Large electrocaloric responses in [Bi1/2(Na,K)1/2]TiO3‐based ceramics with giant electro‐strains

The electrocaloric effect (ECE) is investigated through indirect measurement in two lead‐free [Bi_1/2(Na,K)_1/2]TiO₃‐based ceramics that were previously reported to display giant electro‐strains. In the Nb‐doped ceramic, denoted as BNKT‐2.5Nb, a decent temperature change of ΔT=1.85 K and an electrocaloric responsivity of ΔT/ΔE=0.37 (10^?6Km V^?1) are found around room temperature (32°C). While in the Ta‐doped ceramic, BNKT‐1.5Ta, a wide operation temperature range (T_span ~55 K) is observed near room temperature. Additional electrical measurements, as well as transmission electron microscopy experiments, are performed to identify the mechanisms of the ECE in both ceramics.     

Disorders and oxygen vacancies in p-type Sn2B2O7 (B = Nb,Ta): Role of the B-site element

Sn₂Nb_2−xTa_xO₇ (x = 0.0–2.0) with pyrochlore structure is a promising material for p-type oxide semiconductors. A systematic study of its Nb/Ta ratio indicated that the hole–generation efficiency of the Nb end (Sn₂Nb₂O₇) was an order of magnitude lower than that of the Ta end (Sn₂Ta₂O₇). Although this occurs due to differences in oxygen-vacancy formation, the origins of the hole–generation efficiencies remain unclear due to limited information on local and global crystal-structure disorders in pyrochlore Sn₂Nb₂O₇ and Sn₂Ta₂O₇. In this study, the crystal structures of Sn₂B₂O₇ (B = Nb, Ta), composed of BO₆ octahedra and Sn₄O tetrahedra, were investigated using X-ray absorption spectroscopy and X-ray diffraction. A detailed investigation of the local and global crystal structures indicated a larger amount of disorder in the Sn₄O tetrahedra in Sn₂Nb₂O₇ compared to Sn₂Ta₂O₇; disorder in the BO₆ octahedra occurred only in Sn₂Ta₂O₇. This study indicates that an appropriate selection of the B-site element is vital for suppressing defect and disorder formation in Sn₄O tetrahedra and subsequently improving the hole–carrier–generation efficiency.     

Main Source of Microwave Loss in Transition‐Metal‐Doped Ba(Zn1/3Ta2/3)O3 and Ba(Zn1/3Nb2/3)O3 at Cryogenic Temperatures

Microwave resonator measurements were performed on high‐performance microwave ceramics Ba(Zn_1/3Ta_2/3)O₃ (BZT) and Ba(Zn_1/3Nb_2/3)O₃ (BZN) containing additives commonly used by commercial manufacturers (i.e., Co, Mn, and Ni). We find that the loss tangent, even in ambient magnetic fields, is dominated by electron paramagnetic resonance (EPR) absorption by exchange‐coupled 3d electrons in transition metal clusters at cryogenic temperatures. The large orbital angular momentum in Co²⁺ and Ni²⁺ ions of L = 3 causes strong anisotropic‐broadened dipolar interactions that extend EPR losses to zero applied field. This effect is greatest in BZN with Co concentrations greater than 0.5 mol%, dominating the losses at liquid nitrogen temperatures (77 K) and below. In samples containing Mn²⁺ ions with L = 0, the dipolar interactions and associated EPR losses in ambient fields are smaller. We show the magnetic‐field‐dependent changes in the EPR losses (i.e., tan δ) and magnetic reactive response (i.e., μ_r) are from the same mechanism, as they follow the Kramers–Kronig relation. Finally, we note that these materials can make ultra‐high Q passive microwave devices with externally controlled transfer functions, as the quality factor (Q) of the composition Ba(Co_1/15Zn_4/15Nb_2/3)O₃ at 77 K can be tuned from 1 100 to 12 000 at 10 GHz by applying practical magnetic fields.     

Thermal,Structural, and Crystallization Properties of New Tantalum Alkali‐Germanate Glasses

In this work, new glass compositions were prepared in the ternary system GeO₂–K₂O–Ta₂O₅. Potassium oxide was added to reach the complete melt of the starting mixture and two composition series were investigated: the first one with a constant K₂O molar content of 10% in the ternary system (90–x)GeO₂–10K₂O–xTa₂O₅ and the second one with the same molar content of K₂O and Ta₂O₅ in the ternary system (100–2x)GeO₂–xK₂O–xTa₂O₅. Homogeneous and transparent glasses could be obtained between x = 0 and 20. X‐ray diffraction analyzes of samples with x = 25 identified orthorhombic Ta₂O₅ in the first series and an isostructure of K_3.8Ge₃Nb₅O_20.4 in the second series where it is assumed that Ta⁵⁺ ions are inserted in the Nb⁵⁺ sites. As one of our goal with these materials is related with the preparation of glass‐ceramics containing Ta₂O₅ nanocrystals, the first series has been selected for further characterizations. An increase in glass‐transition temperatures with increasing Ta₂O₅ content as well as an increase of the thermal stability from x = 0 to 10 has been identified by differential scanning calorimetry. For higher contents, crystallization events were identified. Fourier transform infrared and Raman spectroscopic characterizations allowed to point out the intermediary behavior of Ta₂O₅ in the vitreous network where TaO₆ octahedra are inserted inside the germanate network with TaO₆ clusters identified at higher Ta₂O₅ contents. Heat‐treated samples with high tantalum contents (x = 15 and 20) exhibit preferential precipitation of orthorhombic Ta₂O₅ with nanometric size, suggesting the possibility of obtaining transparent glass‐ceramics for optical applications.     

Optimization of Lithium Content and Sintering Aid for Maximized Li+ Conductivity and Density in Ta‐Doped Li7La3Zr2O12

Ta‐doped cubic phase Li₇La₃Zr₂O₁₂ (LLZ) lithium garnet received considerable attention in recent times as prospective electrolyte for all‐solid‐state lithium battery. Although the conductivity has been improved by stabilizing the cubic phase with the Ta⁵⁺ doping for Zr⁴⁺ in LLZ, the density of the pellet was found to be relatively poor with large amount of pores. In addition to the high Li⁺ conductivity, density is also an essential parameter for the successful application of LLZ as solid electrolyte membrane in all‐solid‐state lithium battery. Systematic investigations carried out through this work indicated that the optimal Li concentration of 6.4 (i.e., Li_6.4La₃Zr_1.4Ta_0.6O₁₂) is required to obtain phase pure, relatively dense and high Li⁺ conductive cubic phase in Li_7?xLa₃Zr_2?xTa_xO₁₂ solid solutions. Effort has been also made in this work to enhance the density and Li⁺ conductivity of Li_6.4La₃Zr_1.4Ta_0.6O₁₂ further through the Li₄SiO₄ addition. A maximized room‐temperature (33°C) total (bulk + grain boundary) Li⁺ conductivity of 3.7 × 10^?4 S/cm and maximized relative density of 94% was observed for Li_6.4La₃Zr_1.4Ta_0.6O₁₂ added with 1 wt% of Li₄SiO₄.     

Semiconductivity in Acceptor‐Doped BaTi1−xHoxO3−x/2−δ/2

Acceptor‐doped BaTiO₃ powders of formula: BaTi_1?xHo_xO_3?x/2?δ/2: x = 0.0001, 0.001, 0.01, 0.03, and 0.07, were prepared by sol‐gel synthesis, fired at 800°C–1500°C and either quenched or slow‐cooled to room temperature. Electrical properties of ceramics depended on firing conditions, Ho content, and cooling rate. Pellets of all x values fired at 800°C–1000°C were insulating and, from the presence of OH bands in the IR spectra, charge balance appeared to involve co‐doping of Ho³⁺ and H⁺ ions without necessity for oxygen vacancy creation. At higher firing temperatures, OH bands were absent. Pellets fired at 1400°C in air and slow cooled were insulating for both low x (0.0001) and high x (0.07) but at intermediate x (0.001 and 0.01) passed through a resistivity minimum of 20–30 Ω cm at room temperature, attributed to the presence of Ti³⁺ ions; it is suggested that, for these dilute Ho contents, each oxygen vacancy is charge compensated by one Ho³⁺ and one Ti³⁺ ion. At higher x, charge compensation is by Ho³⁺ ions and samples are insulating. A second, more general mechanism to generate Ti³⁺ ions, and a modest level of semiconductivity, involves reversible oxygen loss at high temperatures.     

Dielectric and Magnetic Properties of Sr(Fe1/2Ta1/2)O3 Complex Perovskite Ceramics

The dielectric and magnetic properties of Sr(Fe_1/2Ta_1/2)O₃ complex perovskite ceramics were systematically investigated together with the structure. The X‐ray powder diffraction analysis confirmed a B‐site disordered orthorhombic structure in space group Pbnm. Only one broadened dielectric peak with strong frequency dispersion was observed in the present ceramics, which was significantly different from that for the analogue Ba(Fe_1/2Nb_1/2)O₃ and Ba(Fe_1/2Ta_1/2)O₃. The strong dependence of sample thickness and electrode material indicated that the dielectric relaxation behavior at lower frequency was due to the interface effects. The present ceramics were spin glass state with slight ferromagnetic behavior below the Néel temperature (20 K). The co‐presence of Fe³⁺ and Fe²⁺ was confirmed by the μ_eff value.     

Enhancing giant dielectric properties of Ta5+-doped Na1/2Y1/2Cu3Ti4O12 ceramics by engineering grain and grain boundary

Various strategies to improve the dielectric properties of ACu₃Ti₄O₁₂ (A = Sr, Ca, Ba, Cd, and Na_1/2Bi_1/2) ceramics have widely been investigated. However, the reduction in the loss tangent (tanδ) is usually accompanied by the decreased dielectric permittivity (ε′), or vice versa. Herein, we report a route to considerably increase ε′ with a simultaneous reduction in tanδ in Ta⁵⁺–doped Na_1/2Y_1/2Cu₃Ti₄O₁₂ (NYCTO) ceramics. Dense microstructures with segregation of Cu– and Ta–rich phases along the grain boundaries (GBs) and slightly increased mean grain size were observed. The samples prepared via solid-state reaction displayed an increase in ε′ by more than a factor of 3, whereas tanδ was significantly reduced by an order of magnitude. The GB–conduction activation energy and resistance raised due to the segregation of Cu/Ta–rich phases along the GBs, resulting in a decreased tanδ. Concurrently, the grain–conduction activation energy and grain resistance of the NYCTO ceramics were reduced by Ta⁵⁺ doping ions owing to the increased Cu⁺/Cu²⁺, Cu³⁺/Cu²⁺, and Ti³⁺/Ti⁴⁺ ratios, resulting in enhanced interfacial polarization and ε′. The effects of Ta⁵⁺ dopant on the giant dielectric response and electrical properties of the grain and GBs were described based on the Maxwell–Wagner polarization at the insulating GB interface, following the internal barrier layer capacitor model.     

Extended X-ray absorption fine structure,X-ray diffraction and Raman analysis of nickel-doped Ba(Mg1/3Ta2/3)O3

Raman, X-ray diffraction and extended X-ray absorption fine structure (EXAFS) measurements of xBa(Ni_1/3Ta_2/3)O₃ + (1 − x)Ba(Mg_1/3Ta_2/3)O₃ samples with x = 0–0.03 were performed to reveal the nickel doping effect on the microwave properties. EXAFS result clearly shows that the nickel is located on the Mg lattice site. We also found that, as the nickel concentration increases, microwave dielectric constant decreases with the TaO and NiO bond distances. X-ray diffraction shows that the 1:2 ordered structure is degraded with the increasing of nickel concentration. The stretching phonon of the TaO₆ octahedra, that is A_1g(O) phonon near 800 cm⁻¹, are strongly correlated to the microwave properties of xBa(Ni_1/3Mg_2/3)O₃ + (1 − x)Ba(Mg_1/3Ta_2/3)O₃ samples. The large Raman shift and the large width of the A_1g(O) imply rigid but distorted oxygen octahedral structure, therefore, the effect of nickel doping lowers the dielectric constant and the Q × f value of Ba(Mg_1/3Ta_2/3)O₃ ceramic.     

Processing of Dense ζ‐Ta4C3−x by Reaction Sintering of Ta and TaC Powder Mixture

Tantalum carbides are commonly processed by hot pressing, canned hot‐isostatic‐pressing, or spark plasma sintering because of their high melting temperatures and low diffusivities. This paper reports processing of dense ζ‐Ta₄C_3?x by reaction sintering of a Ta and TaC powder mixture (C/Ta atomic ratio = 0.66). ζ‐Ta₄C_3?x is of interest due to its rhombohedral (trigonal) crystal structure that may be characterized as a polytype with both face‐centered‐cubic and hexagonal‐close‐packed Ta stacking sequences interrupted by stacking faults and missing carbon layers. This structure leads to easy cleaving on the basal planes and high fracture toughness. A key step in processing is the hydrogenation of the Ta powder to produce β‐TaH_x, a hard and brittle phase that enables efficient comminution during milling and production of small, equiaxed Ta particles that can be packed to high green density with the TaC powder. Studies of phase evolution by quantitative X‐ray diffraction during sintering revealed several intermediate reactions: (1) decomposition of β‐TaH_x to Ta; (2) diffusion of C from γ‐TaC to Ta leading to the formation of α‐Ta₂C_y' with the kinetics described by the Avrami equation with an exponent, n = 0.5, and an activation energy of 219 kJ/mole; (3) equilibration of α‐Ta₂C_y' and γ‐TaC_0.78 phases; and (4) formation of ζ‐Ta₄C_2.56 from the equilibrated α‐Ta₂C and γ‐TaC_0.78 phases with the kinetics characterized by a higher Avrami exponent (n ≈ 3) and higher activation energy (1007 kJ/mole). The sintered material contained ~0.86 weight fraction ζ‐Ta₄C_2.56 and ~0.14 weight fraction γ‐TaC_0.78 phases. The microstructure showed evidence of nucleation and growth of the ζ‐Ta₄C_2.56 phase in both the α‐Ta₂C and γ‐TaC_0.78 parent phases with distinct difference in the morphology due to the different number of variants of the habit plane.     

ζ‐Ta4C3−x: A High Fracture Toughness Carbide with Rising‐Crack‐Growth‐Resistance (R‐Curve) Behavior

The microstructures and mechanical properties of tantalum carbides containing predominantly the ζ‐Ta₄C_3?x phase are compared with the properties of the monocarbide (γ‐TaC) and the hemicarbide (α‐Ta₂C) and two‐phase composites. It is shown that a Ta and γ‐TaC powder mixture corresponding to a C/Ta at. ratio of 0.66 can be hot‐pressed (1800°C, 2 h) to obtain ~95 wt% of ζ‐Ta₄C_3?x with a density of 98% of theoretical. This material has an attractive combination of high fracture toughness (13.8 ± 0.2 MPa√m) and fracture strength (759 ± 24 MPa) with modest hardness (5.6 ± 0.5 GPa). The fracture toughness and strength measured for this material were the highest among all the materials with C/Ta ratio ranging from 0.5 (hemicarbide) to 1.0 (monocarbide). It is also shown that a material containing 86 wt% ζ‐Ta₄C_3?x can be consolidated by pressureless sintering of a hydrogenated Ta and γ‐TaC powder mixture without significant drop in density (97% of theoretical) or mechanical properties (13.4 ± 0.2 MPa√m, 700 ± 20 MPa, 6.0 ± 0.4 GPa). Materials containing high weight fraction of the ζ‐Ta₄C_3?x phase exhibited rising crack‐growth‐resistance (R‐curve) behavior. Optical and scanning electron microscope observations suggested crack‐face bridging was the dominant toughening mechanism. The crack‐bridging ligaments were lamellae of the basal planes of the ζ‐Ta₄C_3?x phase produced by their easy cleavage. The thickness of the lamellae ranged from 40 to 2000 nm, significantly less than the grain size.     

Synthesis and characterization of mesoporous Ta2O5–TiO2 photocatalysts for water splitting

Two series of Ta₂O₅–TiO₂ photocatalysts (Ta:Ti = 4:1, 1:1 and 1:4) were prepared by sol–gel technique applying triblock copolymer of Pluronic P123 and were tested in platinized form (0.3 wt.%) in photodecomposition of water under ultraviolet and visible light (λ > 300 nm). It was found the mesoporous character of tantalum containing catalysts with relatively high surface area (100–130 m² g⁻¹) of these samples. However, higher concentration of TiO₂ in mixed oxides leads to the destruction of mesoporous character of synthesized photocatalysts. All samples were characterized with thermogravimetry, XRD, N₂ physisorption, DR-UV–vis and FTIR spectroscopy. The mixed oxides of Ta₂O₅–TiO₂ system showed much lower band-gap than pure Ta₂O₅ and relatively high activity in platinized state in photocatalytic hydrogen generation under visible. Doping of pure oxides and mixed systems with sulfur resulted in lowering of the band-gap values below 3 eV and much better activity in H₂ evolution reaction. Non-platinized photocatalysts showed activity in liquid phase cyclohexene photooxidation at 305 K.     

First‐Principle Calculation and Assignment for Vibrational Spectra of Ba(Mg1/2W1/2)O3 Microwave Dielectric Ceramic

Ba(Mg_1/2W_1/2)O₃ ceramic was synthesized using a conventional solid‐state reaction method at 1500°C for 4 h. The face‐centered cubic crystal structure of the material was confirmed by Rietveld refinement of X‐ray diffraction (XRD) data, and vibrational modes were obtained by Raman and Fourier transform far‐infrared (FTIR) reflection spectroscopies. First‐principle calculations based on density functional theory with local density approximation were used to calculate Gamma‐point modes and dielectric properties of Ba(Mg_1/2W_1/2)O₃. The Raman spectrum with nine active modes can be fitted with Lorentzian function, and the modes were assigned as F_2g⁽¹⁾ (126 cm^?1), F_2g⁽²⁾ (441 cm^?1), E_g(O) (538 cm^?1), and A_1g(O) (812 cm^?1). Far‐infrared spectrum with 12 infrared active modes was fitted using both the Lorenz three‐parameter classical and four‐parameter semiquantum models. Consequently, the modes were assigned as F_1u⁽¹⁾ (144 cm^?1), F_1u⁽²⁾ (284 cm^?1), F_1u⁽³⁾ (330–468 cm^?1), and F_1u⁽⁴⁾ (593–678 cm^?1). The active modes were represented by linear combinations of symmetry coordinates that were obtained by group theory analyses. The Raman mode A_1g, which has the highest wave number (812 cm^?1) is dominated by the breath vibration of the MgO₆ octahedron. The infrared modes F_1u⁽²⁾, that can be described as the inverted vibrations of Mg atoms in the MgO₆ octahedron along the x_i, y_i, and z_i axes have the most contributions to the microwave permittivity and dielectric loss.     

Improving piezoelectric properties and temperature stability for KNN‐based ceramics sintered in a reducing atmosphere

Lead‐free 0.955K_0.5Na_0.5Nb_1‐zTa_zO₃‐0.045Bi_0.5Na_0.5ZrO₃+0.4%MnO ceramics (abbreviated as KNNTaz‐0.045BNZ+0.4Mn) were prepared by a conventional solid‐state sintering method in a reducing atmosphere (oxygen partial pressure of 1 × 10^?10 atm). All ceramics with a pure perovskite structure show the two‐phase coexistence zone composed of rhombohedral and tetragonal phase. Ta⁵⁺ ions substitute for Nb⁵⁺ ions on the B‐site, which results in a decrease in the R phase fraction in the two‐phase coexistence zone. The R‐T phase transition temperature moves to room temperature due to the substitution of Nb⁵⁺ ions by Ta⁵⁺ ions. A complex domain structure composed of small nano‐domains (~70 nm) formed inside large submicron domains (~200 nm) exists in KNNTa0.02‐0.045BNZ+0.4Mn ceramics, which can induce a strong dielectric‐diffused behavior and improve the piezoelectric properties. The temperature stability for the reverse piezoelectric constant for the KNNTaz‐0.045BNZ+0.4Mn ceramics can be improved at z = 0.02. Excellent piezoelectric properties (d₃₃ = 328 pC/N, and  = 475 pm/V at E_max = 20 kV/cm) were obtained for the KNNTa_0.02‐0.045BNZ+0.4Mn ceramics.     

p‐Type Optoelectronic and Transparent Conducting Oxide Properties of Delafossite CuAl1/2Fe1/2O2

CuAl_1/2Fe_1/2O₂ delafossite was prepared using a solid‐state reaction method to investigate its optical and electronic transport properties. CuAl_1/2Fe_1/2O₂ formed a hexagonal delafossite structure with an Rm space group. The positive Seebeck coefficient and the direct optical gap of 3.6 eV confirmed that the CuAl_1/2Fe_1/2O₂ delafossite in a p‐type transparent conducting oxide. The fluorescence emission at 390 nm (green emission) confirmed that CuAl_1/2Fe_1/2O₂ has a direct transition band gap. Thermogravimetric analysis indicated a weight loss of 1.2%, caused by the intercalation of O atoms, which produced hole carriers from the different ionic radii at the B sites. The electric conductivity at room temperature was thermally activated, as predicted by the small‐polaron hopping mechanism, with an activation energy of 75 meV and a charge transport energy of 61 meV. CuAl_1/2Fe_1/2O₂ delafossite exhibited p‐type optoelectronic behavior and is a transparent conducting oxide, which may be crucial in the p‐type photonic and electrode industries.     

Thermal and optical properties of La2O3–Ga2O3– (Nb2O5 or Ta2O5) ternary glasses

La₂O₃–Ga₂O₃–M₂O₅ (M = Nb or Ta) ternary glasses were fabricated using an aerodynamic levitation technique, and their glass‐forming regions and thermal and optical properties were investigated. Incorporation of adequate amounts of Nb₂O₅ and Ta₂O₅ drastically improved the thermal stabilities of the glasses against crystallization. Optical transmittance measurements revealed that all the glasses were transparent over a wide wavelength range from the ultraviolet to the mid‐infrared. The refractive indices of the glasses increased and the Abbe number decreased upon substituting Ga₂O₃ with Nb₂O₅, and the decrease in the Abbe number was significantly suppressed when Ta₂O₅ was incorporated into the glass. As a result, excellent compatibility between high refractive index and lower wavelength dispersion was realized in La₂O₃–Ga₂O₃–Ta₂O₅ glasses. Analysis based on the single‐oscillator Drude–Voigt model provided more systematical information and revealed that this compatibility was due to an increase in the electron density of the glass.     

Microwave properties of Ba(Mg1/3Ta2/3)O3, Ba(Mg1/3Nb2/3)O3 and Ba(Co1/3Nb2/3)O3 ceramics revealed by Raman scattering

Five Ba(Co_1/3Nb_2/3)O₃ samples sintered at different temperatures (form 1350 to 1550 °C), one Ba(Mg_1/3Ta_2/3)O₃ and a Ba(Mg_1/3Nb_2/3)O₃ sample were examined by Raman scattering to reveal the correlation of the 1:2 ordered perovskite structure with the microwave properties, such as dielectric constant and Q factors. The Ba(Co_1/3Nb_2/3)O₃ sample sintered at 1400 °C, which possesses the highest microwave Q value and the lowest dielectric constant among five Ba(Co_1/3Nb_2/3)O₃ samples, has the narrowest width and the highest frequency of the stretch mode of oxygen octahedron (i.e. A_1g(O) near 800 cm⁻¹). We found that the dielectric constant is strongly correlated with the Raman shift of A_1g(O) stretch modes, and the width of A_1g(O) stretch mode reflects the quality factor Q × f value in the 1:2 ordered perovskite materials. This concludes that the oxygen octahedron play an important role of the material's microwave performance. Based on the results of Q × f values and the lineshapes of A_1g(O) stretch mode, we found that the propagation of microwave energy in Ba(Mg_1/3Ta_2/3)O₃ and Ba(Mg_1/3Nb_2/3)O₃ shows weak damping behavior, however, Ba(Co_1/3Nb_2/3)O₃ samples sintered at different temperature exhibit heavily damped behavior.     

Enhanced photocatalytic activity of nonstoichiometric crystalline TaO2F and Ta2O5 with carbon coating

Pure and carbon-coated tantalum-based oxides photocatalysts were synthesized via the mesocrystalline precursor transformation method by annealing pure and polydopamine-coated (NH₄)₂Ta₂O₃F₆ mesocrystals in Ar. The oxygen-poor atmosphere thermal annealing process assisted the formation of nonstoichiometric TaO₂F mesocrystals with more F and Ta₂O₅ nanorods with oxygen vacancies and the associated lower valence state Ta ions (Ta⁴⁺). Furthermore, the carbon coating, decomposed from coated polydopamine, helped to control their particle size within 100 nm by isolating the connection of (NH₄)₂Ta₂O₃F₆ subunits. Hence, as-synthesized products, particularly carbon-coated Ta₂O₅ nanosheets, owning large surface area (67.6 m² g^?1), fine particle size (<100 nm), excellent electronic conductivity, decreased bandgap energy, enhanced and extended absorption in the visible range, exhibited preferable photocatalytic activity in the photodegradation of methylene blue, reaching a 76.54 % and 41.71 % removal under ultraviolet and visible light illumination, suggesting a promising candidate for wide-range responsive photocatalytic applications.     

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.     

Achieving enhanced densification and superior ionic conductivity of garnet electrolytes via a co-doping strategy coupled with pressureless sintering

Lithium garnet oxides with 6.5 mol Li, such as Li_6.5La₃Zr_1.5(Ta/Nb)_0.5O₁₂, typically crystallise in cubic structure and exhibit excellent room-temperature ionic conductivity close to 1 mS cm^?1. However, it is challenging to densify garnet oxides. In this work, we investigated how the co-doping of tantalum (Ta) and niobium (Nb) affects the densification of pressureless sintered garnet electrolytes with compositions of Li_6.5La₃Zr_1.5Ta_(0.5?x)Nb_xO₁₂, where x = 0–0.5. The highest densification (94.5% of relative density) was achieved in Li_6.5La₃Zr_1.5Ta_0.1Nb_0.4O₁₂ (TN-LLZO) when it was sintered at 1150 °C for 6 h. This TN-LLZO garnet electrolyte delivers an ionic conductivity of 1.04 × 10^?3 S cm^?1 (at 22 °C) with a low activation energy of 0.41 eV. Our findings demonstrate that the content of dopants (Ta and Nb) plays a critical role in enhancing the sintering performance of garnet ceramics at ambient pressure.