Lanthanum Tungstate Research Articles

Determination of proton- and oxide ion tracer diffusion in lanthanum tungstate (La/W = 5.6) by means of ToF-SIMS

Tracer diffusion of protons and oxide ions, as well as chemical diffusion of water, have been determined for the high temperature proton conductor lanthanum tungstate by means of time-of-flight secondary ion mass spectrometry (ToF-SIMS). The oxygen tracer diffusion and surface exchange coefficients, D(O)* and k(O)*, were measured after exchange anneals in water vapor enriched in H(2)(18)O between 350 and 620 °C, and the apparent activation energies were 176 and 82 kJ mol(-1), respectively. The hydrogen tracer diffusion coefficient (D(H)*) was measured between 220 and 320 °C after exchange anneals in D(2)O-containing atmospheres, and the apparent activation energy was 63 kJ mol(-1). The extracted D(H)* agrees with the results of transient conductivity- and TG-measurements. Chemical diffusion and surface exchange coefficients, D(H)δ and k(H)δ, were measured at 250 and 400 °C, and the result confirms that the material is hydrated by ambipolar sluggish transport of protons and oxide ions. The surface exchange coefficients were compared to the result of TG relaxation, suggesting that access to oxygen vacancies limits the overall surface exchange reaction under incorporation of water and oxygen.

Complete structural model for lanthanum tungstate: a chemically stable high temperature proton conductor by means of intrinsic defects

This is the first paper reporting that lanthanum tungstate, earlier believed to be La6WO12, is in fact La28−xW4+xO54+δ , where tungsten dissolves in lanthanum sites to form a stable solid-state electrolyte, exhibiting proton conduction by hydration at intermediate temperatures.

Thermogravimetric relaxation study of the proton conductor lanthanum tungstate, La28−xW4+xO54+δv2−δ, x = 0.85

Diffusion- and surface exchange coefficients for incorporation and transport of protons and water in lanthanum tungstate, La27.15W4.85O55.28v0.73 have been determined from thermogravimetric relaxation. Tracer diffusion of protons has proved to be considerably faster than chemical diffusion of water, whereas the tracer surface exchange process is somewhat slower than its chemical counterpart. Consequently, tracer relaxations display larger critical thicknesses than the chemical ones and are moreover found to be predominantly surface controlled. The activation energy of the chemical diffusion coefficient changes above 700 °C and this transition and a corresponding change in its water vapor dependency are discussed in light of chemical diffusion of water. The activation energy of the chemical surface exchange coefficient under reducing conditions is the half of that under oxidizing. Platinum nano particles have proved to increase the rate of water incorporation under reducing conditions.

Preparation of lanthanum tungstate membranes by tape casting technique

Lanthanum tungstate materials have been reported to show exceptional mixed proton and electron conducting behaviour at elevated temperatures and making them attractive for dense hydrogen gas separation membranes. In this work preparation of planar asymmetric lanthanum tungstate membranes was addressed. For this purpose carbon black and rice starch pore formers were evaluated for optimum substrate gas permeability. It was found that carbon black pore former results in higher level of effective porosity. Stabilization of fine lanthanum tungstate powder in ethanol based solvent media was carried out to find out optimum surfactant quantity for tape casting slurry. By combining lanthanum tungstate tapes with and without pore former defect free asymmetric membranes were produced by conventional sintering.

Synthesis and luminescence properties of Ln 3+ (Ln 3+ = Er 3+, Sm 3+)-doped barium lanthanum tungstate BaLa 2WO 7 phosphors

BaLa 2WO 7 phosphors activated with the trivalent rare-earth Er 3+ and Sm 3+ were synthesized via a solid-state reaction using a vibrating-mill method, and their luminescence properties were investigated. Er 3+-doped BaLa 2WO 7 and Sm 3+-doped BaLa 2WO 7 emit bright green and reddish-orange light, respectively. The photoluminescence investigations revealed that the phosphors exhibit apparent characteristic emissions from the 2 H 11 / 2 and 4 S 3 / 2 state, respectively, to the 4 I 15 / 2 ground state for Er 3+, and 4 G 5 / 2 to 6 H 5 / 2 , 7 / 2 , 9 / 2 states for Sm 3+ under near-UV excitation. The green emission of the Er 3+-doped BaLa 2WO 7 phosphor had CIE chromaticity coordinates (0.249, 0.717) with a dominant wavelength of 546 nm and a color purity of 92%. The Sm 3+-doped BaLa 2WO 7 phosphor had color coordinates (0.558, 0.441), a dominant wavelength of 606 nm, and 94% color purity. The sharp emission peaks and excellent luminescence properties show that BaLa 2WO 7 is a suitable host for rare-earth-doped phosphors, which may be suitable for optical applications.

Determination of hardness, Young's modulus and fracture toughness of lanthanum tungstates as novel proton conductors

Lanthanum tungstate is a promising material to be used as electrolyte in proton conducting fuel cells, or as a mixed proton–electron conducting membrane for hydrogen separation, and its mechanical properties are crucial for these applications. Lanthanum tungstates with a La/W atomic ratio between 4.8 and 6.0 have been investigated at room temperature at micro/nanoindentation range. Lanthanum tungstates exhibit a strain gradient plasticity at the vicinity of the imprints, which implies that the hardness presents an indentation size effect that was corrected using the Nix and Gao approach. The hardness and Young's modulus have therefore been determined to be 8–9 GPa and 130 ± 15 GPa, respectively. The fracture toughness was estimated to be ∼2 MPa m 1/2 for LWO56 using the Palqmvist equation. Both hardness and Young's modulus did not present a significant dependence with neither the sintering temperature nor the composition. The different imprints were visualized by Atomic Force Microscopy.

Nanoporous Lanthanum Tungstate: A Viable Adsorbent for Heavy Metals and Organic Pollutants

AbstractRemoval of heavy metal cations from synthetic and industrial electroplating effluents by means of mesoporous lanthanum tungstate sorbents was thoroughly investigated. Three types of such sorbents, namely I‐LT, II‐LT and III‐LT, uncalcined and calcined, were produced and characterized by means of X‐ray diffraction, nitrogen sorption isotherms and thermogravimetry analysis (TGA). Results of the experiments for chromium, copper, mercury, iron and lead removal from both synthetic and real industrial wastewater solutions, using these novel sorbents, exhibit promising. Details of this new approach towards wastewater treatment have been discussed and the potentials of technological advancement using mesoporous lanthanum tungstate for environmental purposes addressed. Comparison with the literature shows superiority of the produced adsorbents over reported products.

Third structure determination by powder diffractometry round robin (SDPDRR-3)

The results from a third structure determination by powder diffractometry (SDPD) round robin are discussed. From the 175 potential participants having downloaded the powder data, nine sent a total of 12 solutions (8 and 4 for samples 1 and 2, respectively, a tetrahydrated calcium tartrate and a lanthanum tungstate). Participants used seven different computer programs for structure solution (ESPOIR,EXPO,FOX,PSSP,SHELXS,SUPERFLIP, andTOPAS), applying Patterson, direct methods, direct space methods, and charge flipping approach. It is concluded that solving a structure from powder data remains a challenge, at least one order of magnitude more difficult than solving a problem with similar complexity from single-crystal data. Nevertheless, a few more steps in the direction of increasing the SDPD rate of success were accomplished since the two previous round robins: this time, not only the computer program developers were successful but also some users. No result was obtained from crystal structure prediction experts.

New crystal structure and characterization of lanthanum tungstate “La6WO12” prepared by freeze-drying synthesis

Lanthanum tungstates with a La/W atomic ratio between 6 and 4.8 have been synthesized as polycrystalline materials using the freeze-drying wet-chemical precursor method. Our results show that a single phase material is obtained when the La/W ratio is between 5.3 and 5.7 (T = 1500 degrees C). Outside this compositional range, segregation of either La(2)O(3) (La/W > or = 5.8) or La(6)W(2)O(15) (La/W < or = 5.2) are found. We have solved the crystal structure for the composition with a La/W nominal atomic ratio of 5.6 by combining powder X-ray and powder neutron diffraction techniques. This structure substantially differs from that previously reported for Ln(6)WO(12) (Ln = Y, Ho). The main differences between the two structure types are the crystal symmetry, the different coordination environment of the cations and the formula unit. The formula unit can be written as La(6.63)W(1.17)O(13.43) (Z = 4; calculated density = 6.395 g/cm(3)), well in accordance with the diffraction techniques, He-pycnometry and electron probe microanalysis. These materials can be described as a face centred cubic structure with space group F43m. Lattice parameters vary between 11.173 and 11.188 A, depending on composition. Dense ceramic materials are obtained at 1400 degrees C, and microanalyses measurements indicate that no significant tungsten evaporation occurs compared to the nominal values. Compositions with La(2)O(3) segregation show similar conductivity values as the single phase ones, but those containing segregation of W-rich phases show a considerable drop in conductivity with increasing content of the secondary phase.

Potentiometric and conductometric studies on the system acid-isopolytungstate and the formation of lanthanum tungstates

The stoichiometry of the polyanions formed by the action of nitric acid on sodium tungstate(≤ 0.01M) has been studied by means of electrometric techniques involving pH-potentiometric andconductometric titrations. The well defined inflections and breaks in the titration curves provideevidence for the existence of the polyanions, para-W 12 O 4110- and meta-W 12 O 396- corresponding to theratio of H + :WO 42- as 7:6 and 9:6 in the pH ranges 5.7-6.0 and 3.6-4.1, respectively. The interaction oflanthanum nitrate with sodium tungstate solutions, at specific pH levels 8.0, 5.9 and 4.0 was alsostudied by pH and conductometric titrations, in aqueous and alcoholic media, with each of the reagentsalternatively used as titrant. The electrometric experiments provide definite evidence of the formationof normal-La 2 O 3 .3WO 3 , para-5La 2 O 3 .36WO 3 and meta-La 2 O 3 .12WO 3 tungstates in the vicinity of pH6.3, 5.0 and 4.2, respectively. Analytical investigations on the precipitates formed confirm the resultsof the electrometric study.

Potentiometric and conductometric studies on the system acid-isopolytungstate and the formation of lanthanum tungstates

The stoichiometry of the polyanions formed by the action of nitric acid on sodium tungstate (&lt; 0.01M) has been studied by means of electrometric techniques involving pH-potentiometric and conductometric titrations. The well defined inflections and breaks in the titration curves provide evidence for the existence of the polyanions, para-W12O41(10-) and meta-W12O39(6-) corresponding to the ratio of H+:WO4(2-) as 7:6 and 9:6 in the pH ranges 5.7-6.0 and 3.6-4.1, respectively. The interaction of lanthanum nitrate with sodium tungstate solutions, at specific pH levels 8.0, 5.9 and 4.0 was also studied by pH and conductometric titrations, in aqueous and alcoholic media, with each of the reagents alternatively used as titrant. The electrometric experiments provide definite evidence of the formation of normal-La2O3.3WO3, para-5La2O3.36WO3 and meta-La2O3.12WO3 tungstates in the vicinity of pH 6.3, 5.0 and 4.2, respectively. Analytical investigations on the precipitates formed confirm the results of the electrometric study.

Spectroscopic characterisation of the Tm3+ doped KLa(WO4)2 single crystals

Abstract The spectroscopic characterisation of the Tm3+ doped double potassium–lanthanum tungstate KLa(WO4)2 (KLWO) through the absorption, fluorescence and lifetime measurements is presented. We show absorption spectra in the visible and NIR regions recorded at room and low temperature, the luminescence spectra especially that of the 3H4 → 3H6 transition lying in the 800 nm region which makes it suitable for diode laser pumping. The lifetimes were measured for KLa(WO4)2 with 2 at.% of Tm3+ and the values were 112 μs, 208 μs, 1.7 ms and 5.7 μs for 1G4, 3H4, 3F4, and 1D2, respectively. The results of the Judd–Ofelt analysis are presented and discussed. The obtained intensity parameters were used to determine the radiative decay rates, radiative lifetimes and branching ratios for the Tm3+ transitions from the excited multiplet manifolds to the lower lying manifold states. The infrared cross-section measurements were done.

Laser Performance of Neodymium-Doped Lanthanum Tungstate Crystals

We demonstrated the laser oscillation at 1.06 µm with neodymium-doped lanthanum tungstate crystals under pulsed-laser pumping at 802 nm. Fiber-shaped crystals with Nd concentration of up to 20 at.% were grown using the micro-pulling-down method. A maximum slope efficiency of 35% was achieved for a 3.2 at.%-doped sample at 1059 nm.

Ab Initio Structure Determination of Lanthanum Cyclo-tetratungstate α-La2W2O9 from X-ray and Neutron Powder Diffraction

Lanthanum tungstate α-La2W2O9 has been successfully synthesized by the ceramic route. Its crystal structure was determined ab initio from neutron diffraction data (space group P1; No. 2; a=7.2489(1) Å b=7.2878(1) Å c=7.0435(1) Å α=96.367(1)°, β=94.715(1)°, γ=70.286(1)°; Z=2; reliability factors: Rp=6.0%, Rwp=6.9%, RB=3.0%). The structure exhibits isolated [W4O18]12− groups in the form of rings involving [WO6] octahedra alternating with [WO5] trigonal bipyramids sharing corners. These rings are inserted in the 3D framework built by the 9- and 10-fold coordination polyhedra of the lanthanum atoms.

Doping of an absorbent into a Raman crystal for suppression of higher-order Stokes generation

A Raman-active crystal, lanthanum tungstate doped with praseodymium (Pr) ions, was grown to facilitate the selection of the order of Stokes waves in stimulated Raman scattering. Absorption of the second Stokes wave by the Pr ions suppressed conversion to higher-order Stokes waves, resulting in efficient energy conversion to the first Stokes wave.

Fiber-like lanthanum tungstate crystal for efficient stimulated Raman scattering

A solid-state Raman material, fiber-shaped single crystal lanthanum tungstate (LWO) has been successfully grown by the micro-pulling-down method. The fiber-shaped LWO crystals yielded intense Stokes pulses with an energy shift of 940 cm−1. In stimulated Raman conversion, a conversion efficiency of 6.8% (internal efficiency of 37%) to the first Stokes wave was achieved.

Synthesis, Structure, and Reactivity of Novel Lanthanum Tungstates

Two new compounds, La[sub 2]W[sub 3]O[sub 12] and LaFeW[sub 3]O[sub 12], were prepared from La[sub 2]O[sub 3] and WO[sub 3] or from Fe[sub 2]O[sub 3], La[sub 2]O[sub 3], and WO[sub 3]. La[sub 2]W[sub 3]O[sub 12] has a structure which can be considered as a distorted superstructure of the scheelite structure CaWO[sub 4] in which 1/3 of the Ca sites are vacant. It is monoclinic, with space group C2/c and a = 7.873(2) [angstrom], b = 11.841(2) [angstrom], c = 11.654(2) [angstrom], and [beta] = 109.25(3)[degrees]. LaFeW[sub 3]O[sub 12] is triclinic with a = 7.569(3) [angstrom], b = 7.537(2) [angstrom], c = 32.41(2) [angstrom], [alpha] = 90.13(4)[degrees], [beta] = 94.75(3)[degrees], and [gamma] = 98.90(3)[degrees], and the structure consists of two distinct layers, one of which is a La tungstate which consists of a sheet of La in eight coordination and a sheet of WO[sub 4] tetrahedra and another which consists of a sheet of edge-sharing FeO[sub 6] octahedra sandwiched between two sheets of edge-connected WO[sub 6] octahedra. The reactivity of these compounds with H[sub 2] was investigated by TGA. La[sub 2]W[sub 3]O[sub 12] is reduced at 1100[degrees]C to yield a mixture of La[sub 2]O[sub 3] and W metal. The products ofmore » the reduction of LaFeW[sub 3]O[sub 12] were found to depend on the composition of the atmosphere. In N[sub 2] with 5% H[sub 2] the compound yields a complex mixture of La[sub x](WO[sub 3])[sub y] compounds with W and a W-Fe alloy, while in 100% H[sub 2], La[sub 2]O[sub 3], W, and the alloys Fe[sub 6]W[sub 7] or Fe[sub 2]W are formed.« less

Synthesis and Ion-Exchange Properties of Lanthanum Tungstate, A New Inorganic Ion Exchanger

Synthesis and Ion-Exchange Properties of Lanthanum Tungstate, A New Inorganic Ion Exchanger

Preparation and properties of lanthanum tungstate and studies of its ion-exchange and electrochemical behaviour

A new inorganic ion-exchanger lanthanum tungstate has been prepared by mixing lanthanum nitrate and sodium tungstate. The refluxed product has been found to be most suitable; therefore, most of the electrochemical and ion-exchange studies were performed on this product. The material is a weak cation exchanger with a capacity of 0.80 mmol per gram. Electrochemical studies are performed by preparing a pellet of this exchanger in polystyrene; charge density and membrane potential have been calculated. The results are discussed.

Synthesis and Ion-Exchange Properties of Lanthanum Tungstate, A New Inorganic Ion Exchanger

Abstract Amorphous samples of a new inorganic ion exchanger, lanthanum tungstate, have been prepared under varying conditions. The material prepared by mixing 0.05 M lanthanum nitrate and 0.05-M sodium tungstate in a ratio of 1:2 was studied in detail for its ion-exchange capacity, chemical stability, IR, thermogravimetry, and Kd values. Its columns have been used for the separation of CD2+ from Pd2+, Mn2+, and Cu2+, and Ni2+ from Pd2+.