The crystal structures of Ba2R2/3V2O8 (R = La, Nd) and Sr2La2/3V2O8; palmierite derivatives

The title phases Ba₂R_2/3V₂O₈ (R = La, Nd) and Sr₂La_2/3V₂O₈, synthesised by solid state reaction of oxides at 1350°C, have structures derived from that of the palmierite-type of Ba₃V₂O₈; the stoichiometry is often written as A₃RV₃O₁₂. Ba₂La_2/3V₂O₈ is hexagonal, spacegroup R3m, a = 5.75271(7), c = 21.0473(5) Å, Z = 3; cation ordering was determined by joint Rietveld refinement using X-ray and neutron powder diffraction data, R _wp = 4.45%, R _p = 6.33%, ² = 6.847. In the Ba₃V₂O₈ structure, Ba occupies two sites: 3a and 6c. In Ba₂La_2/3V₂O₈, Ba wholly occupies the 3a site; the 6c site contains both Ba, La and vacancies. Bond valence analysis was inconclusive, but tends to support the presence of Ba on the 3a site. Sr₂La_2/3V₂O₈ and Ba₂Nd_2/3V₂O₈ are isostructural, as confirmed by Rietveld refinement using X-ray powder diffraction data. Ba₂R_2/3V₂O₈ phases could not be synthesised for lanthanides, R, smaller than Sm.     

Structural properties of lead vanadate glasses containing La3+ or Fe3+ ions

Structural properties of lead vanadate glasses containing La³⁺ or Fe³⁺ ions were investigated using X-ray diffraction, Fourier transform infrared spectroscopy and laser Raman spectroscopy. Crystalline Pb₂V₂O₇ was formed for the molar composition 66.7PbO-33.3V₂O₅. Incorporation of greater quantities of La³⁺ into lead metavanadate glass caused the crystallization of Pb₂V₂O₇. Fourier transform infrared and laser Raman spectra also suggested the presence of LaVO₄. Incorporation of Fe³⁺ ions into lead metavanadate glass, up to 20 wt% Fe₂O₃, did not cause crystallization inside the glass matrix. Changes in the vibrational spectra are discussed.     

Synthesis and characterization of nanocrystalline La2Mo2O9 fast oxide-ion conductor by an in-situ polymerization method

A nanocrystalline La₂Mo₂O₉ powder was synthesized via the pyrolysis of polyacrylate salt precursor prepared by an in situ polymerization of the metal salts and acrylic acid. The pyrolysis behavior of the polymeric precursor was studied by thermal (TG/DTA) analysis. The obtained product was characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) analysis. The results revealed that the average particle size is ∼25 nm for La₂Mo₂O₉ with good crystallinity. The synthesized nanocrystalline La₂Mo₂O₉ powder showed good sinterability and reached ∼99% of theoretical density when sintered at 800 °C for 4 h. The La₂Mo₂O₉ sample sintered at 800 °C, yield good microstructure with improved conductivity value of about 0.12 S/cm at 800 °C.     

Effect of La2O3 doping on the microstructure and electrical properties of a SnO2-based varistor

The effect of La₂O₃ addition on the densification and electrical properties of the (0.9895-xSnO₂+0.01CoO+0.0005Nb₂O₅+x La₂O₅ system, where x=0.0005 or 0.00075, was considered in this study. The samples were sintered at 1300 °C for 2 and 4 h and a single SnO₂ phase was identified by X-ray diffraction. Microstructure analysis by scanning electron microscopy showed that the affect of La₂O₃ addition is to decrease the SnO₂ grain size. J versus E curves indicated that the system exhibits a varistor behavior and the effect of La₂O₃ is to increase both the non-linear coefficient () and the breakdown voltage (E₂). Considering the Schottky thermionic emission model the potential height and the width were estimated. The addition of small amounts of La₂O₃ to the basic system increases the potential barrier height and decreases both grain size and potential barrier width. © 2001 Kluwer Academic Publishers     

Phase equilibria in the system CdO-B2O3-GeO2 at 800°C

Phase equilibria in the system CdO-B₂O₃-GeO₂ were investigated at 800° C using X-ray powder diffraction techniques. The binary phases reported previously as produced by solid-state reactions were confirmed and, in addition, a new one CdO·2GeO₂ was found. The ternary phase diagram was solved and a new phase CdO·B₂O₃·GeO₂ was discovered. X-ray powder data are given for both these new phases. Solid solution effects were investigated for the primary and binary phases by comparison of patterns; no solid solutions were detected. The relationship of the phase diagram to the composition of photoconductive oxide glasses is discusssed in the light of previous suggestions made of possible mechanisms responsible for the photoelectric effects.     

Phase equilibria in the high magnesia corner of the system MgO-ThO2-B2O3 at 1200° C

Phase equilibria in the high magnesia corner of the system MgO-ThO₂-B₂O₃ were investigated at 1200°C using X-ray powder diffraction techniques. The two magnesium borates reported previously at this temperature were confirmed, but no magnesium thorate and no ternary phases were found. Solid solution effects were investigated; for the binary phases by comparison of patterns, for magnesium oxide and thorium oxide by comparison of lattice parameters. No solid solutions were detected.Phase equilibria in the system MgO-ThO₂ were investigated at 1500°C, again no binary compound was found and no solid solution detected. A suggestion is made to reconcile the latter observation with previous work.     

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.     

H2S sensing properties of La-doped nanocrystalline In2O3

The nanocrystalline powders of pure and La³⁺-doped In₂O₃ with cubic structure were prepared by a simple hydrothermal decomposition route. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and microstructure by transmission electron microscopy (TEM). All the compositions exhibited a single phase, suggesting a formation of solid solution in the concentration of doping investigated. Gas-sensing properties of the sensor elements were tested by mixing a gas in air at static state, as a function of concentration of dopant, operating temperature and concentrations of the test gases. The pure In₂O₃ exhibited high response towards H₂S gas at an operating temperature 150 °C. Doping of In₂O₃ with La³⁺ increases its response towards H₂S and La³⁺ (5.0 wt.% La₂O₃)-doped In₂O₃ showed the maximum response at 125 °C. The selectivity of the sensor elements for H₂S against different reducing gases was studied. The results on response and recovery time were also discussed.     

High-temperature structural change of CrOx-LaOx-Al2O3 catalyst for lean-burn exhaust treatment

Solid-state reaction in the system of CrOx-Al₂O₃ and CrOx-LaOx-Al₂O₃ and their sintering at 500–1100°C were examined by X-ray diffraction, electron spin resonance (ESR) and surface area measurement for the development of heat-stable catalytic ceramic in lean-burn exhaust treatment. CrOx-LaOx-Al₂O₃ catalyst, even heated at 1000°C in air, showed the removal conversion of 100% for C₃H₆, 95% for CO and 7% for NO at 500°C for high velocity automotive lean-burn exhaust with A/F = 18 and S.V. = 10⁵h^–1. La-modification of catalyst was effective to high surface area stabilization and the improvement of complete oxidation activity of CO and hydrocarbons.     

Phase relations in the system CaO-B2O3-SiO2

Phase relations in the lime-rich portion of the system CaO-B₂O₃-SiO₂ have been studied by microscopy, infrared spectroscopy and X-ray powder diffraction of heated mixtures and quenched charges. Extensive solid solution of B₂O₃ in Ca₂SiO₄ occurs along the Ca₂SiO₄-Ca₃B₂O₆ boundary, which has been studied in detail. It contains a ternary compound, Ca₁₁B₂Si₄O₂₂, which is stable to liquidus temperatures, melting incongruently to Ca₂SiO₄ and liquid at 1420 °C. Ca₁₁B₂Si₄O₂₂forms a eutectic with Ca₃B₂O₆ at 1400 °C and, in the ternary system, with CaO and Ca₃B₂O₆ at 1390 °C.     

Acoustic emission study of phase relations in low-Y2O3 portion of ZrO2-Y2O3 system

The (metastable) tetragonal phase in 3–4 mol% Y₂O₃-ZrO₂ alloys undergoes a transition to the monoclinic form in the 200–300 °C temperature range. Microcracking due to the volume change at this transition has been detected in these compositions by sharp acoustic emission during heating. The phase change was confirmed by X-ray diffraction, dilatometry and scanning electron microscopy. The monoclinic tetragonal transition in ZrO₂-1 mol% Y₂O₃ alloy at 850–750 °C and the same phase change in 2, 3, 4 and 6 mol% Y₂O₃ compositions at the eutectoid temperature of about 560 °C was also clearly signalled by the acoustic emission counts during heating and cooling. There was no significant acoustic emission activity on heating and cooling the 9 and 12 mol% Y₂O₃ compositions, which are cubic. The acoustic emission data thus confirm the phase relations in the 1–12 mol% Y₂O₃ region, established by conventional methods such as differential thermal analysis, dilatometry and X-ray diffraction.     

Al2O3-Ta2O5-rich glass-ceramic with interconnected pores and its sintering

Substitution of SiO₂ in the ternary sodium borosilicate system with Al₂O₃ plus Ta₂O₅ was found to produce glass which decomposed into microphases and/or crystallized after heat treatment. At least one of the phases present was water soluble. The structure of the material was glassy with the presence of a small crystalline content. Crystalline forms found during powder X-ray diffraction analysis of heat treated, leached and then sintered materials were orthorhombic NaTaO₃ plus AIBO₃, orthorhombic NaTaO₃ and orthorhombic Na₂O · 4Ta₂O₅ plus rhombic 9AI₂O₃ · 2B₂O₃, respectively. The specific surface areas of the leached materials ranged between 96 and 304 m²g^–1, while the mean pore radii of interconnected pores were calculated to be between 2.0 and 8.4 nm. A sintering rate of between 1520 and 1580° C for 5 min were estimated from void volume and bulk density measurements.     

Phase equilibria in the system CdO-B2O3-SiO2 at 800° C

Phase equilibria in the system CdO-B₂O₃-SiO₂ were investigated at 800° C using X-ray powder diffraction techniques. The binary phases reported previously were confirmed, but no ternary phases were found. Solid solution effects were investigated for the primary and binary phases by comparison of patterns; no solid solutions were detected. The relationship of the phase diagram to the composition of photoconductive oxide glasses is discussed and suggestions are made of possible mechanisms responsible for the photoelectric effects.     

Structural studies of some V2O5-P2O5-B2O3-Fe2O3 glass systems

X-ray diffraction and infrared measurements were performed on vanadium borophosphate glass containing different amounts of iron ranging from 0–7.5 mol % and heat treated at 300 °C for various times. The structure and phase separation could be determined for each glass composition. V₂O₅ was the main precipitated phase in all heat-treated samples, and its amount was dependent on the heat-treatment time and Fe₂O₃ content. Also FeP was detected in samples heat treated for 24 h. The infrared measurements showed the presence of both V⁴⁺ and V⁵⁺. The symmetry of V₂O ₇ ⁴⁻ and VO ₄ ³⁻ groups was found to increase with increasing Fe₂O₃ content. It was also found that some PO₄ changed to BO₃, forming a non-bridging oxygen.     

Influence of Al2O3 support on the thermal decomposition of ammonium metavanadate

The thermal decomposition of ammonium metavanadate (AMV) supported on aluminium oxide was investigated using differential thermal analysis, thermogravimetry, infrared and X-ray diffraction techniques. The results obtained revealed that the decomposition of AMV supported on alumina proceeded in three decomposition stages. Alumina was found to enhance only the decomposition of the intermediate ammonium hexavanadate to produce V₂O₅. In addition, the values of activation energies of the various decomposition stages were accompanied by a significant decrease on decreasing the concentration of AMV. The infrared spectra indicated that the band corresponds to the surface site V=O strongly affected by the presence of Al₂O₃. Finally, an interaction between Al₂O₃ and V₂O₅ occurred at 660 °C giving well-crystallized AlVO₄.     

Electrochemical characterizations of surface modified LiMn2O4 cathode materials for high temperature lithium battery applications

LiMn₂O₄ spinel cathode materials were coated with 2.0 wt.% of La₂O₃ by polymeric process followed by calcinations at 400 °C and 800 °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 X-ray photoelectron spectroscopy. La₂O₃-coated LiMn₂O₄ 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 and compact coating layer on the surface of the core materials. La₂O₃ was completely coated over the surface of the LiMn₂O₄ core cathode materials. The galvanostatic charge and discharge of the La₂O₃-coated LiMn₂O₄ cathode materials were carried out at 0.1 mA/g in the range of 3.0 and 4.5 V at 30 °C and 60 °C. Based on the results, La₂O_3-coated spinel LiMn₂O₄ cathode at 800 °C has improved the structural stability, high reversible capacity and excellent electrochemical performances of the rechargeable lithium batteries.     

Metastable phase relation and phase equilibria in the Cr2O3-Fe2O3 system

In order to ascertain the metastable phase relation in the Cr₂O₃-Fe₂O₃ system, the existing phases were investigated by X-ray analysis using samples obtained by heating the coprecipitated powders for 1 h at 600–1000°C. There was a metastable two-phase region of Cr₂O₃-rich (CC) and Fe₂O₃-rich (FC) phases below about 940°C. Equilibrium state of 1:1 composition at 600–900°C was considered to be a single phase of the corundum solid solution. The metastable two-phase CC + FC region was suggested to appear probably due to the compositional inhomogeneity in the coprecipitated powders.     

Crystallization behavior of amorphous solid solutions and phase sepration in the Cr2O3-Fe2O3 system

Fine particles of the amorphous Cr₂O₃-Fe₂O₂ solid solutions were prepared by dehydration of coprecipitated hydroxides and their crystallization behavior was studied by differential thermal analysis and X-ray diffraction. The peak temperature for crystallization attained a maximum at a composition near Fe₂O₃ content of about 60 mol % and the activation energy for crystallization attained a minimum at a composition near Fe₂O₃ content of about 50 mol % in this quasibinary system. Phase separation occurred in a range of Fe₂O₃ content from about 35 to 80 mol % in the corundum-type solid solutions heat treated at 600 °C for 2 h. Crystallization behavior was discussed briefly related with phase separation and diffusion in fine particles.     

Preparation and characterization of binary V2O5-Bi2O3 glasses

Characterization of the binary V₂O₅-Bi₂O₃ glasses prepared by rapidly quenching the melt has been made from the studies of X-ray diffraction, scanning electron microscopy, infrared absorption, differential thermal analysis, electron paramagnetic resonance, chemical analysis, density and electrical properties. Stable glasses are obtained for 95 to 75 mol % V₂O₅ by quenching on a stainless steel substrate, while quenching on a copper substrate extends the glass formation range from 95 to 70 mol % V₂O₅. The V-O bond vibration in the glasses occurs at 1020 cm^–1 and the V^5% ion exists in six-fold coordination as in crystalline V₂O₅. All the glasses appear to be in single phase. The spin concentration in the glasses is found to be independent of temperature. A second heat-treatment at 255° C develops crystalline phase in the glasses. Unlike infrared absorption, electron paramagnetic resonance, density and chemical compositions, the electrical and thermal (DTA) properties are found to be slightly sensitive to the thermal history of preparation of the glasses. The high-temperature (300 to 500 K) conduction in the glasses seems to be due to adiabatic hopping of polarons. The thermopower is observed to be independent of temperature and provides evidence for small polaron formation in the glasses.     

A novel broadband emission phosphor Ca2KMg2V3O12 for white light emitting diodes

A novel broadband emission phosphor Ca₂KMg₂V₃O₁₂ was first synthesized by solution combustion method. The X-ray diffraction showed that Ca₂KMg₂V₃O₁₂ phase can be obtained at 600-900 °C through combustion route. The crystal structure of this material was refined by Rietveld method using powder X-ray diffraction. It crystallizes in cubic system and belongs to space group Ia3d with z = 8, a = 0.12500 nm. The excitation band of Ca₂KMg₂V₃O₁₂ peaks at 320 nm in a region between 260 nm and 425 nm, and the emission spectrum exhibits an intense band centered at about 528 nm covering from 400 nm to 800 nm. The colour coordinates of samples prepared at different ignition temperatures are in a range of x = 0.323-0.339, y = 0.430-0.447.