Single Pyrochlore Phase Research Articles

Synthesis, Different Characterizations and Pigment Application of Nanoceramic Zirconate Powder.

In this work, several attempts were made to prepare nanoceramic zirconate (Nd₂Zr₂O7) powder for pigment applications. The single pyrochlore phase, Nd₂Zr₂O7 was successfully synthesized with the aid of flux at relatively low temperature (1000 °C) upon milling the reaction mixture made from Nd₂O₃ and ZrO₂ in a tungsten carbide vial. Several characterizations were done on the thermal analysis (TG/DTA), X-ray diffraction (XRD), High-resolution scanning electron microscope (HRSEM), energy dispersive X-ray (EDX) analysis, Fourier-transform infrared (FT-IR) analysis, and near-infrared (NIR spectroscopy). A single-phase nanocrystalline (˜28 nm) pyrochlore structure (Fd3m) was confirmed through XRD analysis. A highly uniform particle in the size of ˜110 nm was observed for the Nd₂Zr₂O7 phase in the microimages. The vibrational (FT-IR) peaks at 423 cm-1 and 510 cm-1 in FTIR spectroscopic study confirmed the formation of pyrochlore structure. Higher NIR reflectivity recorded for this material in the 750-2500 nm region shows its novel application in color cool pigments for energy-saving paints reminiscent of Bi2-xYxCe²O7, Bi²Ce2-xTbxO7, and Gd²Ti²O7.

Synthesis, characterization, and single-crystal growth of a high-entropy rare-earth pyrochlore oxide

The synthesis and single-crystal growth of a rare-earth high-entropy oxide ${({\mathrm{Yb}}_{0.2}{\mathrm{Tb}}_{0.2}{\mathrm{Gd}}_{0.2}{\mathrm{Dy}}_{0.2}{\mathrm{Er}}_{0.2})}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ in the cubic pyrochlore phase $Fd\overline{3}m$ are achieved by solid-state reaction and optical floating-zone growth techniques. X-ray diffraction and a structure refinement analysis confirm a single-phase pyrochlore structure and lattice parameter of $a=10.1152(3)\phantom{\rule{0.16em}{0ex}}\AA{}$. Additional characterization techniques, including scanning transmission electron microscopy and nanoscale electron-energy loss spectroscopy mapping, support a single-phase pyrochlore structure with a homogeneous mixture down to \ensuremath{\sim}2.3 nm. Magnetization measurements on a single crystal reveal antiferromagnetic correlations with a spin-glass ground state. These results show that it is possible to grow large single crystals of a high entropy pyrochlore oxide, expanding the avenues for future high-entropy oxide research.

Synthesis of Er2Ir2O7 pyrochlore iridate by solid-state-reaction and CsCl flux method

The solid-state reaction, hydrothermal synthesis and CsCl flux methods were employed preparing Er2Ir2O7 pyrochlore iridate. A mixture of initial Er2O3 and IrO2 oxides was thermally treated, following the temperature evolution of individual phases by means of x-ray diffraction and energy dispersive x-ray spectroscopy. Reactions by hydrothermal synthesis, using various commonly used mineralising agents, showed negligible increase in the pyrochlore phase fraction. The solid-state reaction method allowed a preparation of Er2Ir2O7, however the reacted mixture contained a high percentage (60%) of initial unreacted Er2O3. Adding an excess of IrO2 to the initial content improved the composition of the reacted sample (50% of Er2Ir2O7 for initial Er2O3:IrO2 ratio 0.9:2.1). Nevertheless, a secondary Er–Ir–O phase with slightly larger crystallographic unit cell was also created as an additional product. An optimised flux synthesis, consisting of repeated heating and regrinding cycles at 800 °C and using CsCl as a flux, provided the best conditions for single phase pyrochlore preparation. A sample with 94% Er2Ir2O7 was further improved by repeated reaction with additional excess IrO2 oxide. A successful preparation route for (at least) the heavy-rare earth pyrochlore iridates is established.

White light emitting stannate pyrochlore based single phase phosphor CaLa1−xSnNbO7:xDy3+ for pc-WLED applications

A new series of single phase pyrochlore based white phosphors, CaLa1−xSnNbO7:xDy3+ (x = 0.01, 0.02, 0.03, 0.04) and Ca1−ySryLa0.97SnNbO7:0.03Dy3+ (y = 0.1, 0.2, 0.3) were developed by a high temperature ceramic route. The phase purity and the crystalline structure of the as-prepared samples were characterised by powder X-ray diffraction method and the phosphors were found to crystallize into the cubic pyrochlore type structure. The photoluminescence properties were investigated and the results indicated that phosphors show a strong excitation levels in near UV (388 nm) and blue (452 nm) regions and emits intense complimentary blue (487 nm and 477 nm) and yellow (589 nm) light upon UV or blue excitation which can be assigned to the characteristic 4F9/2 → 6HJ (J = 13/2 and 15/2) transitions of Dy3+ respectively. Thus the suitable combination of the above complimentary colors, yellow and blue is an approach to realize a full color emission and the Commission Internationale de I’Eclairage (CIE) color coordinates of the resultant phosphors lie in the white light region (0.38, 0.36) close to the standard chromaticity coordinates (white light) of NTSC. The substitution of bigger ion like Sr2+ for Ca2+ enhanced significantly the photoluminescence properties by way of their excitation and emission intensities due to increased polarizability and distortion of the Dy3+ environment. These results demonstrate that CaLa1−xSnNbO7:xDy3+ and Ca1−ySryLaSnNbO7:0.03Dy3+ are promising single phased white phosphor for pc-WLED applications.

Solid-state reactive sintering of La2-xGdxZr2O7 transparent ceramics and their optical properties

Abstract Transparent polycrystalline La2-xGdxZr2O7 (x = 0.4–2.0) ceramics were fabricated by solid-state reactive sintering using commercially available La2O3, Gd2O3 and ZrO2 nanopowders as the raw materials. The phase composition, microstructure evolution, densification and optical transmittance of the resulting ceramics were investigated. XRD and Raman results reveal that both the powders and ceramics are in single-phase pyrochlore structure. With the promotion of Gd content, the disorder degree of pyrochlore structure increases gradually while the cell parameters decrease. The x = 1.0, 1.2 and 1.6 samples exhibit high optical transmittance in the 450 nm-6.0 μm range, and the La0.4Gd1.6Zr2O7 sample shows the highest transmittance of 83.84%. The transparent La2-xGdxZr2O7 ceramics with high optical quality are available as scintillator matrice and high temperature window material.

Vanadium-substituted Sm2Ti2O7 pyrochlore. Insight into the structure and electrical conductivity under oxidizing and highly reducing atmosphere

With the aim to increase the conductivity of Sm2Ti2O7 in reducing atmosphere characteristic to the anodic side of the SOFC by partial substitution of titanium site with vanadium, crystalline phases, reduction behavior and transport properties of vanadium-doped Sm2Ti2O7 were investigated in this study. Sm2Ti2-xVxO7±δ (x = 0; 0.1; 0.2) powders have been synthesized using a modified Pechini method. After thermal treatment in air, the presence of monoclinic secondary phase was evidenced by X-ray diffraction analysis and Raman spectroscopy, while thermal treatment in 2% H2/Ar mixture led to single-phase pyrochlore structures. H2-TPR analysis revealed the high reducibility of vanadium-substituted sample. The electrical conductivities of the investigated sample in air are quite small, mainly due to the presence of secondary phase. The decrease in oxygen partial pressure led to a high increase in overall conductivity due to the n-type electronic component. Sample with 5% V on Ti exhibited the highest increase in conductivity when switching from air to hydrogen-containing atmosphere.

Titanium substitution in Gd2Zr2O7 for thermal barrier coating applications

Abstract The study underlines the impact of Ti4+ substitution in Gd2Zr2O7 for applications in thermal barrier coatings (TBC). Depending on the Ti4+ content, two different crystal structures of Gd2Zr2O7 namely pyrochlore and fluorite were determined. Ti4+ substitutions in the increasing order induced a gradual contraction of Gd2Zr2O7 unit cell; however, with the accomplishment of concentration dependent crystal structures of either single phase pyrochlore or mixtures of pyrochlore and fluorite. Absorption measurements enunciated the enhanced infra-red reflectance behaviour of Gd2Zr2O7 due to Ti4+ substitutions. A gradual increment in the concentration of Ti4+ substitutions in Gd2Zr2O7 envisaged a simultaneous porous to dense morphological features, which reflected in the resultant mechanical data. Hot corrosion studies ensure the critical role of Ti4+ to retain the crystal structure of Gd2Zr2O7.

Fabrication and optical properties of transparent LaErZr2O7 ceramic with high excess contents of La and Er

Abstract In this study, transparent LaErZr2O7 ceramic with high excess La and Er contents (nominally La1.28Er1.28Zr2O7.84) was successfully prepared by vacuum sintering at 1850 °C for 6 h using nanosized powder. The XRD, SEM, EDX and TEM results reveal that the single pyrochlore phase in the powder sample transforms to the coexistence of La-rich pyrochlore phase and Er-rich defect fluorite phase after high temperature sintering. The high excess amounts of La and Er favor the formation of pyrochlore structure. Despite the coexistence of two phases, the sample with 1 mm thickness shows excellent in-line transmittance in the visible to mid-infrared region (as high as 81% at 1100 nm). The upconversion and infrared emission under 980 nm exciting were measured and discussed as well.

Phase evolution and chemical durability of Zr1-xNd xO2-x/2 (0 ≤ x ≤ 1) ceramics

A series of Zr1-xNd xO2-x/2 (0≤x≤1) ceramics was prepared by solid-state reaction method. The effects of Nd content on the phase evolution were investigated. The chemical durability of resulting waste forms was also examined. The results show that the ceramics with x<0.1 show monoclinic and cubic zirconia phase, with 0.2≤x<0.4 exhibit a single cubic phase, with 0.4≤x≤0.6 exhibit a single pyrochlore phase, with 0.6<x<0.8 exhibit a single cubic phase and remain cubic phases and hexagonal Nd2O3 when 0.8≤x≤1. The unit cell parameters of the Nd-doped zirconia samples increase as the Nd content increases. Moreover, the normalized element release rates of Nd element in Nd-doped zirconia ceramics firstly decrease with leaching time and almost no change after 21days (∼0−6gm−2d−1), demonstrating its good chemical durability.

Molten salt synthesis of titanate pyrochlore waste-forms

Actinide chlorides, such as those arising from pyrochemical reprocessing operations can be problematic to immobilise, as the high chloride content often makes their solubilities in melts very low, and even in small quantities can seriously affect the properties of the waste-form. Rather than attempt to immobilise the chlorine, one potential approach is to utilise the chloride salt as a reaction medium from which the An(III) cations can be extracted and immobilised. To this end, lanthanide titanate pyrochlores have been prepared by molten salt synthesis in CaCl2:MgCl2, CaCl2:NaCl and MgCl2:NaCl eutectics. Single-phase pyrochlore is found to be formed at temperatures as low as 650°C in the CaCl2:NaCl system, whereas in the CaCl2:MgCl2 and MgCl2:NaCl eutectics reaction with Mg produces a magnesium titanate secondary phase. Compositions of Yb2Ti2O7 doped with Sm3+ as an actinide surrogate have been synthesised, and cold-pressing and sintering at 1500°C yields fully dense pellets.

A novel thermal barrier coating for high-temperature applications

A new thermal barrier coating system based on La1.7Dy0.3Zr2O7 (LDZ), which had a lower thermal conductivity for applications above 1773K, was prepared by the air plasma spraying (APS). The phase composition, thermal expansion coefficient, thermal conductivity, the actual heat insulation and antioxidant ablation of the as-sprayed coatings were investigated. XRD results reveal that single pyrochlore phase LDZ coating is prepared and no new phase appears after ablation at 1573K and 1773K. Compared to 8wt% yttria partially stabilized zirconia (YSZ) coatings, LDZ has better phase stability, lower thermal conductivity, better actual heat insulation, antioxidant ablation and heat resistance performance. These results imply that the LDZ ceramics can be explored as candidate material for the ceramic layer in TBC system.

An electron spin resonance and photoluminescence investigation of the effect of annealing temperature on Gd-doped La2Zr2O7 nano-ceramics

Lanthanum zirconate pyrochlore (La2Zr2O7=LZO) is considered as a potential ceramic host matrix for nuclear waste immobilization. In this context, incorporation of Gd ions, a surrogate for trivalent minor actinides, in the LZO host was investigated. Undoped and Gd doped LZO samples were synthesized using a low temperature gel-combustion route and characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), dynamic light scattering (DLS), electron spin resonance (ESR) and photoluminescence (PL) spectroscopic techniques. It was observed that even at 500°C, a single phase pyrochlore could be stabilized without any impurity phase. The average particle size of the synthesized product was estimated to be 85–90nm. At lower temperatures, it was observed that, the Gd ions were present mostly on the surface of the LZO host which diffused into the lattice and replaced the La3+ ions after annealing at 700°C for 5h. Due to difference in the ionic radii, the environment around the Gd ion was highly asymmetric. This was further confirmed by PL investigations wherein upon excitation with UV radiation, the nano ceramics exhibited strong and sharp transition at 310nm suggesting the existence of Gd3+ions in a D2d geometry. Upon annealing, agglomeration of the particles was taking place reducing the surface defects and increasing the PL intensity as well as life time values.

Epitaxial growth of tin(II) niobate with a pyrochlore structure

Tin(II) niobate with a pyrochlore structure has a unique valence electronic structure associated with the hybridization of the O-2p and Sn-5sp orbitals. It is represented as the chemical formula of Sn2Nb2O7 for the ideal stoichiometry while reported to have a highly non-stoichiometric tendency. In this study, we have fabricated tin(II) niobate films with a pyrochlore structure on yttria-stabilized zirconia (100) substrates by means of pulsed laser deposition and investigated their optical properties. The single pyrochlore phase has been obtained for the films with Sn/Nb ratios of 0.5 and 0.9. The film with the higher Sn/Nb ratio of 0.9 is epitaxially grown on the substrate with a cube-on-cube orientation relationship. It has a flat surface comparable to that of the substrate. The absorption coefficient of the film is evaluated from optical transmittance and reflectance measurements. The optical direct band gap is estimated to be 2.6eV and its absorption coefficient reaches 1.0×105cm−1 at about 3.0eV above the absorption threshold.

Characterization and phase stability of pyrochlore (Nd1−xCex)2Zr2O7+y (x = 0–1)

Powders of (Nd1−xCex)2Zr2O7+y (x=0–1) with pyrochlore structure were prepared by sol-spray pyrolysis and subsequent graphite reduction at 1200°C. The effects of cerium content on crystal structure, phase stability and microstructure of the (Nd1−xCex)2Zr2O7+y (x=0–1) inert matrix materials were studied. XRD and Raman results presented that all samples were in a single pyrochlore phase. XPS results indicated the cerium was in a single oxidation state of +4. After sintering at 1500°C for 24h in air, the sample of x=1 underwent a phase segregation to Zr1−xCexO2 solid solutions of tetragonal, metastable tetragonal and cubic phases, the sample of x=0.9 contained a few tetragonal phase in addition to fluorite phase, the samples of x=0.5–0.8 had a single phase fluorite structure, the samples of x=0–0.4 maintained the pyrochlore structure.

Synthesis and characterization of Y2Ti2O7 and ErxY2−xTi2O7 nanofibers

1-D nanostructures are deemed to be important building blocks for future optical and optoelectronic nanodevices. We have used electrospinning methods to synthesize nanofibers and freestanding, non-woven nanofibers membranes of single phase yttrium titanate and ErxY2−xTi2O7 (x = 0–12 %) with diameters less than 150 nm and have characterized the physical, thermal and optical properties. We have demonstrated that such fibers can be synthesized at a relatively low cost and high production volume with a narrow diameter range. No clear phases are observed up to a calcination temperature of 700 °C and pyrochlore phase appear at 725 °C which are also confirmed by the TGA. Single pyrochlore phase is observed at temperatures greater than 900 °C. No other oxide phases are present.

Cobalt Doping Effects on the Mechanical and Electrical Parameters of Bi1.5Zn0.92Nb1.5O6.92 Solid Solution

The cobalt doping effects on the lattice constant, strain, grain size, dislocation density and electrical conduction are investigated by means of X-ray diffraction and electrical resistivity measurements on the Bi1.5Zn0.92Nb1.5–xCoxO6.92–x (x=0.03-0.20) ceramics, respectively. Increasing cobalt content sharply increases compressing strain and dislocation density and decreases both the lattice constant and the grain size of the pyrochlore. At a doping content of 0.05 new minor phase of ZnO appears. The ZnO grains increase with increasing cobalt content. When the cobalt doping is repeated in accordance with the formula Bi1.5Zn0.92Nb1.5–3x/5CoxO6.92, a single phase pyrochlore is obtained with cobalt content up to 0.10. The electrical resistivity analysis reflects increasing activation energy with increasing cobalt content. The cobalt creates an impurity level in the energy gap of the pyrochlore that shifts towards the mid gap converting the extrinsic nature of conductivity to intrinsic at a cobalt content of 0.10.

Effect of fluorite phase on the electrical conductivity of (Sm1−Dy )2Zr2O7 pyrochlore

Abstract A single pyrochlore phase for x = 0 to 0.4 in (Sm1 − xDyx)2Zr2O7 was obtained and a fluorite phase for x = 0.5 to 1.0. The occupancy of oxide ion on the 8b site which is originally a vacant site in Fd 3 ¯ m increased linearly from x = 0 to 0.4. The substitution of Dy3 + for Sm3 + increased the electrical conductivity and showed an abrupt decrease at x = 0.5 and further substitution decreased the conductivity. The Raman spectra for x = 0.4 showed a possibility for fluorite symmetry to exist in addition to a pyrcohlore whereas XRD patterns a single pyrochlore phase. A small amount of Al2O3 addition eliminated the fluorite phase observed by XRD and stabilized the pyrochlore phase. Additionally, the Al2O3 addition improved the electrical conductivity of 6.6 × 10− 3 Scm− 1 at 1073 K.

Synthesis and Conductivity Measurement of Lanthanum Zirconate Doped with Rare Earth Dopants

In this work, La2Zr2O7 with a pyrochlore structure was doped with Sc, Y, Nd, Sm and Gd into its Zr-site to investigate the dependence of conduction behavior on the dopant. A systematic study was also performed to study the relationship between Y content and the proton conductivity. The results revealed that La2Zr2O7 doped with Y had higher conductivity, compared with those doped with Sc, Nd, Sm and Gd. Among the samples doped with various content of Y, La2(Zr0.9Y0.1)2O7−δ was identified to have a single pyrochlore phase, and exhibited the highest bulk conductivity of 3.68 × 10−4 Scm−1 at 600°C in wet H2. But such value is still not enough for the application as an electrolyte in fuel cells. Poor capability for water incorporation is one of the reasons restricting a further increase of the proton conductivity.

Green Up-Conversion Photoluminescence and Dielectric Relaxation of Ho3+ /Yb3+ -Codoped Bi2 Ti2 O7 Pyrochlore Thin Films

Ho3+/Yb3+-codoped Bi2Ti2O7 pyrochlore thin films were prepared by a chemical solution deposition method, and their visible up-conversion (UC) photoluminescence and dielectric relaxation were studied. Ho and Yb can be doped into Bi2Ti2O7 lattice and single pyrochlore phase is maintained. Intense visible UC photoluminescence can be observed under the excitation of a 980-nm diode laser. Two UC emission bands centered at 551 nm and 665 nm in the spectra can be assigned to 5F4, 5S2→5I8 and 5F5→5I8 transitions of Ho3+ ions, respectively. The dependence of their UC emission intensity on pumping power indicates that both the green and red emissions of the thin films are two-photon process. In addition, a Stokes near-infrared emission centered at 1200 nm can be detected, which is due to 5I6→5I8 transition of Ho3+ ions. The thin films prepared on indium tin oxide–coated glass substrates exhibit a relatively high dielectric constant and a low dielectric loss as well as a good bias voltage stability. The dielectric relaxation of the thin films was also analyzed based on the temperature- and frequency-dependent dielectric properties. This study suggests that Ho3+/Yb3+-codoped Bi2Ti2O7 thin films are promising materials for developing multifunctional optoelectronic thin film devices.

Structural, Magnetic, and Electrical Properties in the Metallic Pyrochlore Pr2+xIr2-xO7+δ

Pr2Ir2O7 is the metallic pyrochlore magnet which has been revealed to form a spin liquid phase by recent studies. Here we report results of structural, magnetic, and electrical transport properties of a newly synthesized polycrystalline sample of Pr2Ir2O7. Our powder x-ray diffraction indicates the sample to be a single pyrochlore phase with a “Pr-stuffing” into the Ir site. The resistivity measurement suggests a phase transition into some sort of ordered phase, in contrast to the previous report. We discuss the possible origin of the phase transition.