Pyrochlore Structure Research Articles

Structure, morphology and functional characteristics of Gd2Ti2O7 nanocrystals formed under various annealing conditions

This article is devoted to the study of the processes of formation and growth of nanocrystals with the pyrochlore structure in the TiO2-Gd2O3 system. A mixture of an aqueous solution of gadolinium nitrate and a solution of titanium isopropoxide with polyethylene glycol in isopropyl alcohol was evaporated to a gel state, and then it was heat-treated using a one- or two-stage scheme. The study showed that the material being formed has a trabecular structure with a developed system of macropores. The use of the two-stage roasting method allows a uniform distribution of titanium and gadolinium atoms in the sample. Gadolinium titanate with a pyrochlore structure, which is formed using an aggregation mechanism at a temperature T = 700 °C from an X-ray amorphous precursor, is a variable composition compound. It has been shown that in terms of obtaining a monophasic product, the possibility of varying the size of nanocrystals, and defects in the pyrochlore structure, the most promising heat treatment mode is synthesis in the temperature range of 750–900 °C. In this case, the possibility of controlling the above characteristics of Gd2Ti2O7 nanoparticles can be implemented by varying the ratio of titanium- and gadolinium-containing precursors in the initial mixture, the duration of the isothermal exposure process.

Effect of Ce-doping on the phase composition and aqueous stability of Gd2Hf2O7 ceramic as nuclear waste forms

A series of Ce-doping nanograin Gd2CexHf2-xO7 (0.0 ≤ x ≤ 2.0) ceramic samples as nuclear waste forms were prepared, and their microstructure, phase evolution, and aqueous durability were systematically studied. The results revealed that the original Gd2Hf2O7 sample comprised a single pyrochlore structure after sintering at 1673 K, but all the Gd2CexHf2-xO7 (0.5 ≤ x ≤ 2.0) samples assumed defective fluorite structures with Ce-doping. As the xCe value increased from 0.0 to 2.0, the average grain size of the samples gradually increased from 162 nm to 599 nm, whereas the relative density (measured density/theoretical density) of the samples gradually increased from 91.12% to 93.93%. The normalized release rates of Ce element in the Gd2Ce2O7 sample after 42 days were slightly lower than those of Hf element in the Gd2Hf2O7 sample owing to the lower porosity of the Gd2Ce2O7 sample, and the bond dissociation energy of Ce–O (∼795 kJ/mol) was slightly higher than that of Hf–O (∼791 kJ/mol). Moreover, the normalized release rates of all considered elements were lower than 10−5 g m−2 d−1, indicating that the samples had excellent chemical stability.

Response of nonstoichiometric pyrochlore composition Nd1.8Zr2.2O7.1 to electronic excitations

AbstractIn response of swift heavy ion (100 MeV I9+) irradiation, the irradiation‐induced disordering in nonstoichiometric pyrochlore composition (Nd1.8Zr2.2O7.1) was compared to that of stoichiometric composition Nd2Zr2O7. Both compositions were prepared through auto gel‐combustion followed by sintering under the identical conditions. Systematic analysis of the compositions before and after irradiation was performed with X‐ray diffraction (XRD), Raman spectroscopy, and plane‐view high‐resolution transmission electron microscopy (HRTEM) techniques. Irradiation caused pyrochlore to amorphous phase transformation was observed in both compositions except the slower rate of amorphization in Nd1.8Zr2.2O7.1. The amorphization was achieved as a consequence of isolated disordered track overlapping at higher ion fluence with the estimated track diameter 2.73 ± 0.05 and 3.46 ± 0.30 nm for Nd1.8Zr2.2O7.1 and Nd2Zr2O7, respectively, employing the framework of single‐ion impact model to XRD results. HRTEM micrographs also revealed the less prevalence of irradiation‐induced amorphization in Nd1.8Zr2.2O7.1 with the observed irradiation‐induced modified track region composed of defect‐rich pyrochlore structure, anion‐deficient fluorite structure, and amorphous domains; with the diameter of 3.0 ± 1.0 nm and 5.0 ± 1.0 nm in Nd1.8Zr2.2O7.1 and Nd2Zr2O7, respectively. The preexisting anion‐deficient fluorite structure in Nd1.8Zr2.2O7.1 helps in its epitaxial growth as recovered structure from melted ion track during irradiation‐induced rapid cooling.

Novel composite materials with high near-infrared reflectance properties to retard the thermal aging of polyethylene: Pr3+ and Er3+ doped La2Zr2O7 compounds

In this paper, Er3+ and Pr3+ successfully replaced La3+ in the lattice without changing the structure of La2Zr2O7 pyrochlore, and finally formed the substituted solid solution La2-xMxZr2O7+δ (M = Er, Pr; x = 0–0.25). SEM analysis shows that the pigments were in the shape of nearly spherical particles with a particle size between 10 and 60 nm. La2-xPrxZr2O7+δ and La2-xErxZr2O7 materials have high NIR reflectance properties, with the NIR reflectance of 68.46%–92.51% and 82.79%–90.89%, which are related to the variation of free carrier concentration due to doping content. Compared with pure polyethylene material, the polyethylene plastic samples containing La2-xPrxZr2O7+δ and La2-xErxZr2O7+δ not only had lower internal surface temperatures, 5.9–11.4 °C and 6.9–13.2 °C lower but also had significantly lower thermal aging. In conclusion, the use of synthetic pigments with high NIR reflectivity as additives can delay the thermal aging of plastics and increase the aging resilience of plastic.

Enhanced electrochemical hydrogen storage performance of lanthanum zirconium oxide ceramic microstructures synthesized by a simple approach

Two rare-earth zirconate oxides RE2Zr2O7 (RE = La and Nd) with pyrochlore structures were fabricated, employing a simple and modified solid-state route. After confirmation of their microstructure, physicochemical features, and morphologies, the fabricated pyrochlores were applied for electrochemical hydrogen storage, and their capacity was compared in an alkaline medium. The La2Zr2O7 microstructure exhibited an enhanced electrochemical hydrogen storage performance (maximum discharge capacity = 2350 mAh/g) compared to the Nd2Zr2O7 structures. Having a higher specific surface, as well as a more favorable physical and chemical nature, could be responsible for this more suitable performance. Lanthanum zirconate ceramic microstructure could be introduced as an active material with an appropriate capacity for electrochemical hydrogen storage.

Influence of the Precursor Structure on the Formation of Tungsten Oxide Polymorphs.

Understanding material nucleation processes is crucial for the development of synthesis pathways for tailormade materials. However, we currently have little knowledge of the influence of the precursor solution structure on the formation pathway of materials. We here use in situ total scattering to show how the precursor solution structure influences which crystal structure is formed during the hydrothermal synthesis of tungsten oxides. We investigate the synthesis of tungsten oxide from the two polyoxometalate salts, ammonium metatungstate, and ammonium paratungstate. In both cases, a hexagonal ammonium tungsten bronze (NH4)0.25WO3 is formed as the final product. If the precursor solution contains metatungstate clusters, this phase forms directly in the hydrothermal synthesis. However, if the paratungstate B cluster is present at the time of crystallization, a metastable intermediate phase in the form of a pyrochlore-type tungsten oxide, WO3·0.5H2O, initially forms. The pyrochlore structure then undergoes a phase transformation into the tungsten bronze phase. Our studies thus experimentally show that the precursor cluster structure present at the moment of crystallization directly influences the formed crystalline phase and suggests that the precursor structure just prior to crystallization can be used as a tool for targeting specific crystalline phases of interest.

High Throughput Studies of Doped Li-La-Zr-O Garnet Solid Electrolytes

Lithium lanthanum zirconium oxide (LLZO) is a leading candidate for solid Li-batteries due to its high lithium-ion conductivity, stability in air and against Li metal, and compatibility with high-voltage cathodes.1,2 Despite significant research being done, our understanding of LLZO is limited by the relatively small number of compositions which have been studied; both in the larger Li-La-Zr-O system and in the doped cubic LLZO system, which can accommodate an extremely wide range of dopants.3To study this large number of compositions, we have applied a high-throughput methodology for synthesizing, characterizing, and testing sets of 64 LLZO electrolytes at the mg-scale. We employ a citrate sol-gel synthesis method whereby reagent solutions are dispensed across a well-plate to give a composition gradient. After drying and calcining the gels, the resulting powders are pelleted and then sintered at the desired temperature. High-throughput characterization techniques utilized include powder X-ray diffraction, impedance spectroscopy, DC polarization, and electrochemical stability window testing.Using our methodology, we have recently studied over 700 samples to produce a full phase stability diagram for the Li-La-Zr-O pseudoternary system.4 We found there is significant solubility of Li in the La2Zr2O7 pyrochlore structure, and that both cubic and tetragonal undoped LLZO appearing throughout the system have similarly poor bulk conductivities. This previous study again emphasizes the key role dopants play in LLZO. Herein, our methodology is applied to a comprehensive doping study where 60 different dopants are evaluated under identical synthesis conditions to allow for a thorough understanding of dopant effects on structure, ionic conductivity, electronic conductivity, and electrochemical stability window. Our samples achieve high conductivities over 1 mS/cm and relative densities above 90% with our combinatorial synthesis. Dopant content is optimized for each promising dopant to compare best-case results and correlate performance with structural properties like lattice parameters and density. Since comparing dopants in the literature is often difficult due to varying synthesis methods, this systematic work is essential in rational design of these electrolytes.1.Q. Liu, et al., J. Power Sources 2018, 389, 120-134.2.T. Thompson, et al., ACS Energy Letters 2017, 2, 462-468.3.F. Zheng, et al., J. Power Sources 2018, 389, 198-213.4.E. Anderson et al., DOI:10.1016/j.ssi.2022.116087.

Tunable luminescence and energy transfer in Tb3+,Eu3+ co-doped Gd2Zr2O7 phosphors with high thermal stability for WLEDs

Red phosphors with good thermal stability, high color temperature and color purity are critical in high power warm white LED devices. Herein, a series of single and co-doped Tb3+,Eu3+ polychromatic phosphors with pyrochlore structure based on Gd2Zr2O7 have been prepared for the first time by a conventional solid phase reaction method. Compared to single doped Eu3+ phosphors, Tb3+,Eu3+ co-doped phosphors possess following advantages: improved UV absorption, increased photoluminescence intensity, extended luminescence range and improved thermal stability. At room temperature, the Gd2Zr2O7:Tb3+,Eu3+ phosphors exhibited a gradual shift from yellow-green emission at 544 nm to orange-red emission at 613 nm with increasing doping of Eu3+ ions. Energy transfer efficiency has been evaluated to be 71% by analyzing the fluorescence lifetime of Tb3+ ions. The concentration quenching mechanism under different conditions was analyzed in detail and it was concluded that the quenching of Tb3+ is caused by dipole-dipole interaction. In addition, the activation energy (ΔEa) of the Gd1.92Zr2O7:0.03Tb3+,0.05Eu3+ phosphor was calculated to be as high as 0.2674 eV compared to other Tb3+, Eu3+ co-doped phosphors, demonstrating its high thermal stability. WLEDs devices based on Gd1.92Zr2O7:0.03Tb3+,0.05Eu3+ phosphors have been prepared, which have low correlated color temperature (CCT) (5283 K), high color rendering index Ra = 85.5 K) and CIE coordinates chromaticity coordinate (0.3371,0.3333). All of the results show that Gd2Zr2O7:Tb3+,Eu3+ phosphors provide a new avenue to develop high quality red emission and high thermal stability phosphors for WLEDs.

Preparation and thermal physical properties of MgO-(Nd1-xYx)2(Zr1-xCex)2O7 composite ceramics as a candidate for inert matrix fuel

In recent years, the MgO-Nd2Zr2O7 composite ceramics inert matrix (IM) was proposed as a candidate for the transmutation of Pu and minor actinides (MA) in light water reactors (LWR) or accelerator driven sub-critical system (ADS). To take full advantage of the pyrochlore structure of Nd2Zr2O7 to accommodate different types of MA and Pu, in this work, Y and Ce co-doped ωMgO-(1-ω)(Nd1-xYx)2(Zr1-xCex)2O7 (M-NYZC) composite ceramics used as a candidate IM for inert matrix fuel were prepared by solid-state one-step sintering method at 1500 °C for 3 h. The phase structure and microstructure of M-NYZC composite ceramics were characterized by XRD and SEM-EDS. Also, the thermal physical properties of M-NYZC composite ceramics including the thermal expansion coefficient and thermal conductivity were investigated systematically. Results from XRD and SEM-EDS showed that the magnesium oxide and NYZC pyrochlore solid solution are intermixed well with each other, and the M-NYZC samples present uniform element distribution with a grain size of about 1 μm. Also, the as-prepared samples possessed well densified microstructure, and the average relative density of all samples could exceed 95 %. Moreover, the thermal expansion coefficient and thermal conductivity of 0.5M-0.5NYZC (x = 0, 0.15, 0.2, 0.5, 0.9) samples were located in the range of 13.4 × 10-6–14.4 × 10-6 K-1 and 16.34–3.70 W·m-1·K-1 respectively. By comparison, the thermal physical properties of 0.5M-0.5NYZC samples are superior to that of traditional UO2 and MOX fuels. It is suggested that the M-NYZC composite ceramics prepared in this study can meet the thermal physical performances of inert matrix fuel.

Sintering resistance and phase stability of (Y0.2La0.2Nd0.2Sm0.2Eu0.2)2Zr2O7 for ultra-high temperature thermal barrier coatings

High-temperature stability, including sintering resistance and phase stability, is critical for thermal barrier coatings (TBCs) applied at ultra-high temperature over 1500 °C. In this work, we reported a high-entropy rare-earth zirconate (Y02La0.2Nd0.2Sm0.2Eu0.2)2Zr2O7 (YLNSE) synthesized by solid-state reaction method. X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive spectrometer (EDS) mapping results confirmed that YLNSE formed a single-phase pyrochlore structure with uniform rare earth elements distribution. YLNSE maintained more uniform microstructure, higher porosity retention, lower grain growth rate and better thermomechanical properties during sintering at 1500 °C, as the results of the ordered crystal structure and sluggish diffusion effect, indicating a better sintering resistance than conventional TBC material yttria-stabilized zirconia (YSZ). In addition, YLNSE also demonstrated high coefficient of thermal expansion (CTE, 11.04 × 10-6K-1, 25–1500 °C) and low thermal conductivity (1.95 W m-1 K-1, 1500 °C), as well as good phase stability at 1500 °C. In view of the excellent high-temperature stability of YLNSE, it is a promising candidate for ultra-high temperature TBCs.

Crystal Growth and Magnetism of Transition Metal Pyrochlore Fluorides.

Geometric magnetic frustration arises when the geometry of a structure prevents the simultaneous fulfillment of nearest-neighbor antiferromagnetic interactions and is commonly observed in lattices that exhibit a triangular topology, such as those found in the pyrochlore structure. Via a mild hydrothermal route, we have synthesized seven quaternary β-pyrochlore-related fluorides AxM2+xM3+(2-x)F6 (A = Cs and Rb; M2+ = Co2+, Ni2+, and Zn2+; and M3+ = V3+ and Fe3+). Crystal structures and compositions were determined using a combination of single-crystal X-ray diffraction and energy-dispersive spectroscopy. After adjusting the reaction conditions, phase-pure products of AxM2+xM3+(2-x)F6 were obtained. The magnetic susceptibility and isothermal magnetization data for all seven compounds were collected to interpret the magnetic behavior, which ranged from paramagnetic to antiferromagnetic with and without a ferromagnetic component. We found that the magnetic behavior of the AxM2+xV3+(2-x)F6 pyrochlore structures strongly depends on the presence or absence of unpaired electrons on the M2+ position. The titled pyrochlore compounds, with the exception of the Zn-analogue, can be considered frustrated materials, with frustration indices in the range of 6-13.

Preparation of alkaline earth element doped La2Hf2O7 materials and application in hydrogen separation

In order to improve the properties, alkaline earth element doped La1.95M0.05Hf2O7-δ (M = Ca, Sr, Ba) are prepared by high-temperature solid state method. The experimental results indicate that Ca is a suitable doping element. The prepared La2-xCaxHf2O7-δ (x = 0, 0.025, 0.05, 0.1) series samples are of pure pyrochlore structures. Among them, La1.95Ca0.05Hf2O7-δ has the densest structure, the highest conductivity, which is 2.86 × 10−3 S cm−1 in humid air at 800 °C, and outstanding chemical stability in H2O, CO2, H2 and 300 ppm H2S.The hydrogen separation performance of La1.95Ca0.05Hf2O7-δ membrane is tested using the external short-circuit method. The hydrogen permeation flux of 1.25 mm thick La1.95Ca0.05Hf2O7-δ membrane is 0.15 mL·min−1·cm−2 in 70% H2/He feed gas at 900 °C. Humidification increases the hydrogen permeation flux and CO2 or H2S reduces the hydrogen permeation flux.

Phase equilibria in the ZrO2–HfO2–Nd2O3 system at 1500 °C and 1700 °C

The phase equilibria in the ZrO2–HfO2–Nd2O3 ternary system at 1500 °C and 1700 °C were studied over the whole concentration range by X-ray diffraction and microstructural analyses. Corresponding isothermal sections were constructed from the data obtained. It was found that solid solutions in this system are derived from a tetragonal (T) modification of ZrO2, a monoclinic (M) modification of HfO2, a hexagonal (A) modification of Nd2O3, a cubic phase with a fluorite (F) structure of ZrO2 (HfO2), and an ordered phase with a pyrochlore (Py) structure of Nd2Zr2O7 (Nd2Hf2O7). The phase boundaries and unit cell lattice parameters were determined. The solubility of Nd2O3 in M − HfO2 is pretty low and about less than 1 mol%, as confirmed by XRD and microstructural analyses. The ordered pyrochlore-type (Py) phase of Nd2Zr2O7 (Nd2Hf2O7) forms continuous series of solid solutions at 1500 °C and 1700 °C. The homogeneous region of these continuous series of solid solutions changes insignificantly with increasing temperature. The changes in the construction of the isothermal section of the ZrO2-HfO2-Nd2O3 phase diagram at 1500 °C compared to 1700 °C are associated with the thermal stability of cubic fluorite-type (F) solid solutions. Other phases in the ZrO2-HfO2-Nd2O3 ternary system are not detected at the studied temperatures.

Iron-containing polyoxotungstophosphates and their thermolysis products

Thermolysis of ferrocenium tungstophosphate [(C5H5)2Fe]3[PW12O40]∙H2O and tungstophosphatoferrates with the general formula of Cat4[PW11O39Fe(H2O)]∙ m H2O, where Cat = (NH4)+, Rb+, Cs+, (CH3)4N+, with the Keggin anion structure were studied using differential scanning calorimetry, infrared spectroscopy, X-ray phase analysis, and electron microscopy. The crystalline products of their thermal decomposition - phases with the structure of pyrochlore and tungsten bronzes - were identified. The synthesized compounds are catalysts for the oxidation reactions of organic compounds and the production of carbon nanomaterials.

Synthesis and characterization of a new ordered rhombohedral pyrochlore family Ln3Sb3Cu2O14 (Ln=Pr,Nd, Sm, Eu, Tb, Dy) with more detailed study of the Sm compound

New polycrystalline compound with a rhombohedral pyrochlore structure and chemical formula Sm3Sb3Cu2O14 was synthesized in air by solid-state reaction method. The compound crystallizes in the rhombohedral system, with R-3m space group and Z = 3. Lattice parameters are a = 7.41681(2) Å and c = 17.24845(5) Å. The electrical resistivity, measured by impedance spectroscopy, highlights a semiconductor behavior with an activation energy Ea = 0.47 eV. The dielectric properties show a regular decrease of permittivity with temperature, the dielectric loss decreases at low temperature and remain stable around 0.8 at high temperature. Using UV–vis Diffuse Reflectance, we can assume that the band gap is around 3.28 eV. The magnetic susceptibility of Sm3Sb3Cu2O14, measured from 5 K to 800 K, shows Curie-Weiss paramagnetism behavior with a significant Van Vleck contribution from the Sm+3 magnetic cation.

Phase Formation of Co and Cr Co-Doped Bismuth Niobate with Pyrochlore Structure

The formation mechanisms of pyrochlore-type Bi2Co1/2Cr1/2Nb2O9+Δ (space group Fd-3m, a = 10.4838(8) Å), in the temperature range from 400 to 1050 °C were studied by employing X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy. An extensive reaction between the binary metal oxides was found to begin at temperatures above 550 °C, following the transition of monoclinic α-Bi2O3 into a tetragonal β-Bi2O3 polymorph. The synthesis process occurs in several stages when Bi-rich intermediate products (Bi6CrO12, Bi6Cr2O15, and Bi5Nb3O15) transform into bismuth-depleted BiNbO4 and a chromium–cobalt spinel is formed. The formation of a single pyrochlore phase occurs at the final reaction stage at 1050 °C via the doping of bismuth ortho-niobate, BiNbO4, by the transition metal cations. The observed mechanism is essentially similar to the mechanism of tantalate-based phases except for the formation of Bi5Nb3O15 at the intermediate reaction stages.

(La0.2Dy0.2Nd0.2Sm0.2Eu0.2)2GdTaO7: a promising candidate oxides for high-temperature coating applications

In the paper, the (La0.2Dy0.2Nd0.2Sm0.2Eu0.2)2GdTaO7 oxide was prepared employing sol–gel and solid-reaction method. Its crystal-structure, microstructure, element-distribution, mechanical and thermophysical properties are investigated. The analytical conclusions reveal that this high-entropy oxide shows single pyrochlore structure, dense microstructure, and even element distribution. Owing to the strong phonon scattering originated from high-entropy effect and numerous oxygen vacancies, it has low thermal conductivity and ranges from 1.41 to 1.27 W/m. K. Its coefficient of thermal expansion is less than those of rare-earth cerium oxides and greater than that of 7YSZ. The Young's modulus of this high-entropy oxide is in the same order with 7YSZ, its Vickers hardness and fracture-toughness are lower than those of 7YSZ. This high-entropy oxide is also of excellent lattice-stability up to 1200 °C.

Crystal chemical design, synthesis and characterisation of U(IV)-dominant betafite phases for actinide immobilisation

Crystal chemical design principles were applied to synthesise novel U4+ dominant and titanium excess betafite phases Ca1.15(5)U0.56(4)Zr0.17(2)Ti2.19(2)O7 and Ca1.10(4)U0.68(4)Zr0.15(3)Ti2.12(2)O7, in high yield (85–95 wt%), and ceramic density reaching 99% of theoretical. Substitution of Ti on the A-site of the pyrochlore structure, in excess of full B-site occupancy, enabled the radius ratio (rA/rB = 1.69) to be tuned into the pyrochlore stability field, approximately 1.48 ≲ rA/rB ≲ 1.78, in contrast to the archetype composition CaUTi2O7 (rA/rB = 1.75). U L3-edge XANES and U 4f7/2 and U 4f5/2 XPS data evidenced U4+ as the dominant speciation, consistent with the determined chemical compositions. The new betafite phases, and further analysis reported herein, point to a wider family of actinide betafite pyrochlores that could be stabilised by application of the underlying crystal chemical principle applied here.

Quantum paramagnetism in a non-Kramers rare-earth oxide: Monoclinic Pr2Ti2O7

Little is so far known about the magnetism of the ${A}_{2}{B}_{2}{\mathrm{O}}_{7}$ monoclinic layered perovskites that replace the spin-ice supporting pyrochlore structure for ${r}_{A}/{r}_{B}>1.78$. We show that high quality monoclinic ${\mathrm{Pr}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ single crystals with a three-dimensional network of non-Kramers ${\mathrm{Pr}}^{3+}$ ions that interact through edge-sharing superexchange interactions, form a singlet ground-state quantum paramagnet that does not undergo any magnetic phase transitions down to, at least, 1.8 K. The chemical phase stability, structure, and magnetic properties of the layered perovskite ${\mathrm{Pr}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ were investigated using x-ray diffraction, transmission electron microscopy, and magnetization measurements. Synthesis of polycrystalline samples with the nominal compositions of ${\mathrm{Pr}}_{2}{\mathrm{Ti}}_{2+x}{\mathrm{O}}_{7}$ ($\ensuremath{-}0.16\ensuremath{\le}x\ensuremath{\le}0.16$) showed that deviations from the ${\mathrm{Pr}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ stoichiometry lead to secondary phases of related structures including the perovskite phase ${\mathrm{Pr}}_{2/3}{\mathrm{TiO}}_{3}$ and the orthorhombic phases ${\mathrm{Pr}}_{4}{\mathrm{Ti}}_{9}{\mathrm{O}}_{24}$ and ${\mathrm{Pr}}_{2}{\mathrm{TiO}}_{5}$. No indications of site disordering (stuffing and antistuffing) or vacancy defects were observed in the ${\mathrm{Pr}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ majority phase. A procedure for growth of high-structural-quality stoichiometric single crystals of ${\mathrm{Pr}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ by the traveling solvent floating zone method is reported. Thermomagnetic measurements of single-crystalline ${\mathrm{Pr}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ reveal an isolated singlet ground state that we associate with the low-symmetry crystal electric-field environments that split the ($2J+1=9$)-fold degenerate spin-orbital multiplets of the four differently coordinated ${\mathrm{Pr}}^{3+}$ ions into 36 isolated singlets resulting in an anisotropic temperature-independent van Vleck susceptibility at low $T$. A small isotropic Curie term is associated with 0.96(2)% noninteracting ${\mathrm{Pr}}^{4+}$ impurities.

Synthesis and Oxygen Mobility of Bismuth Cerates and Titanates with Pyrochlore Structure

Synthesis and study of materials based on bismuth cerates and titanates were carried out. Complex oxides Bi1.6Y0.4Ti2O7 were synthesized by the citrate route; Bi2Ce2O7 and Bi1.6Y0.4Ce2O7—by the Pechini method. The structural characteristics of materials after conventional sintering at 500–1300 °C were studied. It is demonstrated that the formation of a pure pyrochlore phase, Bi1.6Y0.4Ti2O7, occurs after high-temperature calcination. Complex oxides Bi2Ce2O7 and Bi1.6Y0.4Ce2O7 have a pyrochlore structure formed at low temperatures. Yttrium doping of bismuth cerate lowers the formation temperature of the pyrochlore phase. As a result of calcination at high temperatures, the pyrochlore phase transforms into the CeO2-like fluorite phase enriched by bismuth oxide. The influence of radiation-thermal sintering (RTS) conditions using e-beams was studied as well. In this case, dense ceramics are formed even at sufficiently low temperatures and short processing times. The transport characteristics of the obtained materials were studied. It has been shown that bismuth cerates have high oxygen conductivity. Conclusions are drawn about the oxygen diffusion mechanism for these systems. The materials studied are promising for use as oxygen-conducting layers in composite membranes.