La substitution in four-layers Aurivillius phase SrBi4Ti4O15

Ceramics with the nominal composition of SrBi_4−xLa_xTi₄O₁₅ have been prepared within the range of 0≤x≤1.8, and the dielectric properties are investigated. Single phase SrBi_4−xLa_xTi₄O₁₅ solid solution exists until the secondary phase of La_2/3TiO₃ appears at x=1.6. The Curie temperature is 520 °C for pure SrBi₄Ti₄O₁₅, and it shifts to low temperature with increasing x, which is due to the smaller structural distortion caused by La³⁺ substitution. In addition, the dielectric constant anomaly is suppressed with the substitution. No dielectric relaxation behavior is observed. When x≥1.2, the paraelectric state is attained in the present ceramics. La³⁺ substitution effects on the SrBi₄Ti₄O₁₅ ceramics are also compared with the previous work on Bi_4−xLa_xTi₃O₁₂.     

Compositional dependence of ferroelectric properties of highly c-axis oriented Bi4−xNdxTi3O12 film capacitors

Highly c-axis oriented neodymium-modified bismuth titanate (Bi_4−xNd_xTi₃O₁₂) films having a variety of neodymium (Nd) contents were successfully grown on Pt/TiO₂/SiO₂/Si(100) substrates using metal-organic sol decomposition. After systematically examining ferroelectric properties of the c-axis oriented Bi_4−xNd_xTi₃O₁₂ film capacitors as a function of the Nd-content, we concluded that the capacitor with x=0.85 had the largest remanent polarization. The Bi_3.15Nd_0.85Ti₃O₁₂ capacitor fabricated using a top Pt electrode showed well-saturated polarization-electric field (P-E) switching curves with the remanent polarization (P_r) of 51 μC/cm² and the coercive field (E_c) of 99 kV/cm at an applied voltage of 10 V. More importantly, the Pt/Bi_3.15Nd_0.85Ti₃O₁₂/Pt capacitor exhibited fatigue-free behavior up to 4.5×10¹⁰ read/write switching cycles at a frequency of 1 MHz. The capacitor also demonstrated an excellent charge-retaining ability and a strong resistance against the imprinting failure.     

Ferroelectric properties of Bi3.25−x/3La0.75Ti3−xNbxO12 films prepared by pulsed laser deposition

Bi₄Ti₃O₁₂ (BiT), Bi_3.25La_0.75Ti₃O₁₂ (BLT), Bi_4−x/3Ti_3−xNb_xO₁₂ (BTN) and Bi_3.25−x/3La_0.75Ti_3−xNb_xO₁₂ (BLTN) thin films have been prepared by pulsed laser deposition. BTN and BLTN films exhibit a maximum in the remanent polarization P_r at a Nb content x=0.018. At this Nb content, the BLTN film has a P_r value (25 μC/cm²) that is much higher than that of BiT and a coercive field similar to that of BiT. The polarization of this BLTN film is fatigue-free up to 10⁹ switching cycles. The high fatigue resistance is mainly due to the substitution of Bi³⁺ ions by La³⁺ ions at the A site and the enhanced P_r arises largely from the replacement of Ti⁴⁺ ions by Nb⁵⁺ ions at the B site. The mechanisms behind the effects of the substitution at the two sites are discussed.     

Properties of vanadium-doped SrBi4Ti4O15 ferroelectric ceramics

Using the standard solid-state reaction method, several vanadium-doped ferroelectric ceramics of type SrBi_4−x/3Ti_4−xV_xO₁₅ (SBTV−x) were synthesized. The vanadium doping content, x, rangs from 0.00 to 0.06. The crystal structure of SrBi₄Ti₄O₁₅ is not affected by V-doping. The electric breakdown voltage of the samples increases with V content. Meanwhile, V-doping results in a notable enlargement of remnant polarization (2P_r). The 2P_r of STBV−0.03 reaches a very large value, which is over 50 μC/cm² and is nearly twice greater than that at zero doping. The Curie temperatures of V-doped samples decrease slightly in comparison with that of SrBi₄Ti₄O₁₅. V-doping can improve the electric properties of SrBi₄Ti₄O₁₅ without sacrificing its thermal stableness.     

Properties of vanadium-doped SrBi4Ti4O15 ferroelectric ceramics

Using the standard solid-state reaction method, several vanadium-doped ferroelectric ceramics of type SrBi_4−x/3Ti_4−xV_xO₁₅ (SBTV-x) were synthesized. The vanadium doping content, x, ranges from 0.000 to 0.06. The crystal structure of SrBi₄Ti₄O₁₅ is not affected by V-doping. The electric breakdown voltage of the samples increases with V content. Meanwhile, V-doping results in a notable enlargement of remnant polarization (2P_r). The 2P_r of STBV-0.03 reaches a very large value, which is over 50 μC/cm² and is nearly twice greater than that at zero doping. The Curie temperatures of V-doped samples decrease slightly in comparison with that of SrBi₄Ti₄O₁₅. V-doping can improve the electric properties of SrBi₄Ti₄O₁₅ without sacrificing its thermal stableness.     

La掺杂SrBi4Ti4O15铁电材料性能研究

按x=0.00，0.10，0.25，0.50，0.75和1.00，采用固相烧结工艺，制备了不同La掺杂量的SrBi_4-xLa_xTi₄O₁₅的陶瓷样品. 用x射线衍射对其微结构进行了分析，并测量了铁电、介电性能.结果发现，La掺杂未改变SrBi₄Ti ₄O₁₅的晶体结构.随掺杂量的增加，样品的矫顽场(E_c)下降，剩余极化(2P_{关键词： 4-x}La_xTi₄O₁₅')" href="#">SrBi_4-xLa_xTi₄O₁₅ La掺杂 铁电性能 相变温度 弛豫铁电     

The size and strain effects on the Raman spectra of Ce1−xNdxO2−δ (0≤x≤0.25) nanopowders

Ultrafine Ce_1−xNd_xO_2−δ (x=0-0.25) powders were synthesized by self-propagating room temperature synthesis. Raman spectra were measured at room temperature in the 300-700 cm⁻¹ spectral range. The shift and asymmetric broadening of the Raman F_2g mode at about 454 cm⁻¹ in pure and doped ceria samples could be explained with combined size and inhomogenous strain effects. Increased concentration of O²⁻ vacancies with doping is followed by an appearance of new Raman feature at about 545 cm⁻¹.     

Effects of vanadium doping on structure and electrical properties of SrBi4Ti4O15 thin films

The effects of vanadium(V) doping into SrBi₄Ti₄O₁₅ (SBTi) thin films on the structure, ferroelectric, leakage current, dielectric, and fatigue properties have been studied. X-ray diffraction result showed that the crystal structure of the SBTi thin films with and without vanadium is the same. Enhanced ferroelectricity was observed in the V-doped SrBi₄Ti₄O₁₅ (SrBi_4−x/3Ti_4−xV_xO₁₅, SBTiV-x (x = 0.03, 0.06, and 0.09)) thin films compared to the pure SrBi₄Ti₄O₁₅ thin film. The values of remnant polarization (2P_r) and coercive field (2E_c) of the SBTiV-0.09 thin film capacitor were 40.9 μC/cm² and 105.6 kV/cm at an applied electric field of 187.5 kV/cm, respectively. The 2P_r value is over five times larger than that of the pure SBTi thin film capacitor. At 100 kHz, the values of dielectric constant and dielectric loss were 449 and 0.04, and 214 and 0.06 for the SBTiV-0.09 and the pure SBTi thin film capacitors, respectively. The leakage current density of the SBTiV-0.09 thin film capacitor measured at 100 kV/cm was 6.8 × 10⁻⁹ A/cm², which is more than two and a half orders of magnitude lower than that of the pure SBTi thin film capacitor. Furthermore, the SBTiV-0.09 thin film exhibited good fatigue endurance up to 10¹⁰ switching cycles. The improved electrical properties may be related to the reduction of internal defects such as bismuth and oxygen vacancies with changes in the grain size by doping of vanadium into SBTi.     

Influence of mobile space charges on electrical properties of ferroelectric Bi3.5La0.5Ti3O12 thin films

Lanthanum-substituted bismuth titanate, Bi_3.5La_0.5Ti₃O₁₂ (i.e., x=0.5 in Bi_4−xLa_xTi₃O₁₂), thin films have been grown on Pt/Ti/SiO₂/Si substrates using pulsed laser deposition. The frequency dependence of the real part ε′(ω) and the imaginary part ε″(ω) of the dielectric constant has been studied. The ε′(ω) does not show any sudden change within the frequency range of 10²-10⁶ Hz. In contrast, the ε″(ω) shows a large dispersion as frequency decreases. The observed relaxation behavior in ε″(ω) can be explained in terms of a migration of oxygen vacancies in (Bi₂O₂)²⁺ layers, not in Bi₂Ti₃O₁₀ perovskite layers.     

Ferroelectric properties of highly c-axis oriented Bi4−xLaxTi3O12 film-based capacitors

Highly c-axis oriented lanthanum-modified bismuth titanate (Bi_4−xLa_xTi₃O₁₂) films having a variety of lanthanum (La) contents were grown on Pt/TiO₂/SiO₂/Si(100) substrates using metal-organic sol deposition and subsequent annealing at 650 °C for 1 h. After systematically examining the ferroelectric properties of Bi_4−xLa_xTi₃O₁₂ films as a function of the La-content, it was concluded that the film with x=0.85 had the largest remanent polarization in the direction parallel to the c-axis. The Pt/Bi_3.15La_0.85Ti₃O₁₂/Pt capacitor showed a well-saturated polarization-electric field (P-E) switching curve with the switching remanent polarization (2P_r) value of 33 μC/cm² and the coercive field (E_c) of 68 kV/cm at an applied voltage of 10 V. More importantly, the capacitor exhibited fatigue-free behavior up to 6.5×10¹⁰ read/write switching cycles at a frequency of 1 MHz. The capacitor also demonstrated an excellent charge-retaining ability and a strong resistance against the imprinting failure.     

Phase transformation behavior and dielectric characteristics of BaTi1−x(Al1/2Nb1/2)xO3 (0.01≤x≤0.40) ceramics

Microstructure, phase transformation behavior and dielectric properties of BaTi_1−x(Al_1/2Nb_1/2)_xO₃ (0.01≤x≤0.40) ceramics were investigated. A high level of (Al_1/2Nb_1/2)⁴⁺ substitution for Ti⁴⁺ ions was not conducive to the stability of the perovskite structure and resulted in the formation of BaAl₂O₄. As x was increased, lattice constants and unit cell volume decreased, reached a minimum at x=0.10 and then increased. The BaTi_1−x(Al_1/2Nb_1/2)_xO₃ ceramics at room temperature experienced a transformation from ferroelectric to paraelectric phase with increasing (Al_1/2Nb_1/2)⁴⁺ concentration. Meanwhile, permittivity of the BaTi_1−x(Al_1/2Nb_1/2)_xO₃ ceramics was markedly reduced, while Q value was slightly increased. Frequency dispersion of dielectric peak was obviously increased as x was increased from 0.01 to 0.10. It is of great interest that a dielectric abnormity represented by a broad dielectric peak at 200-400 K was observed for the composition with x=0.40.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Dielectric and piezoelectric properties of lead-free (Na0.5K0.5)NbO3-SrTiO3 ceramics

In this article, we report successful preparation of dense [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ (x=0.005-0.100) ceramics by ordinary sintering in air. The dependence of phase structure on doping content of SrO and TiO₂ has been determined by the X-ray diffraction technique. It was found that the crystal structure changed from orthorhombic to tetragonal at x≈0.040. Dielectric study revealed that the dielectric relaxor behavior was induced by doping of SrO and TiO₂ into (Na_0.5K_0.5)NbO₃. The samples in the composition range from x=0.005 to 0.020 exhibited excellent electrical properties, piezoelectric constant of electromechanical planar and thickness coupling coefficients of k_p=26.6-32.5% and k_t=39.8-43.8%. The results show that the [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ ceramics are one of the promising lead-free materials for electromechanical transducer applications.     

High-temperature thermoelectric properties of Ca3Co4O9+δ with Eu substitution

We have prepared polycrystalline Ca_3−xEu_xCo₄O_9+δ (x=0, 0.15, 0.3 and 0.45) samples using a sol-gel process followed by SPS sintering and investigated the Eu substitution effects on their high-temperature thermoelectric properties. With the Eu substitution, both the electrical resistivity and thermopower increase monotonously. This could be attributed to the decrease of hole concentrations by substitution of trivalent Eu³⁺ for divalent Ca²⁺. The Eu substituted samples (x=0.15, x=0.3) have lower thermal conductivity than Ca₃Co₄O_9+δ due to their lower electronic and lattice thermal conductivity. The dimensionless figure of merit ZT reaches 0.3 at 1000 K for the sample of Ca_2.7Eu_0.3Co₄O_9+δ.     

Dielectric properties of lanthanum-doped CaCu3Ti4O12 synthesized by semi-wet route

Effect of La³⁺ doping at Ca²⁺ site in CaCu₃Ti₄O₁₂ has been examined. Compositions with x=0.10, 0.20 and 0.30 were synthesized in the system Ca_(1−3x/2)La_xCu₃Ti₄O₁₂ by semi-wet method. Powder X-ray diffraction confirmed the formation of monophasic compounds. The structure remains cubic similar to CaCu₃Ti₄O₁₂. Lattice parameter increases slightly with increasing La³⁺ concentration. Microstructure has been studied using scanning electron microscopy (SEM). Average grain size is in the range 2-4 μm for various compositions. Energy-dispersive spectrometer (EDS) studies confirm the stoichiometry of the synthesized materials. Dielectric constant, dielectric loss and conductivity of the samples decrease with increasing lanthanum concentrations.     

Enhanced ferroelectric properties of (Pb0.90La0.10)Ti0.975O3 multilayered thin films prepared by RF magnetron sputtering

The (Pb_0.90La_0.10)Ti_0.975O₃/PbTiO₃ (PLT/PT), PbTiO₃/(Pb_0.90La_0.10)Ti_0.975O₃/PbTiO₃ (PT/PLT/PT) multilayered thin films with a PbO_x buffer layer were in situ deposited by RF magnetron sputtering at the substrate temperature of 600 °C. With this method, highly (1 0 0)-oriented PLT/PT and PT/PLT/PT multilayered thin films were obtained. The PbO_x buffer layer leads to the (1 0 0) orientation of the films. The dielectric, ferroelectric and pyroelectric properties of the PLT multilayered thin films were investigated. It is found that highly (1 0 0)-oriented PT/PLT/PT multilayered thin films possess higher remnant polarization 2P_r (44.1 μC/cm²) and better pyroelectric coefficient at room temperature p (p = 2.425 × 10⁻⁸ C/cm² K) than these of PLT and PLT/PT thin films. These results indicate that the design of the PT/PLT/PT multilayered thin films with a PbO_x buffer layer should be an effective way to enhance the dielectric, ferroelectric and pyroelectric properties. The mechanism of the enhanced ferroelectric properties was also discussed.     

Electromechanical properties and dielectric behavior of (Bi1/2Na1/2)(1−1.5x)BixTiO3 lead-free piezoelectric ceramics

Bismuth doped bismuth sodium titanate ceramics [(Bi_1/2Na_1/2)_(1−1.5x)Bi_xTiO₃, x=0 to 0.06] were prepared, and the resulting effects on the microstructure and dielectric properties were examined. All of the Bi-doped ceramics exhibited a single phase of perovskite structure with rhombohedral symmetry. The poling leakage current was significantly reduced by the doping of Bi, facilitating the poling process of the ceramics. The doping with Bi enhances the piezoelectric properties and increases the dielectric constant and the dielectric loss of the ceramics. At 2 mol% Bi-doping level, the ceramics exhibit a large remanent polarization of 47 μC/cm² and a relatively low coercive field of 71 kV/cm, while their d₃₃ and k_p reach a maximum value of 95 pC/N and 21%, respectively.     

Spectral properties of Nd:Ca2Nb2O7 crystal

This paper reports the spectral properties of Nd³⁺:Ca₂Nb₂O₇. The spectral parameters of Nd³⁺ in Nd³⁺:Ca₂Nb₂O₇ crystal have been investigated based on Judd-Ofelt theory. The spectral parameters were obtained. The parameters of line strengths Ω_λ are Ω₂=4.967×10⁻²⁰ cm², Ω₄=5.431×10⁻²⁰ cm², Ω₆=5.693×10⁻²⁰ cm². The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 122 μs, 103 μs and 84.4%, respectively. The fluorescence branch ratios calculated: β₁=0.425, β₂=0.479, β₃=0.091, β₄=0.004. The emission cross section at 1068 nm is 6.204×10⁻²⁰ cm².     

Radio-frequency (RF) studies of the magneto-dielectric composites: Cr0.75Fe1.25O3 (CRFO)-Fe0.5Cu0.75Ti0.75O3 (FCTO)

This paper concerns a study about the electrical properties of Cr_0.75Fe_1.25O₃ (CRFO)/Fe_0.5Cu_0.75Ti_0.75O₃ (FCTO) magneto-dielectric composites. These compounds were prepared by the conventional solid-state reaction synthesis. The samples synthesized, as well their two-phase composites [Cr_0.75Fe_1.25O₃]_Z-[Fe_0.5Cu_0.75Ti_0.75O₃]_100−Z (Z=17, 34, 50, 66, 83), were characterized by X-ray powder diffraction technique (XRD). Rietveld's method was employed to verify the quantitative phase abundances in the composites’ and their theoretical densities, which were compared with the experimental densities (pycnometer method).To predict the effect of the phases in the composites effective dielectric function (κ), traditional dielectric mixing models such as parallel, series, and Lichtenecker's model were observed. An alternative approach, a sigmoidal fitting function based on the Boltzmann equation, was proposed to fit the experimental data.     

Vacuum ultraviolet luminescence from a micro-pulling-down method grown Nd:(La0.9,Ba0.1)F2.9

Vacuum ultraviolet (VUV) luminescence from a Nd³⁺:(La_1−x,Ba_x)F_3−x (x=0.1) and a Nd³⁺:LaF₃ single crystal grown by the micro-pulling-down method modified for fluoride crystal growth is discussed. Emission resulting from excitation with 157 nm pulses of a F₂ laser and by 290 nm femtosecond pulses of a Ti:sapphire laser show that the luminescence spectral and temporal characteristics are similar for both excitation cases and that they have good prospects as a VUV laser material.