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.     

X-ray diffraction study of La3Ga5.5Ta0.5O14 and La3Ga5.5Nb0.5O14 langasite-type single crystals

The structure of langatate (as-grown and vacuum-annealed: LGT-I and LGT-II, respectively) and langanite (seed- and tail-end portions: LGN-III and LGN-IV, respectively) single crystals grown by the Czochralski technique from charges of nominal composition La₃Ga_5.5M_0.5O₁₄ = La₃(Ga_0.5M_0.5)(1)Ga₃(2)Ga₂(3)O₁₄ (M=Ta⁵⁺, Nb⁵⁺) is studied by x-ray diffraction. The LGT-I and LGT-II crystals are shown to differ in the Ga and Ta distributions over crystallographic sites: La₃(Ga_0.52)Ta_0.48(2) ⁵⁺Ga₃(Ga_0.94Ta_0.06(1) ³⁺)₂O₁₄ in LGT-I (Ta⁵⁺ in the octahedral site Ga(1) and Ta³⁺ in the trigonal-pyramidal site Ga(3)) and La₃(Ga_0.55Ta_0.45(2) ⁵⁺)Ga₃Ga₂O_13.93(2)0.07 in LGT-II (Ta⁵⁺ in the octahedral site Ga(1) and oxygen vacancies in O(1)). The increased Ta content is responsible for the lower structural perfection of LGT-I. LGN-III and LGN-IV have essentially identical compositions, La₃(Ga_0.47Nb_0.53(1) ⁵⁺)Ga₅O₁₄ and La₃(Ga_0.48Nb_0.52(1) ⁵⁺)Ga₅O₁₄, respectively, but differ in polarity.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 485–492.Original Russian Text Copyright © 2005 by Kuzmicheva, Tyunina, Domoroshchina, Rybakov, Dubovskii.     

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.     

Effect of starting melt composition on growth of La3Ta0.5Ga5.5O14 crystal

The phase diagram of the La₂O₃-Ta₂ O₅-Ga₂ O₃ system around La₃ Ta_0.5 Ga_5.5 O₁₄ (LTG) was investigated by a differential thermal analysis and the quenching method. La₃ Ta_0.5 Ga_5.5 O₁₄ single crystals were subsequently grown from different starting melt compositions using the Czochralski technique. The effect of the starting melt composition on crystal quality was examined by measuring the variation in the chemical composition and lattice parameters along the growth axis. On the basis of these experimental results, we have grown LTG single crystals over 2 inches in diameter.     

Microstructure and microwave dielectric characteristics of (1 − x)Ba(Zn1/3Ta2/3)O3-xBaTi4O9 ceramics

(1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ (0.1 ≤ x ≤ 0.85) composites are prepared by mixing 1150 °C-calcined BaTi₄O₉ with 1150 °C-calcined Ba(Zn_1/3Ta_2/3)O₃ powders. The crystal structure, microwave dielectric properties and sinterabilites of the (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics have been investigated. X-ray diffraction patterns reveal that BaTi₄O₉, ordered and disordered Ba(Zn_1/3Ta_2/3)O₃ phases exist independently over the whole compositional range. The sintering temperatures of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics are about 1240 - 1320 °C and obviously lower than those of Ba(Zn_1/3Ta_2/3)O₃ ceramics. The dielectric constants (?_r) and the temperature coefficient of resonant frequency (τ_f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of BaTi₄O₉ content. Nevertheless, the bulk densities and the quality values (Q × f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of Ba(Zn_1/3Ta_2/3)O₃ content. The results are attributed to the higher density and quality value of Ba(Zn_1/3Ta_2/3)O₃ ceramics, the better grain growth, and the densification of sintered specimens added a small BaTi₄O₉ content. The (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramic with x = 0.1 sintered at 1320 °C exhibits a ?_r value of 31.5, a maximum Q × f value of 68500 GHz and a minimum τ_f value of 4.1 ppm/°C.     

Synthesis and electrochemical characterizations of nano-La2O3-coated nanostructure LiMn2O4 cathode materials for rechargeable lithium batteries

LiMn₂O₄ spinel cathode materials were coated with 1.0, 2.0 and 3.0 wt.% of La₂O₃ by polymeric process, followed by calcinations at 850 °C for 6 h in air. The surface coated LiMn₂O₄ cathode materials were physically characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy and XPS. XRD patterns of La₂O₃-coated LiMn₂O₄ revealed that the coating did not affect the crystal structure and space group Fd3m of the cathode materials, compared to the uncoated LiMn₂O₄. The surface morphology and particle agglomeration were investigated using scanning electron microscopy and the TEM image showed a compact coating layer on the surface of the core materials that had average thickness of about 100 nm. XPS data illustrated that the La₂O₃ was completely coated over the surface of the LiMn₂O₄ core cathode materials. The galvanostatic charge and discharge of the uncoated and La₂O₃-coated LiMn₂O₄ cathode materials were carried out in the potential range of 3.0 and 4.5 V at 30 °C and 60 °C. Among them, 2.0 wt.% of La₂O₃-coated spinel LiMn₂O₄ cathode has improved the structural stability, high reversible capacity and excellent electrochemical performances of the rechargeable lithium batteries.     

Effect of B-site doping on Sm0.5Sr0.5MxCo1−xO3−δ properties for IT-SOFC cathode material (M = Fe, Mn)

The crystal structure, thermal expansion rate, electrical conductivity and electrochemical performance of Sm_0.5Sr_0.5M_xCo_1−xO_3−δ (M = Fe, Mn) have been investigated. Two crystal structures have been observed in the specimens of Sm_0.5Sr_0.5Fe_xCo_1−xO_3−δ (SSFC) at room temperature, the perovskite structure of SSFC has an orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5 ≤ x ≤ 0.9. The specimens of Sm_0.5Sr_0.5Mn_xCo_1−xO_3−δ (SSMC) crystallize in an orthorhombic structure. The adjustment of thermal expansion rate to electrolyte, which is one of the main problems of SSC, can be achieved to lower TEC values with more Fe and Mn substitution. Especially, Sm_0.5Sr_0.5Mn_0.8Co_0.2O_3−δ exhibits good thermal compatibility with La_0.8Sr_0.2Ga_0.8Mg_0.2O₃. High electrical conductivities are obtained for all the specimens and they demonstrate above 100 S/cm at 800 °C in SSFC system. The polarization resistance increases with increasing Mn content, Nevertheless, the polarization resistance of SSFC increases with increasing Fe content, but when the amount of Fe reaches to 0.4, the maximum is obtained while the resistance will decrease when the amount of Fe reaches above 0.4. Sm_0.5Sr_0.5Fe_0.8Co_0.2O_3−δ electrode exhibits high catalytic activity for oxygen reduction operating at temperature from 700 to 800 °C.     

Effect of tantalum substitutions on microstructures and dielectric properties of calcium copper titanate (CaCu3Ti4O12) ceramic

Tantalum (Ta⁵⁺) doped CaCu₃Ti_4−xTa_xO₁₂ (x = 0.05 and 0.10) ceramics were prepared by semi-wet route using solid Ta₂O₅ powder and metal nitrate solutions. Powder X-ray diffraction confirmed the major phase formation of cubic CaCu₃Ti₄O₁₂ ceramic in the doped samples sintered at 900 °C for 6 h. Microstructure has been studied using field-emission scanning electron microscopy. The grain size is in the range of 150 nm to 1 μm for both compositions. Dielectric constant of the samples increases with increasing tantalum concentrations.     

Bi4LnNb3O15 (Ln = La, Pr, Nd) and Bi4LaTa3O15: New intergrowth Aurivillius related phases

We describe the synthesis and characterization of new intergrowth Aurivillius related phases, Bi₄LnNb₃O₁₅ (Ln = La, Pr, Nd) and Bi₄LaTa₃O₁₅. Both powder X-ray diffraction and electron microscopy investigations show that the compounds adopt orthorhombic structures with the cell parameters a ∼ 5.5 Å, b ∼ 5.5 Å and c ∼ 20.9 Å, suggesting an ordered intergrowth structure that consists of n = 1 [Bi₂NbO₆]⁻ and n = 2 [Bi₂LnNb₂O₉]⁺ Aurivillius fragments which are stacked alternately along the c-axis. The oxides do not show a second harmonic generation (SHG) response toward 1064 nm laser radiation; they do not show a ferroelectric-paraelectric transition either between 30 and 900 °C in dielectric measurements, indicating a centrosymmetric structure. Optical absorption studies show that the intergrowth phases possess considerably smaller band gaps than the parent Nb₂O₅ and Ta₂O₅.     

Processing, dielectric properties and impedance characteristics of Na0.5Bi0.5Cu3Ti4O12 ceramics

Na_0.5Bi_0.5Cu₃Ti₄O₁₂ (NBCTO) ceramics were prepared by conventional solid-state reaction method. The phase structure, microstructure and dielectric properties of NBCTO ceramics sintered at various temperatures with different soaking time were investigated. Pure NBCTO phase could be obtained with increasing the temperature and prolonging the soaking time. High dielectric permittivity (13,495) and low dielectric loss (0.031) could be obtained when the ceramics were sintered at 1000 °C for 7.5 h. The ceramics sintered at 1000 °C for 7.5 h also showed good temperature stability (−4.00 to −0.69%) over a large temperature range from −50 to 150 °C. Complex impedances results revealed that the grain was semiconducting and the grain boundaries was insulating. The grain resistance (R_g) was 12.10 Ω cm and the grain boundary resistance (R_gb) was 2.009 × 10⁵ Ω cm when the ceramics were sintered at 1000 °C for 7.5 h.     

In situ observation of the change in domain structure due to phase transformation in a perovskite La2MnGaO6

The microstructure of a perovskite, La₂MnGaO₆, was studied by Transmission Electron Microscopy (TEM). An idiomorphic grain exhibited a domain structure at ambient temperature, and each domain possessed a common b-axis and axial relation, [1 0 0]//[0 0 1]. When the grain was heated by a convergent electron beam, the domains vanished after the orthorhombic to rhombohedral phase transformation taking place at 423 K. The domains reappeared when cooled, although domain boundaries moved and the domains rotated by 90° with respect to the b-axis.     

Synthesis, crystal structure and thermal decomposition of [La2(CH3CH2COO)6·(H2O)3]·3.5H2O precursor for high-k La2O3 thin films deposition

Lanthanum acetylacetonate La(C₅H₇O₂)₃·xH₂O has been used in the preparation of the precursor solution for the deposition of polycrystalline La₂O₃ thin films on Si(1 1 1) single crystalline substrates. The precursor chemistry of the as-prepared coating solution, precursor powder and precursor single crystal have been investigated by Fourier Transformed Infrared Spectroscopy (FTIR), differential thermal analysis coupled with quadrupole mass spectrometry (TG-DTA-QMS) and X-ray diffraction. The FTIR and X-ray diffraction analyses have revealed the complex nature of the coating solution due to the formation of a lanthanum propionate complex. The La₂O₃ thin films deposited by spin coating on Si(1 1 1) substrate exhibit good morphological and structural properties. The films heat treated at 800 °C crystallize in a hexagonal phase with the lattice parameters a = 3,89 Å and c = 6.33 Å, while at 900 °C the films contain both the hexagonal and cubic La₂O₃ phase.     

Low-temperature synthesis and characterization of yttrium-gallium garnet Y3Ga5O12 (YGG)

To obtain yttrium-gallium garnet (Y₃Ga₅O₁₂, YGG) a simple “chimie douce” method has been developed. This sol-gel method yielded excellent starting gel precursor for the fabrication of YGG phase, which could be used as host material for optical applications. The pattern of X-ray diffraction analysis of the ceramic sample sintered for 10 h at 1000 °C showed the formation of monophasic Y₃Ga₅O₁₂ phase. The phase transformations, composition and micro-structural features in the gels and polycrystalline sample were studied by thermoanalytical methods (TGA/DTA), powder X-ray diffraction analysis (XRD), infrared spectroscopy (IR) and scanning electron microscopy (SEM). The quality of the resulting products (homogeneity, crystallisation temperature, grain size, grain size distribution, etc.) is discussed.     

Structural and ferroelectric properties of La modified Sr0.8Bi2.2Ta2O9 thin films

La modified SBT (Sr_0.8La_0.1Bi_2.1Ta₂O₉) thin films of different thickness were fabricated on Pt/Ti/SiO₂/Si substrates by the metalorganic decomposition technique. All the films were annealed layer-by-layer at 800 °C using a rapid thermal annealing furnace. X-ray diffraction analysis indicated that the relative intensity of the (2 0 0) diffraction peak [I(2 0 0)/I(1 1 5)] increased with the increase of the film thickness. Eventually, an a-axis preferentially oriented SLBT film was obtained. These results are discussed with respect to the anisotropy of the grain growth. The a-axis preferentially oriented SLBT film, whose relative intensity of the (2 0 0) peak [I(2 0 0)/I(1 1 5)] was 1.05, had a remanent polarization (2P_r) value of 21 μC/cm² and a coercive field (2E_c) value of 70 kV/cm under the electric field of 200 kV/cm.     

Structure and lithium ion conductivity of garnet-like Li5La3Sb2O12 and Li6SrLa2Sb2O12

Oxides with the nominal chemical compositions Li₅La₃Sb₂O₁₂ and Li₆SrLa₂Sb₂O₁₂ were prepared by solid-state reaction. The structures were refined by the Rietveld method using powder X-ray diffraction data. The synthesis of Li₅La₃Sb₂O₁₂ resulted in the well known garnet-related structure plus 5 wt.% of La₂LiSbO₆ in the bulk. In contrast to that, Li₆SrLa₂Sb₂O₁₂ could be synthesised in single garnet-related type phase. Lithium ion conductivities of Li₅La₃Sb₂O₁₂ and Li₆SrLa₂Sb₂O₁₂ were studied by the ac impedance method. The grain-boundary contribution to the total (bulk + grain-boundary) resistance is very small and about 5 and 3% for Li₅La₃Sb₂O₁₂ and Li₆SrLa₂Sb₂O₁₂, respectively, at 24 °C and decreases further with increase in temperature. Among the investigated compounds, Li₅La₃Sb₂O₁₂ exhibits the highest total (bulk + grain-boundary) and bulk ionic conductivity of 7.8 × 10⁻⁶ and 8.2 × 10⁻⁶ S cm⁻¹, respectively, at 24 °C. The structural data indicate that the coupled substitution Li + Sr ⇒ La leads to a closure of the bottle neck like O-O distances of the shared edges of neighbouring Li octahedra and therefore reduces the mobility of Li ions in Li₆SrLa₂Sb₂O₁₂. Scanning electron microscope (SEM) images of the Li₆SrLa₂Sb₂O₁₂ compound revealed well crystallised large homogeneous grains (∼4.8 μm) and the grains were in good contact with the neighbouring grain, which leads to a smaller grain-boundary contribution to the total resistance.     

Effect of BiFeO3 additions on the dielectric and piezoelectric properties of (K0.44Na0.52Li0.04)(Nb0.84Ta0.1Sb0.06)O3 ceramics

(1 − x) (K_0.44Na_0.52Li_0.04)(Nb_0.84Ta_0.1Sb_0.06)O₃ − x BiFeO₃ (x = 0, 0.002, 0.004, 0.006, 0.008, 0.01) lead-free piezoelectric ceramics were prepared by the conventional ceramic processing. The compositional dependence of the phase structure and the electrical properties of the ceramics were studied. A morphotropic phase boundary between the orthorhombic and tetragonal phases was identified in the composition range of 0.004 < x < 0.006. The ceramics near the morphotropic phase boundary exhibit a strong compositional dependence and enhanced piezoelectric properties. The ceramics with 0.6 mol.% BiFeO₃ exhibit good electrical properties (d₃₃ ∼ 246 pC/N, k_p ∼ 43%, T_c ∼ 285 °C, ?_r ∼ 1871, and tan δ ∼ 1.96%). These results show that the (1 − x) (K_0.44Na_0.52Li_0.04)(Nb_0.84Ta_0.1Sb_0.06)O₃ − x BiFeO₃ ceramic is a promising lead-free piezoelectric material for applications in different devices.     

Improved hydrogen evolution activities under visible light irradiation over NaTaO3 codoped with lanthanum and chromium

Na_1−xLa_xTa_1−xCr_xO₃ and NaTa_1−xCr_xO₃ (x = 0.01, 0.03, 0.05 and 0.10) have been synthesized by a solid state reaction method. These photocatalysts can produce H₂ in the presence of methanol under visible light irradiation (λ > 420 nm). The photocatalytic activities of Na_1−xLa_xTa_1−xCr_xO₃ are much higher than those of NaTa_1−xCr_xO₃, respectively. Especially, the H₂ evolution rate of Na_0.9La_0.1Ta_0.9Cr_0.1O₃ is 2.2 μmol h⁻¹, which is nearly 4 times higher than that of NaTa_0.9Cr_0.1O₃ (0.6 μmol h⁻¹). The improved activities of Na_1−xLa_xTa_1−xCr_xO₃ compared with NaTa_1−xCr_xO₃ can be ascribed to two factors: one is smaller particle size and higher specific surface area which is caused by the doping of lanthanum; the other is that Na_1−xLa_xTa_1−xCr_xO₃ has less Cr⁶⁺, which is induced by codoping of lanthanum and chromium.     

Effect of PbO-SiO2 and PbO-B2O3 flux systems on the crystalline and magnetic properties of Ni0.5Zn0.5Fe2O4 ferrite prepared from the mixed powders

Lead borate and lead silicate were added to lower the sintering temperature of a Ni_0.5Zn_0.5Fe₂O₄ ferrite prepared from the blend of two types of powders and to homogenize the grain size. 5PbO·SiO₂ and 5PbO·B₂O₃ flux systems were added to lower the sintering temperature and diminish the magnetic loss at high frequencies. The ferrites were studied by bulk density, scanning electron microscopy and impedance analysis. It was found that the addition of PbO markedly accelerated the grain growth, while SiO₂ and B₂O₃ were found to be effective to obstruct the movement of grain boundaries and to minimize the grain size. Doping with PbO in the mixed powders appropriately increased the densification and initial permeability. The ferrite doped with 1% of 5PbO·SiO₂ possessed the lowest loss tangent (tgδ) in the range of 5 M-40 MHz and the highest threshold frequency.     

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。     

Preparation and crystal structure of H-BaTa2O6-type K1.83Ba4.17Nb12.18O36 and dielectric properties of the related compounds

Single crystal of a novel compound, K_1.83Ba_4.17Nb_12.18O₃₆, has been synthesized in the course of investigation on the K₂O-BaO-Nb₂O₅ system. The crystal structure was determined by single crystal X-ray diffraction data. The space group of this compound was found to be P6/mmm (#191) with the lattice parameters of a = 21.109(4) and c = 3.967(1) Å. The final R-factors were R = 0.039 and Rw=0.042 for unique 508 reflections. The crystal structure had the same tunnel structure as that of hexagonal BaTa₂O₆ (H-BaTa₂O₆), which is the high temperature form in three modifications of BaTa₂O₆. The chemical composition of K_1.83Ba_4.17Nb_12.18O₃₆ was close to that of the tetragonal tungsten bronze (TTB) type KBa₂Nb₅O₁₅ and the powder samples within this composition were prepared so as to clarify the boundary between H-BaTa₂O₆ and TTB-type structures. The H-BaTa₂O₆-type structure appears in (K + Ba)/Nb ≤ 0.500 and the TTB-type structure is in (K + Ba)/Nb ≥ 0.575. The dielectric constants of these samples were measured from room temperature to 500 °C for sintered body. The TTB-type compounds exhibited ferroelectric temperature dependence with the Curie points of 284-372 °C and the H-BaTa₂O₆-type compounds were not ferroelectrics as predicted from the crystal structure analysis.