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In this work, [KNbO3]0.98-[A(Ni1/2Sn1/2)O3]0.02(A = Ba, Sr, Ca) photocatalytic materials were prepared using a solid-state method. The results show that changing the A-site doping element while keeping the B-site doping element constant leads to systematic variations in the band structure of KNbO3-based materials. The analysis reveals that as the ionic radius of the A-site dopant decreases, the conduction band and valence band of the samples exhibit an overall negative shift along the energy axis. The more negative conduction band energy level is favorable for electron reduction and hydrogen gas production. The hydrogen evolution rates of [KNbO3]0.98-[A(Ni1/2Sn1/2)O3]0.02(A = Ba, Sr, Ca) under simulated sunlight are 165.798 µmol/(g·h) (A = Ca), 151.697 µmol/(g·h) (A = Sr) and 119.487 µmol/(g·h) (A = Ba), respectively. [ABSTRACT FROM AUTHOR]

The inversion asymmetry of polar crystals enables ferroelectric ceramics to possess unique physical properties, among which the anomalous photovoltaic effect drives their potential application in photovoltaic conversion. Semiconducting (1−x) KNbO3−xBaNi.5Hf.5O3−δ (KNBNH, x = 2, 4, 6, 8%) ferroelectric ceramics with enhanced photovoltaic performance were prepared by utilizing conventional solid‐state sintering strategies. In these ceramics, the defect‐induced bandgap intermediate state is derived from the 3d split state of Ni and plays a dominant role in lowering the bandgap of KN. The defective bandgap state induced by Ni in KNBNH promotes its absorption of light and the separation of photogenerated carriers, thus enhancing its photovoltaic response. The KNBNH6 shows a maximum value of 138.7 nA/cm2 for short‐circuit photocurrent density (Jsc) among these ceramics, which is further enhanced to 702.9 nA/cm2 after 30 kV/cm polarization. Structural investigations after polarization indicate that polarization induced lattice distortion in KNBNH6, leading to an increase in the polarity of its cells. This work provides an understanding of defect‐induced bandgap states and high‐field polarization to enhance ferroelectric photovoltaic properties. [ABSTRACT FROM AUTHOR]

Ti4+-ion-doped (n + 1)SrO − nCeO2 (n = 2) ceramic systems were prepared with the conventional solid-state reaction method, and the effects of the phase structures and compositions, sintering behaviors, microstructures and microwave dielectric properties of these ceramic systems were investigated in detail as a function of TiO2 content. The analytical results of the XRD patterns show that the pure (n + 1)SrO − nCeO2 (n = 2) system is a composite-phase ceramic system with coexisting SrCeO3 and Sr2CeO4 phases (represented as a SrCeO3 + Sr2CeO4 system), which belong to the orthogonal structures of the Pmcn (62) and Pbam (55) space groups, respectively. For the xTiO2-(1 − x) (SrCeO3 + Sr2CeO4) (x = 0.1–0.4) ceramic samples, the secondary phase Sr2TiO4 can also be detected within the range of the investigated components. Meanwhile, the Raman spectroscopy, SEM-EDS, and HRTEM (SAED) analysis results also verified the correctness and consistency of the phase structures and compositions for all the given specimens. In addition, complex impedance spectroscopy was used to detect the conductive behavior of these compound ceramic systems, and the calculation results show that the appropriate addition of Ti4+-ions can make the SrCeO3 + Sr2CeO4 system have better thermal stability. The composition of x = 0.2 multiphase structural ceramic sample sintered at 1330 °C for 4 h has a near zero τf value of ~−4.6 ppm/°C, a moderate εr of ~40.3 and a higher Q × f~44,020 GHz (at 6.56 GHz). The relatively superior-performing ceramics developed in this work are expected to provide a promising microwave dielectric material for communication components. [ABSTRACT FROM AUTHOR]

Dielectric capacitors have attracted extensive attention due to their high power density along with fast charge/discharge rate. Despite the high energy storage performance were obtained in lead-based ceramics, we still need to find lead-free ceramic alternatives considering the environmental requirements, and AgNbO3 has received extensive attention owing to its ferroelectric–antiferroelectric (FE–AFE) phase transition. In our study, the AFE properties of the samples were improved by tuning the grain size and polarizability of ions, and excellent energy storage performance was obtained in Bi/Ta co-doped AgNbO3. The BANT ceramic exhibited a remarkably enhanced recoverable energy density of 3.9 J/cm3 and acceptable efficiency of 61%. Morphological and structural analyses and electrical properties revealed that this co-substitution can optimize the AFE properties of the material, which can be demonstrated by the slim P–E loops and the reduced grain sizes. Of note, the BANT6 ceramic exhibited fast discharge speed accompanied by the actual energy storage density of up to 3.5 J/cm3 under 260 kV/cm. These findings indicate that AgNbO3-based AFE ceramics are a prospective alternative for energy storage materials applied to high-pulse-power fields. [ABSTRACT FROM AUTHOR]

A depolarized electric field (Edp) of ferroelectrics to boost the carrier separation and migration rate is critical to promote the field of visible light photocatalytic hydrogen evolution (PHE). Here, the PHE performance of high-field poled semiconducting (1 − x)KNbO3–xBa(Ni1/2Nb1/2)O3−δ (abbreviated as (1 − x)KN-xBNN, x = 0, 0.02, 0.05, 0.10, and 0.20) ferroelectric ceramics were systematically studied under full spectrum (λ > 320 nm) and visible light (λ > 420 nm), respectively. Benefiting from their narrow bandgap and sub-bandgap, all the Ba, Ni-modified KN ceramic powders exhibited a PHE response under visible light irradiation. Notably, the visible light PHE rate of the high-field poled 0.95KN–0.05BNN ceramic powder can be promoted to 415.16 μmol g−1 h−1, which is 17.8-fold higher than that of the unpoled sample (23.26 μmol g−1 h−1). The boosting PHE rate in poled samples can be attributed to the high-field polarization of the material, which can significantly affect the lattice structure and cause an ordered ferroelectric-domain structure to form a depolarized electric field (Edp). EPR spectroscopy indicated that the poled 0.95KN–0.05BNN sample exhibited a superior catalytic activity of ˙OH evolution compared with its unpoled counterpart, suggesting the dependence of high photocatalytic performance on the unique field-induced polarization structure by an outer electric field. Moreover, a higher photo-excited transient photocurrent in the poled 0.95KN–0.05BNN specimen further proved the positive effect of Edp on the photovoltaic response. The formation of Edp in the semiconducting (1 − x)KN–xBNN ferroelectric favors the separation and transportation of photogenerated carriers. Thus, the synergistic effect of narrow bandgap and high-field polarization ability is the key to enhance the visible light photocatalysis of semiconducting ferroelectrics. Boosting the visible light photoelectrochemical catalytic activity of semiconducting (1 − x)KN–xBNN ferroelectrics through high-field polarization may provide a useful strategy to design new ferroelectric-based semiconductor materials. [ABSTRACT FROM AUTHOR]

In the process of exploring ferroelectric semiconductors, a new system of (1−x) KNbO3–xSrFeO3−δ (x = 0.00‐0.20) was successfully synthesized via solid‐state reaction. The crystal structures, ferroelectric, dielectric, optical, and electrical properties were systematically characterized. The orthorhombic phase with Amm2 space group is detected in all the ceramics. In addition, the orthorhombic and tetragonal phases coexist in 0.80KNbO3‐0.20SrFeO3‐δ ceramic. The decrease in oxygen octahedron distortion induces a weak ferroelectric polarization. The existence of long‐range ferroelectric polarization order in all the ceramics is verified and the bandgap of the ceramics can be tuned to ~2.18 eV. The improved short‐circuit photocurrent density (Jsc) and open‐circuit voltage (Voc) of the poled 0.95KNbO3‐0.05SrFeO3−δ ceramic at 30 kV/cm are ~6.90 nA/cm2 and 0.04 V, respectively. The activation energies for electrical conductivity of the grains and grain boundaries from 0.90KN–0.10SF ceramic are 0.67 and 0.77 eV, respectively, which indicate the doubly ionized oxygen vacancies. This work provides a new way to tune the optical bandgap/ferroelectric properties of KNbO3‐based ceramics for potential application in ferroelectric photovoltaic and energy fields. [ABSTRACT FROM AUTHOR]

Perovskite-oxide (1 − x)KNbO3-xBaCo1/2Nb1/2O3−δ (KN-BCN; x = 0.00–0.20) ferroelectric semiconductor ceramics with oxygen defects are successfully prepared via a conventional solid-state sintering method. X-ray diffraction data indicate that the crystal symmetry evolves from orthogonal to tetragonal at increasing x values. Raman spectroscopic analysis confirms the long-range polarization of all compositions. X-ray photoelectron spectroscopy shows that the detailed chemical formula of 0.90KN-0.10BCN ceramics is 0.90KNbO3-0.10BaCo1/2Nb1/2O2.90. Room-temperature ferroelectricity weakens when the x value increases. The optical band gap narrows from 3.25 eV for x = 0.00 to 1.57 eV for x = 0.20, and the minimum value of ∼ 1.28 eV occurs in the 0.90KN-0.10BCN ceramic. Impedance analysis illustrates that the conduction mechanism of grains is mainly internal electron conduction, and that of the grain boundary is intrinsic conduction. The conducting mechanism of the ceramic system follows ohmic behavior by log I–log U curves. The maximum short-circuit photocurrent density and open-circuit photovoltage are 6.68 nA cm−2 and 0.80 V, and stable output is maintained. The KN-BCN ceramic system can be used in photovoltaic materials. [ABSTRACT FROM AUTHOR]

(1 − x) (Bi0.49Na0.49Ca0.02TiO3)-xCaHfO3 (BCNTH-x, x = 0.01, 0.02, 0.03, 0.035, 0.04) systems were prepared by solid-state reaction method. Their microstructures, dielectric/piezoelectric/ferroelectric properties and impedance spectroscopy were investigated. All compositions demonstrated a single orthorhombic structure with Pbnm space group. Temperature-dependent permittivity curves displayed one anomaly, which shifts to low temperature from 278 to 208 °C with increased CaHfO3 content, because of the replacement of Hf4+ for Ti4+ at the B-site that reduced B-site cation ordering. Analysis of dielectric and ferroelectric properties indicated that a crossover from typical ferroelectrics to relaxor ferroelectrics occurred in BCNTH-0.03 and BCNTH-0.035 ceramics. The largest electrostrictive strain of 0.22% and the lowest coercive field of 37.7 kV/cm were achieved in BCNTH-0.035 and BCNTH-0.03 ceramics under 100 kV/cm, respectively. Ferroelectric domain and domain wall were fitted by impedance complexes, and the corresponding activation energy was 0.46 and 1.18 eV, which may be correlated to the combination of motion of the first ionization of oxygen vacancies and doubly ionized oxygen vacancies, respectively. [ABSTRACT FROM AUTHOR]

Na5RE(MoO4)4 (RE = La, Gd, Dy, Er) ceramics have been synthesized by the conventional solid-state reaction method. The crystallizations, microstructures and microwave dielectric properties of Na5RE(MoO4)4 ceramics were also investigated. The room-temperature x-ray diffraction patterns of Na5RE (MoO4)4 ceramics illustrate a tetragonal structure with a I 41/a space group. The grain size of Na5La(MoO4)4 is larger than that of Na5RE(MoO4)4 (RE = Gd, Dy, Er) ceramics, and uneven grains are observed in the systems. Na5RE (MoO4)4 ceramics sintered at 570-690°C for 4 h possess excellent microwave dielectric properties with εr = 6.4-8.4, τf = − 27.7-34.3 ppm/°C, and Q × f = 4073-24411 GHz at their optimum sintering temperature. The nonreactivity of Na5Er(MoO4)4 with silver is evidenced by x-ray diffraction, and SEM and EDS analysis reveal that silver can be a co-firable electrode material for the present ceramics. [ABSTRACT FROM AUTHOR]

xSr0.7Ce0.2TiO3–(1 − x)Sr(Mg1/3Nb2/3)O3 ceramics, referred to xSCT–(1 − x)SMN, were successfully produced by conventional solid-state sintered technology. The compounds, belonging to perovskites with a secondary phase of CeO2, can be detected even with x down to 0.1 of SCT composition. The overall trend for grain growth illustrates the increase with increasing SCT doping level. The Raman peak at 825 cm−1 splits into two peaks and causes red shift phenomenon. XPS spectra indicate that Ti and Nb ions exist respectively in tetravalence and pentavalence, and Ce ions exist in trivalence and tetravalence. Dielectrics constant (εr) of SCT–SMN ceramics gradually increases with increasing theoretical dielectric polarizabilities. A wider width of the 825 cm−1 for FWHM of A1g mode Raman peaks suggests to a lower Q × f value. The increasing tolerance factor in agreement with temperature coefficient of resonant frequency (τf), denotes that the rise of perovskite symmetry. The 0.1SCT–0.9SMN ceramic sintered at 1450 °C for 4 h illustrates excellent microwave dielectric properties with εr ~ 35.4, Q × f ~ 11282 GHz and τf ~ 1.7 ppm/°C. Activation energies of 0.1SCT–0.9SMN ceramic at 100, 300 and 500 V, are ~0.436, 0.427 and 0.331 eV, respectively, indicative of a decreased trend with external electric field. [ABSTRACT FROM AUTHOR]