroscopy, thermal analysis, X-ray diffraction, electron microscopy and vibrating sample magnetometry methods. The single phase bismuth ferrite nanoparticles synthesized with the ethylene glycol to nitrate anions molar ratio of 5 showed the weak ferromagnetism behavior with saturation magnetization of 1.3 emu/g, due to the size confinement effect. Furthermore, the BiFeO₃ nanoparticles were used for the degradation of methylene blue (MB) as a typical dye pollutant under direct sunlight irradiation.     

Structural phase transition,electrical and photoluminescent properties of Pr3+-doped (1-x)Na0.5Bi0.5TiO3-xSrTiO3 lead-free ferroelectric thin films

Lead-free ferroelectric Pr³⁺-doped (1-x)Na_0.5Bi_0.5TiO₃-xSrTiO₃ (x?=?0–0.5) (hereafter abbreviated as Pr-NBT-xSTO) thin films were prepared on Pt/Ti/SiO₂/Si and fused silica substrates by a chemical solution deposition method combined with a rapid thermal annealing process at 700?°C, and their structural phase transition, dielectric, ferroelectric, and photoluminescent properties were investigated as a function of STO content. Raman analysis shows that with increasing STO content, the phase structures evolve from rhombohedral phase to coexistence of rhombohedral and tetragonal phases (i.e. morphotropic phase boundary), and then to tetragonal phase. The structural phase transition behavior has been well confirmed by temperature- and frequency- dependent dielectric measurements. Meanwhile, the variation in photoluminescence intensity of Pr³⁺ ions with different STO content in the NBT-xSTO thin films also indicates that there exists a clear structural phase transition when the film composition is close to the morphotropic phase boundary. Superior dielectric and ferroelectric properties are obtained in the Pr-NBT-0.24STO thin films due to the formation of morphotropic phase boundary. Our study suggests that Pr-NBT-xSTO thin films be promising multifunctional materials for optoelectronic device applications.     

A comparative study of the lattice structure,optical band gap,electrical conductivity and polarization at different stages of the heat treatment of chemical routed Al(OH)3

Aluminium hydroxide (Al(OH)₃) was prepared by chemical reaction of the Al(NO₃)₃ in alkaline medium. The as-prepared powder was heated in the temperature range 250 °C to 1250 °C for studying the structural phase transformation at different stages of the heat treatment. The synchrotron x-ray diffraction patterns confirmed a structural transformation of Al(OH)₃ through different (Boehmite, γ, θ, δ, and α) polymorphic phases of Al₂O₃ on increasing the heat treatment temperature. The samples in Boehmite (γ-AlOOH) and α- Al₂O₃ phases showed Raman active modes, whereas the intermediate (meta-stable) multi-phased structure showed weak Raman active peaks. The analysis of UV–visible spectra of the samples indicated two optical band gap energy values in the high energy range 4.50–4.73 eV and low energy range 3.06–3.84 eV. The voltage dependence of current, capacitance and electrical polarization were recorded to study electrical properties in heat treated samples. The capacitance value, derived from the polarization, showed a usual increasing trend on decreasing the measurement frequency (inverse of the time) of driving electric voltage. The measured electrical polarization in the samples was found to be highly correlated to their electrical conductivity and the results are helpful to understand the role of electrical conductivity on exhibiting the apparently ferroelectric properties in high conductive and low polarizable dielectric oxides.     

Structural phase transition and high temperature phase structure of Friedels salt, 3CaO · Al2O3 · CaCl2 · 10H2O

Friedels salt, the chlorinated compound 3CaO · Al₂O₃ · CaCl₂ · 10H₂O (AFm phase), presents a structural phase transition at about 30°C from a monoclinic to a rhombohedral phase. It has been studied by X-ray powder diffraction and optical microscopy in transmitted light with crossed polarisers on single crystals prepared by hydrothermal synthesis. The high temperature phase was determined at 37°C from X-ray single crystal diffraction data. The compound crystallises in the space group R c with lattice parameters of a = 5.7358(6)Å and c = 46.849(9)Å (Z = 3 and D_x = 2.111 g/cm³). The refinement of 498 independent reflections with I > 2σ(I) led to a residual factor of 7.1%. The Friedels salt can be described as a layered structure with positively charged main layers of composition [Ca₂Al(OH)₆]⁺ and negatively charged layers of composition [Cl⁻,2H₂O]. The chloride anions are surrounded by 10 hydrogen atoms, of which six belong to hydroxyl groups and four to water molecules. The structural phase transition may be related to the size of the chloride anions, which are not adapted to the octahedral cavity formed by bonded water molecules.     

Defect mechanisms,oxygen vacancy trapping ability in rare-earth doped BaTiO3 from first-principles and thermodynamics

The defect mechanisms of rare earth (RE) doped BaTiO₃ have a strong impact on the electrical performance of the multilayer ceramics capacitors (MLCCs). Oxygen vacancy is the main reason for the device degradation over longtime use, while the effect of the doping strategy on controlling the oxygen vacancies is not yet quantitatively understood. In this work, the grand canonical thermodynamic defect model based on first-principle calculations is applied to evaluate the defect mechanism of RE-doped BaTiO₃ under practical experimental condition. The charge compensation and prior site occupancy of RE are found not only associated with ionic size but also exhibit transitions with oxygen partial pressure and doping concentration. Furthermore, the oxygen vacancy trapping ability of RE ions is evaluated from the perspectives of thermodynamics and kinetics. The migration barrier among first nearest oxygen sites dramatically changed depending on the RE site occupancy. The large trapping ability is contributed by the relatively large negative binding energy of the defect complex and comparable RE concentrations substituted on Ba and Ti sites. The two conditions can be achieved in amphoteric ions doped systems, while in pure donor doped BT only one of these conditions can be satisfied. Although the self-compensated defect complexes exhibit the highest binding energy, the trapping ability contributed by different defect complexes (${\mathrm{RE}}_{\mathrm{Ti}}^{{}^{\prime }}$, ${\mathrm{RE}}_{\mathrm{Ba}}^{·}$, RE_Ba − RE_Ti) is generally comparable in these systems. This feature of amphoteric RE ions accounts for the improvement of the lifetime and reliability of MLCCs.