Growth and piezoelectric properties of Al-substituted langasite-type La3Nb0.5Ga5.5O14 crystals

The influences of aluminum substitution for gallium in the langasite-type La₃Nb_0.5Ga_5.5O₁₄ (LNG) crystals on their growth and electric properties were investigated. Al-substituted LNG (La₃Nb_0.5Ga_5.5−xAl_xO₁₄; LNGAx) single crystals up to the solubility limit x = 0.2 have been grown by the conventional Czochralski technique. The electric properties of the LNGAx crystals were investigated and compared to those of LNG. With Al substitution, the piezoelectric constants, d₁₁ and d₁₄, were slightly higher. The LNGAx crystals showed a temperature dependence of d₁₁ similar to that of the LNG crystal.     

Grain boundary effect on electron-beam-induced current in La3Ga5.5Ta0.5O14 crystal

La₃Ga_5.5Ta_0.5O₁₄ crystal recently attracted more attention due to its superior electromechanical properties and high Q × f product. We report that the first electron-beam-induced current experiment on La₃Ga_5.5Ta_0.5O₁₄ single crystal. This method is employed to study the effect of the crystal's grain boundary on the incident electron beam. The experimental results clearly show that when the electron beam scans over the grain boundary of the crystal, a fraction of the carriers recombine at the grain boundary and is unavailable for the current generation. This recombination rate will be enhanced when the electron beam was close to the boundary and cause a dip in the collected current. Although the crystal is an insulator, this effect still can be observed if the coating metal is proper to be chosen. It is also pointed out that the different diffusion lengths of the crystal might be due to the tilted grain boundary.     

Synthesis of a novel langanite-type compound La3Al5.5Nb0.5O14 by the sol-gel process

Langanite (La₃Ga_5.5Nb_0.5O₁₄, LGN) and its isomorphs are a few piezoelectric materials which have the unique temperature compensation and piezoelectric properties. But the high Ga₂O₃ content makes them very expensive and limits their applications. We reported a new langanite-type compound La₃Al_5.5Nb_0.5O₁₄ (LAN) which has the advantage of no Ga₂O₃ content. Chemically homogeneous La₃Al_5.5Nb_0.5O₁₄ sol was synthesized using La (NO₃)₃·6H₂O, Al (NO₃)₃·9H₂O and niobium citrate as starting materials. Single-phased LAN powder was prepared by decomposition of a citrate polymer precursor and subsequent reactions at high temperatures. TG-DTA, XRD and FTIR were employed to investigate the transformation process of gel to LAN powder. The results showed that, after calcination at 900 °C, pure La₃Al_5.5Nb_0.5O₁₄ polycrystalline powder with a narrow particle size distribution was obtained, which has the same structure with La₃Ga_5.5Nb_0.5O₁₄.     

Preparation and characterization of new visible-light-driven BaCo0.5Nb0.5O3 photocatalyst

A new type of visible-light-driven photocatalyst BaCo_0.5Nb_0.5O₃ was successfully synthesized via a sol-gel process in this study. After heating the precursors at 1000 °C, a pure perovskite phase was obtained. The particle size and crystallinity of BaCo_0.5Nb_0.5O₃ powders markedly increased with a rise in the calcination temperature. The band gap of BaCo_0.5Nb_0.5O₃ calculated from the UV-visible spectra was found to be less than that of titania. BaCo_0.5Nb_0.5O₃ was demonstrated to have photocatalytic activity under visible light irradiation and this activity significantly depended on the synthesis temperature. The sol-gel derived powders were found to have better photocatalytic activity than the solid-state derived powders because of the reduced particle size and increased surface area.     

Time-resolved spectroscopy of intrinsic luminescence of Y3Ga5O12 and (LaLu)3Lu2Ga3O12 single crystals

The nature of intrinsic luminescence of Y₃Ga₅O₁₂ (YGG) and (LaLu)₃Lu₂Ga₃O₁₂ (LLGG) single crystals grown from a melt was determined. In the case of a YGG single crystal containing Y_Ga antisite defects with a concentration of 0.25–0.275 at.% the intrinsic luminescence was considered as a superposition of luminescence of self-trapped excitons (STE), luminescence of excitons localized near antisite defects (LE(AD) centers) and luminescence caused by a recombination of an electron with a hole captured at Y_Ga antisite defects. Due to a large (2–3%) concentration of Lu_La antisite defects in LLGG single crystals the intrinsic luminescence was a superposition mainly of the LE(AD) center emission and the recombination luminescence of Lu_La antisite defects. The energy structure of the mentioned centers in YGG and LGGG hosts was determined from the excitation spectra of their luminescence under excitation by synchrotron radiation in the range of the fundamental absorption edge of these garnets.     

Dielectric properties of low temperature sintered LiF doped BaFe0.5Nb0.5O3

Sintering of BaFe_0.5Nb_0.5O₃:BFN requires the use of high temperatures to achieve satisfactory densification of this material. The aim of this study is to determine the effect of LiF on the sintering and electrical properties of BFN ceramics (the LiF was added as a sintering agent). The results show that LiF lowers the sintering temperature by 150-200 °C without affecting the formation of BFN. Ceramics doped with 2-3% LiF show optimum densities of about 93-94% of the theoretical value when sintered at low temperatures (1000-1100 °C). Samples containing 2-3% LiF show the following dielectric behaviour. The dielectric constant curves are very broad over a wide temperature range, with room-temperature values of 7154 in the 2% LiF sample and 2527 in the 3% LiF sample. The dielectric constants gradually increase up to 300 °C to values of about 38,862 in the 2% LiF sample and 40,471 in the 3% LiF sample. Furthermore, the addition of 2-3% LiF to BFN causes a reduction in the room-temperature dielectric loss from 4.29 in undoped BFN to less than 1.2 for LiF-containing samples.     

Dielectric properties and electrical conductivity of Li0.07Na0.93Ta0.1Nb0.9O3 and Li0.07Na0.93Ta0.111Nb0.889O3 ferroelectric solid solutions

The dielectric properties and electrical conductivity of the ferroelectric perovskite solid solutions Li_0.07Na_0.93Ta_0.1Nb_0.9O₃ and Li_0.07Na_0.93Ta_0.111Nb_0.889O₃ have been studied at temperatures from 290 to 700 K and frequencies from 25 Hz to 1 MHz. In this temperature range, the solid solutions undergo a first-order ferroelectric phase transition. Li_0.07Na_0.93Ta_0.111Nb_0.889O₃ (prepared from coprecipitated Ta_2y Nb_{2(1 − y)}O₅ pentoxides) has a markedly lower Curie temperature (by ∼75 K) and higher ionic conductivity and high-frequency dielectric permittivity in comparison with Li_0.07Na_0.93Ta_0.1Nb_0.9O₃ (prepared from a mechanical mixture of Ta₂O₅ and Nb₂O₅).     

Growth and properties of Ca0.28Ba0.72Nb2O6 single crystals

Calcium barium niobate Ca_0.28Ba_0.72Nb₂O₆ (CBN-28) crystals were successfully grown by the Czochralski method. X-ray powder diffraction experiments indicated that CBN single crystals are tetragonal with a = 12.432(±0.002) Å and c = 3.957(±0.001) Å, which have almost the same structure as the Sr_0.50Ba_0.50Nb₂O₆ (SBN-50) crystal. The thermal expansion coefficient perpendicular to Z-direction had been measured to be 1.25 × 10⁻⁵ K⁻¹ between 293.15 and 572.15 K, and along Z-axis was negative between 298.15 and 543.15 K. The specific heat of the crystal had been measured by the differential scanning calorimetric experiments. The transmittance spectra from 200 to 3200 nm were also measured. The measured temperature dependence of dielectric constants showed that the Curie temperature of the CBN-28 crystals is 260 °C, which is about 200 °C higher than that of the (SBN) crystal.     

硅酸镓镧晶体谐振器的高频振动分析

利用Mindlin板理论分析了硅酸镓镧晶体板强耦合的厚度剪切振动和弯曲振动，获得了硅酸镓镧晶体板高频振动的色散关系、频谱关系和振动模态位移图。数值计算结果表明，Mindlin板理论可以获得硅酸镓镧晶体板厚度剪切振动的一阶精确截止频率，无需修正系数。基于石英晶体谐振器设计晶片最佳长厚比的选取方法，确定了硅酸镓镧晶片的最佳尺寸，避免了厚度剪切振动模态和弯曲振动模态的强耦合。通过绘制硅酸镓镧晶体板在最佳尺寸时的各振动模态位移图，发现厚度剪切振动模态是主振模态，具有很好的能陷效应。Mindlin板理论在硅酸镓镧晶体板高频振动的应用分析可以指导硅酸镓镧晶体谐振器的实际研发。     

Structural and magnetic properties of LaFe0.5Cr0.5O3 studied by neutron diffraction, electron diffraction and magnetometry

The structural and magnetic properties of the perovskite type compound LaFe_0.5Cr_0.5O₃ have been studied by temperature dependent neutron powder diffraction and magnetization measurements. Rietveld refinement of the neutron diffraction data shows that the compound crystallizes in an orthorhombic perovskite structure with a random positioning of the Fe and Cr cations at the B sublattice. The magnetic structure at 10 K is a collinear antiferromagnetic one with the magnetic moment per site being equal to 2.79(4) μ_B. Magnetisation measurements confirm the overall antiferromagnetic behaviour. Moreover, it indicates a weak uncompensated magnetic moment close to the transition temperature T_N ≈ 265 K. This moment can be described by a magnetic cluster state, which remains up to 550 K. Electron diffraction patterns along with high-resolution transmission electron microscopy images reveal that the crystallites are composed by domains of different orientation, which share the same cubic perovskite sub-cell reflections.     

Structural study of Sr0.3Ba0.7Nb2O6 and La0.030Sr0.255Ba0.700Nb2O6 ceramic systems

Sr_0.3Ba_0.7Nb₂O₆ (SBN) and La_0.030Sr_0.255Ba_0.700Nb₂O₆ (LSBN) ceramic compounds have been prepared using the traditional ceramic method at two different calcination temperatures (900 and 1000 °C) and later sintered both at 1400 °C. A study of the effects of the calcination temperatures and La substitution on the morphological, compositional, and structural properties of SBN and LSBN is presented using scanning electronic microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis. From Rietveld refinement processes, the XRD patterns were interpreted to evaluate such effects in the structural parameters and the site occupation factors of the heavy metals and oxygen atoms. The effect of the incorporation of La resulted in a 0.25% cell contraction and turned out to be higher than the 0.08% dilation effect produced by the increase of calcination temperature. The La ion with similar effective ionic radius and higher electronegativity is incorporated into the structure occupying the A₁ site just like the Sr ions in the SBN compound. Differences in the site occupation factors between the SBN and LSBN samples lead to substantial changes in the physical properties such as temperature of relative dielectric constant maximum, relative dielectric constant, and dielectric loss, correlated with the distortion and the relative orientation of the oxygen octahedra.     

High-pressure substitutions for Cd In Cd2Nb2O7 and Cd2Ta2O7

Substitutions into cadmium niobate of the type Cd_2?xA_xNb₂O₇ (x≤0.5) have been carried out at 58 kbars and 1100°C. The substituting A cations were Mg, Mn, Fe, Co, Ni, Cu, and Zn which are all smaller than Cd. Rhombohedral or triclinic distortions derived from cubic Cd₂Nb₂O₇ were achieved at values of x as low as 0.05. All the substituted phases appear to be centric at room temperature. Analogous substitutions into Cd₂Ta₂O₇ were also made.     

X-ray investigation of cleavage plane of single layered manganite La0.5Sr1.5MnO4

The top layer of a cleaved surface of the single layered manganite La_0.5Sr_1.5MnO₄ is measured with crystal truncation rod scattering. Knowledge of the surface structure of strongly correlated electron systems is needed for nano-science and device application of such systems. The result shows that the cleaved surface is terminated by La/Sr layer and has little surface roughness.     

Strain and stress build-up in He-implanted UO2 single crystals: an X-ray diffraction study

The strain and stress build-up in 20-keV He-implanted UO₂ single crystals have been determined by means of X-ray diffraction through reciprocal space mapping, with the use of a model dedicated to the analysis of the strain/stress state of ion-irradiated materials. Results indicate that the undamaged part of the crystals exhibits no strain or stress; on the other hand, the implanted layer undergoes a tensile strain directed along the normal to the surface of the crystals and a compressive in-plane stress. The build-up of both strain and stress with He fluence exhibits a two-step process: (i) a progressive increase up to a maximum level of ~1% for the strain and ~−2.8 GPa for the stress, followed by (ii) a dramatic decrease. The origin of the strain and stress build-up is the formation of both self-interstitial defects and small He-vacancy clusters. The strain, and stress relief is tentatively attributed to the formation of extended defects (such as dislocations) that induce a plastic relaxation.     

The effect of Bi1.5Zn0.5Nb0.5Ti1.5O7 cover layer on the dielectric and tunable properties of Ba0.6Sr0.4TiO3/Bi1.5Zn0.5Nb0.5Ti1.5O7 bilayered thin films

Bi_1.5Zn_0.5Nb_0.5Ti_1.5O₇ (BZNT) thin films with different thicknesses as cover layers were deposited on the Ba_0.6Sr_0.4TiO₃ (BST) thin films on the Pt/Ti/SiO₂/Si substrates by radio frequency magnetron sputtering method. The microstructure, surface morphology, dielectric and tunable properties of BST/BZNT heterogeneous bilayered films were investigated as a function of the thickness of BZNT films and the effect of BZNT films on the asymmetric electrical properties of BST/BZNT bilayered films was discussed. It was found that BZNT cover layer significantly improved the leakage current and the dielectric loss, and the dielectric constant and tunability of BST/BZNT bilayered thin films simultaneously decreased with the increasing thickness of BZNT films. The BST/BZNT bilayered thin film with a 50 nm BZNT cover layer gave the largest figure of merit (FOM) of 33.48 with the upper tunability of 55.38%. The asymmetric electrical behavior of BST/BZNT bilayered films is probably related to an internal electric field caused by built-in voltages at Pt/BST and BZNT/Au interfaces.     

X-ray diffraction of polycrystalline Ti4O7

X-ray powder diffraction of powder and sintered EBONEX (nominally Ti₄O₇) is reported. The patterns show clearly the existance of another Magneli phase (Ti₅O₉) in both forms of the material. Its concentration was determined from the integrated intensity ratios of the (101) vs. (101) the (121) vs. (121) and the (206) vs. (208) lines of Ti₄O₇ vs. Ti₅O₉, respectively. For samples taken from one batch, the powder form of EBONEX contains ca. 25±5 a% of Ti₅O₉ while the sintered form contains about twice as much. Samples from another batch, prepared under improved process control, showed ca. 10a% of Ti₅O₉ in the powder form of the material. Both forms contain small traces of other phases as well, e.g., Ti₃O₅ in the powder and Ti₆O₁₁ in the sintered material. These results agree with recently reported ESR line-broadening measurements. We also observed aging effects such as gradual oxidation in the sintered material.     

Transient light induced grating study of excitation diffusion in Y3Sc2Ga3O12 laser crystals

Excitation migration over the lower energy levels in Y₃Sc₂Ga₃O₁₂ (YSGG) laser crystals doped with Cr, Er, Tm, and Ho ions was studied using the two-wave mixing light induced grating (LIG) technique. The results of the LIG measurements were compared to the data obtained from the luminescence kinetics and spectra overlap studies of the migration. The comparison of the three different spectroscopic techniques allowed more detailed analysis of the complicated pattern of the energy transfer processes in the crystals than any one method alone. The migration was found to be very efficient, especially in highly doped crystals. The diffusion coefficients determined from direct LIG measurements were much higher than those calculated in dipole-dipole approximation from absorption and emission spectral measurements. This implies stronger than dipole-dipole coupling between interacting ions. In Tm doped crystals a surprisingly strong increase of the migration efficiency between Tm concentrations 8 × 10²⁰ cm⁻³ and 1.5 × 10²¹ cm⁻³ was observed both in LIG diffusion and luminescence kinetics experiments. This too cannot be explained in terms of low multipolar energy transfer. Exchange energy transfer and the percolation model are discussed as possible explanations for the experimental results.     

High energy-storage properties of Bi0.5Na0.5TiO3-BaTiO3-SrTi0.875Nb0.1O3 lead-free relaxor ferroelectrics

New lead-free ferroelectric (0.94-x)Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-xSrTi_0.875Nb_0.1O₃ (BNBT-STN, x = 0 and 0.2) are synthesized by using a solid state reaction process. In this work, an obvious evolution of dielectric relaxation behavior and slim P–E hysteresis loops with high P_max and low P_r is observed for BNBT-0.2STN, indicating the dominant of ergodic relaxor phase with dynamic polar nano-regions (PNRs). A relatively large recoverable energy density (W_rec = 1.17 J/cm³) with high energy efficiency (η = 91%) is obtained. Furthermore, it shows small variation (9%) in the temperature range of 30–150 °C and fatigue-free behavior, which can be attributed to the absence of ferroelectric domain in the relaxor phase. The achievement of these characteristics provides that tailoring by B-site vacancies is a potential route when designing a new energy-storage system for BNT-based relaxor ferroelectric materials.     

Comparison of Bi3Fe5O12 film giant Faraday rotators grown on (111) and (001) Gd3Ga5O12 single crystals

Bismuth iron garnet (Bi₃Fe₅O₁₂, BIG) epitaxial thin films were grown on single crystal (Gd₃Ga₅O₁₂, GGG) (111) and (001) substrates by rf-magnetron sputtering technique. Processing parameters have been optimized to obtain high deposition rate (2.74 μm/h) and the surface rms roughness less than 10 nm. X-ray diffraction reveals films epitaxial quality: exclusive (111) or (001) orientation with narrow rocking curves and strong in-plane texture. Films possess low optical loss and magneto-optical Faraday rotation (FR) as high as 5 deg/μm at 677 nm wavelength. Comparative analysis of films grown on (111) and (001) substrates clearly shows significant superiority of BIG/GGG(001) film. For this film, the coercive field ∼100 Oe appears to be 2.5 times lower while the optical transmission to be 10% higher than that for BIG/GGG(111) film. Enhanced magneto-optical performance of BIG/GGG(001) films relies upon better accommodation of the film-to-substrate mismatch strain through the tetragonal BIG lattice distortions compared to the rhombohedral one in BIG/GGG(111) films.