Lanthanum Tantalate Research Articles

Effect of background gas composition on the stoichiometry and lithium ion conductivity of pulse laser deposited epitaxial lithium lanthanum tantalate (Li3xLa1/3−xTaO3)

Lithium lanthanum tantalate (Li3xLa1/3−xTaO3, x = 0.075) thin films were grown via pulsed laser deposition using background gas atmospheres with varying partial pressures of oxygen and argon. The background gas composition was varied from 100% to 6.6% oxygen, with the pressure fixed at 150 mTorr. The maximum ion conductivity of 1.5 × 10−6 S/cm was found for the film deposited in 100% oxygen. The ion conductivity of the films was found to decrease with reduced oxygen content from 100% to 16.6% O2 in the background gas. The 6.6% oxygen background condition produced ion conductivity that approached that of the 100% oxygen condition film. The lithium transfer from the target to the film was found to decrease monotonically with decreasing oxygen content in the background gas but did not account for all changes in the ion conductivity. The activation energy of ion conduction was measured and found to correlate well with the measured ion conductivity trends. Analysis of x-ray diffraction results revealed that the films also exhibited a change in the lattice parameter that directly correlated with the ion conduction activation energy, indicating that a primary factor for determining the conductivity of these films is the changing size of the ion conduction bottleneck, which controls the activation energy of ion conduction.

Ultrathin Lanthanum Tantalate Perovskite Nanosheets Modified by Nitrogen Doping for Efficient Photocatalytic Water Splitting.

Ultrathin nitrogen-doped perovskite nanosheets LaTa2O6.77N0.15- have been fabricated by exfoliating Dion-Jacobson-type layered perovskite RbLaTa2O6.77N0.15. These nanosheets demonstrate superior photocatalytic activities for water splitting into hydrogen and oxygen and remain active with photon wavelengths as far as 600 nm. Their apparent quantum efficiency under visible-light illumination (λ ≥ 420 nm) approaches 1.29% and 3.27% for photocatalytic hydrogen and oxygen production, being almost 4-fold and 8-fold higher than bulk RbLaTa2O6.77N0.15. Their outstanding performance likely stems from their tiny thickness (single perovskite slab) that essentially removes bulk charge diffusion steps and extends the lifetime of photogenerated charges. Theoretical calculations reveal a peculiar 2D charge transportation phenomenon in RbLaTa2O6.77N0.15; thus, exfoliating RbLaTa2O6.77N0.15 into LaTa2O6.77N0.15- nanosheets has limited impact on charge transportation properties but significantly enhances the surface areas which contributes to more reaction sites.

Preparation and characterization of ceramic nanofibers based on lanthanum tantalates

This paper is focused on a modified sol–gel synthesis of polymeric complex and controlled assembly of inorganic nanofibers based on perovskite lanthanum tantalate La1/3TaO3 (LT) first time prepared by needleless electrospinning method. One advantage of the electrospinning method implementing PVA/LT polymer solution and subsequent thermal treatment provides a controlled patterning of the LT ceramic nanofibers. The structural properties and morphology of the composite PVA/LT were characterized by X-ray diffraction (XRD), differential scanning calorimetry/thermogravimetric analysis, Raman spectroscopy and scanning and transmission electron (SEM and TEM) microscopy. The surface microstructures of composite PVA/LT fibers were smooth, and their diameters were higher (about 100–200 nm) than those of the LT nanofibers. The mechanism of phase transformation from amorphous La4.67Ta22O62 at 400 °C via pyrochlore LaTa7O19 to perovskite La1/3TaO3 at 900 °C was determined. XRD analysis verified the formation of the major tetragonal La1/3TaO3 and minor orthorhombic LaTa7O19 phases at 1100 °C.

Structural properties and phase transformation of sol–gel prepared lanthanum tantalates

Perovskite lanthanum tantalate La1/3TaO3 (LT) precursors were prepared by a new polymeric complex sol–gel method using Ta-citrate or -tartrate complexes in different solvents (ethanol or methanol) and calcination between 750 and 1050 °C. The structural properties of precursors were characterised by X-ray diffraction (XRD), differential scanning calorimetry/thermogravimetric analysis (DSC/TG), Fourier transform infrared (FTIR) and Raman spectroscopy, scanning and transmission electron microscopy (SEM, TEM). The fluorite orthorhombic tungsten bronze (TB) of La4.67Ta22O62 and perovskite La1/3TaO3 phases with a small amount of pyrochlore monoclinic LaTaO4 phase were formed after calcination at 900–1050 °C respectively, irregardless of solvent. The particle morphology changed from larger agglomerates composed of spherical 5–20 nm amorphous clusters without visible crystallization at 750 °C to plate-like shaped of well-ordered LT nanoparticles (ethanol) or individual spherical clusters with the fine inner structure (methanol) at 1050 °C. The same effect of alcohol solvent on phase transformation during annealing was shown, whereas the effect of chelating agent was insignificant. Selected area electron diffraction (SAED) patterns of La1/3TaO3 precursors after calcination at 900 °C (a) metastable fluorite La4.67Ta22O62, (b) perovskite La1/3TaO3 and (c) LaTaO4 phases and (d) multidomain structure of super cell of La2/3Ta2O9 .

ChemInform Abstract: The Energetics of Lanthanum Tantalate Materials.

AbstractThe enthalpies of formation of La2Ta2O7(OH)2 and two polymorphs of LaTaO4 are measured by high‐temperature oxide melt solution calorimetry.

The energetics of lanthanum tantalate materials

Lanthanum tantalates are important refractory materials with application in photocatalysis, solid oxide fuel cells, and phosphors. Soft-chemical synthesis utilizing the Lindqvist ion, [Ta 6O 19] 8−, has yielded a new phase, La 2Ta 2O 7(OH) 2. Using the hydrated phase as a starting material, a new lanthanum orthotantalate polymorph was formed by heating to 850 °C, which converts to a previously reported LaTaO 4 polymorph at 1200 °C. The stabilities of La 2Ta 2O 7(OH) 2 (LaTa−OH), the intermediate LaTaO 4 polymorph (LaTa-850), and the high temperature phase (LaTa-1200) were investigated using high-temperature oxide melt solution calorimetry. The enthalpy of formation from the oxides were calculated from the enthalpies of drop solution to be −87.1±9.6, −94.9±8.8, and −93.1±8.7 kJ/mol for LaTa−OH, LaTa-850, and LaTa-1200, respectively. These results indicate that the intermediate phase, LaTa-850, is the most stable. This pattern of energetics may be related to cation–cation repulsion of the tantalate cations. We also investigated possible LnTaO 4 and Ln 2Ta 2O 7(OH) 2 analogues of Ln=Pr, Nd to examine the relationship between cation size and the resulting phases.

Chemical Solution-Based Epitaxial Oxide Films on Biaxially Textured Ni-W Substrates with Improved Out-of-Plane Texture for YBCO-Coated Conductors

Epitaxial films of rare-earth (RE) niobates (where the rare earth includes La, Ce, and Nd) and lanthanum tantalate with pyrochlore structures were grown directly on biaxially textured nickel-3 at.% tungsten (Ni-W) substrates using a chemical solution deposition (CSD) process. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis revealed the surface morphology of the films to be smooth and homogeneous. Detailed X-ray diffraction analysis showed that the films of pyrochlore RE niobate and La-tantalate are highly textured with cube-on-cube epitaxy. The overall texture quality of the films was investigated by measuring the full-width half-maximum (FWHM) of the (004) and (222) rocking curves. We observed a sharper texture for both lanthanum niobate (La3NbO7) and lanthanum tantalate (La3TaO7) films compared to the underlying Ni-W substrate, though they have a larger lattice misfit with the Ni-W substrates. These results were comparable to the texture improvement observed in vacuum-deposited Y2O3 seed layers. Texture improvement in the seed layer is the key towards obtaining YBCO films with a␣higher critical current density. Hence, solution-deposited La3NbO7 and La3TaO7 films can be used as a seed layer towards developing all metalorganic-deposited (MOD) buffer/YBCO architectures.

Peculiarities of the Synthesis of Lithium Ion Conducting Lanthanum Tantalate by Solid‐State Reaction and Precipitation from Solutions

AbstractSynthesis of lithium ion conducting tantalates has been carried out by conventional solid‐state reaction (SSR) techniques and by precipitation from solutions (PS). It is shown that the latter chemical route leads to the formation of a pure perovskite phase, made up of nanosized particles, and uses milder conditions when compared with the solid‐state reaction technique. Thermal analyses, X‐ray powder diffraction (XRD) and infrared (IR) spectroscopy are used to investigate the phase transformations that occur during the preparation of these tantalates from powder precursors. After sintering, the nanosized particles of the powder obtained by the (PS) route are preserved and pellets with good compactness (89 %) are obtained. The electrical properties of the sintered ceramics have been investigated by ac impedance spectroscopy. These experiments showed that the total ionic conductivity (bulk and grain boundaries) of the (PS) pellet is slightly lower than that of the (SSR) pellet. This decrease of conductivity is attributed to an enhancement of the blocking effect of the grain boundaries resulting from the microstructure of the fine‐grain ceramics. The bulk conductivity is not affected by the synthetic route. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Deposition of rare earth tantalate buffers on textured Ni-W substrates for YBCO coated conductor using chemical solution deposition approach

Epitaxial films of rare-earth (RE = La, Ce, Eu, and Gd) tantalates, RE3TaO7 with pyrochlore structures were grown on biaxially textured nickel-3 at.% tungsten (Ni-W) substrates using chemical solution deposition (CSD) process. Precursor solution of 0.3∼0.4 M concentration of total cations were spin coated on to short samples of Ni-W substrates and the films were crystallized at 1050∼1100 °C in a gas mixture of Ar- 4% H2 for 15 to 60 min. X-ray studies show that the films of pyrochlore RE tantalate films are highly textured with cube-on-cube epitaxy. Improved texture was observed in case of lanthanum tantalate (La3TaO7) film grown on Ni-W substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) investigations of RE3TaO7 films reveal a fairly dense and smooth microstructure without cracks and porosity. The rare-earth tantalate layers may be potentially used as buffer layers for YBa2Cu3O7-δ (YBCO) coated conductors.

Mechanochemical Synthesis of Yttrium and Lanthanum Tantalates

The phase changes in the Y2O3–Ta2O5 and La2O3–Ta2O5 systems during mechanical activation and subsequent heat treatment are investigated. Y2O3 and Ta2O5 are shown to react during mechanical activation to form Y3TaO7 . Calcination of activated mixtures leads to the formation of YTaO4 and LaTaO4 . The observed processes are interpreted in terms of transformations of the fluorite structure. A new method is proposed for the synthesis of rare-earth tantalates for applications in diagnostic x-ray contrast media. The method has a number of important advantages over the conventional ceramic route.