O7 Pyrochlore Research Articles

Porous and B‐site Substituted Y2[Mn0.2Ru0.8]2O7 Pyrochlore for Boosting Acidic Water Oxidation Activity and Stability

Boosting the reaction stability without sacrificing the activity and cost is extremely important but full of challenging for the RuO2‐based oxygen evolution catalysts. Herein, porous and B‐site substituted Y2[Mn0.2Ru0.8]2O7 (p‐Y2[Mn0.2Ru0.8]2O7) pyrochlore toward oxygen evolution reaction is innovatively synthesized. The formed meso/macro‐porous structure can increase the specific surface area and corresponding active sites, meanwhile, Mn‐substitution can modulate the electronic structure, stabilize the morphology, and reduce the dosage of Ru species. Excitingly, the p‐Y2[Mn0.2Ru0.8]2O7 performs 50 h stable operation, significantly outperforming the commercial RuO2(CM) counterpart with less than 2 h life. Furthermore, the required overpotential to achieve 10 mA cm‐2 is only 266 mV, accompanied with favorable reaction kinetics and catalyst utilization.

The study of BMN cubic pyrochlores based multi-layer capacitors

The multi-layer ceramic capacitor (MLCC) is prepared with Bi1.5MgNb1.5O7 (BMN) pyrochlores, which has medium dielectric constant, large dielectric tunability, and especially the very small dielectric loss. The multilayer ceramic capacitors with Bi1.5MgNb1.5O7 as the dielectric material are prepared by a tape-casting method. The dielectric constant of the BMN thick film was observed to be superior to that of the bulk material, indicating its potential application in electronic devices. The increase in dielectric constant during the preparation of multilayer ceramic capacitors was explained by investigating the interdiffusion between the ceramic thick films and the electrodes. The BMN thick films and Ag0.7/Pd0.3 electrodes showed a dense microstructure and serial interfaces when sintering at 1050 °C for 1.5 h. The capacitor exhibited good temperature stability with a dielectric constant temperature coefficient of −249.94 ppm/°C, a relative dielectric constant of 216, and a dielectric loss of 0.00036. Provides insights for the development and optimization of high-performance dielectric tunable electronic device.

The effect of dual-sites high-entropy strategy on thermal conductivity of pyrochlore ceramics

The emergence of high-entropy materials has developed a new strategy for the design and optimization of ceramics with desired properties. In this work, the (Gd1/7La1/7Nd1/7Sm1/7Eu1/7Dy1/7Ho1/7)2(Ti1/4Zr1/4Sn1/4Hf1/4)2O7 pyrochlore ceramic with dual-sites high-entropy design has been successfully prepared by a solid-state reaction sintering method. The samples possess a single-phase of pyrochlore structure, and all introduced elements are distributed homogeneously at A and B sites, respectively. Based on the synergistic high-entropy design of A and B sites, the thermal conductivity value of high-entropy pyrochlore ceramic could be diminished to 1.37 W∙m−1∙K−1.

Large magnetocaloric effect due to spin-glass behavior in the (Mn1/3R2/3)2(Mn1/3Sb2/3)2O7 pyrochlore series

The synthesis, structural characterization and magnetic properties of the (Mn1/3R2/3)2(Mn1/3Sb2/3)2O7 pyrochlore series are detailed herein. The R3+ (Rare earth) and Sb5+ cations occupy the A and B sites respectively, due to their charge/size ratio. Mn is found to be located in a 1/3 ratio in both positions with selective oxidation state due to size restrictions. Mixed valence in Mn2+/Mn3+ was confirmed from EELS spectroscopy. According to their geometric frustration and R3+-Mn2+/Sb5+-Mn3+ disorder, spin glass behavior is observed from dc and ac magnetic susceptibility data and no long-range magnetic order could be observed from NPD data down to 1.5 K. The magnetic entropy variation calculated from field dependent magnetization curves reveals large magnetocaloric effect (MCE) for these compounds, suggesting its relation to the dynamics of the frozen state. These values reach up to ΔSM ∼20.7 J/kgK for the RGd sample, which is comparable to other materials with large MCE induced by FM order. This renders these materials competitive in magnetic refrigeration and opens the path for new enhanced magnetocaloric materials exploiting this mechanism.

High-temperature transport properties of entropy-stabilized pyrochlores

In this report, the high-temperature transport properties of (Dy1−xCax)(Zr0.2Hf0.2Sn0.2Ti0.2Ge0.2)O7 pyrochlore oxides with x = 0, 0.05, and 0.1 are studied in dry and humid air. The phase composition and crystal structure were determined by using x-ray and neutron diffraction. The addition of calcium to the structure caused an increase in the concentration of oxygen vacancies, indicating an ionic charge compensation mechanism. Electrical studies allowed us to determine the total electrical conductivity as a function of the synthesis atmosphere and pH2O. The electrical conductivity turned out to be at the level of ∼10−3 S/cm at 800 °C, and only a slight effect of the presence of protonic defects in the structure on the total electrical conductivity was observed. In general, the samples had a low electrical conductivity with a relatively high activation energy of conduction.

He2+ irradiation response of structural evolution at different depths of MgO-Nd2(Zr1-xCex)2O7 composite ceramics used for inert matrix fuel

This paper reported the irradiation response of a 55 wt.% MgO-45 wt.% Nd2(Zr1-xCex)2O7 (abbreviated to M-NZCx, x = 0, 0.3 and 1) composite ceramics used as candidate inert matrix fuel under 500 keV He2+ ions with fluence of 1 × 1016–5 × 1017 ions/cm2. The irradiation-induced lattice expansion, amorphization of MgO and NZCx phases and order-disorder transition of Nd2(Zr1-xCex)2O7 pyrochlore were systemically explored via GIXRD, Raman and TEM techniques. And the detailed irradiation response at different depths of the typical M-NZC0.3 sample was especially investigated via GIXRD and TEM. The results showed that the lattice expansion ratios (Rs) of MgO (M), ordered pyrochlore (P) and disordered defect-fluorite (F) structures of NZCx pyrochlore in the samples under the same dose of He2+ irradiation were regular, which were ranged as the following order: RsM < RsP < RsF. And there was a belt-shaped damage aggregation zone located near interphase boundary at the maximum damage depth of M-NZC0.3 sample, which could be explained by the defect-absorption of boundary. And this defect-absorption of boundary was also found at any depth of M-NZC0.3 sample. This work systematically analyzed α-decay irradiation response of different phases in M-NZCx composite ceramics applying for inert matrix fuel.

Order-disorder structural transition of Nd2(Zr1-xCex)2O7 pyrochlores prepared by auto-combustion method

A series of pyrochlore solid solutions with general formula Nd2(Zr1-xCex)2O7 were successfully synthesized by the glycine-nitrate process (GNP) auto-combustion method and then sintered at high temperature. The order-disorder structure of the as-prepared ceramic samples was systematically characterized by X-ray diffraction (XRD) and Raman spectroscopy. Rietveld refinements of the typical XRD patterns were performed using Full-prof software. XRD and Raman results confirmed a co-existence structure composed of ordered pyrochlore and disordered defect-fluorite structure for Nd2(Zr0.75Ce0.25)2O7 and Nd2(Zr0.7Ce0.3)2O7 samples. And Nd2Ce2O7 specimen presented as defect-fluorite structure with rare earth C-type microdomains in both XRD and Raman spectroscopy. The effect of 48f oxygen on phase transition was verified by XRD and Raman spectroscopy. The refined x-parameter of 48f oxygen reflected the phase transition of Nd2(Zr1-xCex)2O7 solutions from pyrochlore to fluorite structure. High resolution transmission electron microscope (HRTEM) images and selected area electron diffraction (SAED) analysis of transmission electron microscopy (TEM) proved the representative Nd2(Zr0.7Ce0.3)2O7 sample showed biphasic structure with pyrochlore and defect-fluorite structure SAED rings. Also, the C-type microdomain SAED rings of Nd2Ce2O7 structure was found by TEM analysis.

Emerging disorder in Gd2(Ti1−xZrx)2O7 pyrochlores matrices for radioactive waste disposal: symmetry lowering versus defect clustering

The structure of Gd2(Ti1−xZrx)2O7 pyrochlores was investigated by means of synchrotron radiation powder diffraction, including pair distribution function analysis, Raman spectroscopy and DFT calculations to shed light on their rich defect chemistry.

Investigation of the ordered-disordered structural transition of (Nd1-xYx)2(Zr1-xCex)2O7 pyrochlore by X-ray diffraction, Raman spectroscopy, and Transmission electron microscopy

In the current research, the equal molar Y and Ce co-doped (Nd1-xYx)2(Zr1-xCex)2O7 pyrochlore solid solution was prepared by solid-state method at 1500 °C. And the ordered-disordered structure of the as-prepared samples was investigated systematically by X-ray diffraction, Raman spectroscopy, and Transmission electron microscopy, respectively. The results showed that the lattice constants of ordered or disordered (Nd1-xYx)2(Zr1-xCex)2O7 solid solutions increase with increasing Y and Ce co-doping contents simultaneously from 0 to 1. The phase structure presents monophasic ordered pyrochlore (Fd3‾m space group, Z = 8) when Y and Ce contents are in the range of 0–0.15. While a biphasic region exists when contents of Y and Ce are 0.2, namely, there are ordered pyrochlore and disordered defect fluorite (Fm3‾m space group, Z = 4) structure coexisting in the (Nd0.8Y0.2)2(Zr0.8Ce0.2)2O7 solid solution. With the co-doping content of Y and Ce in the 0.3–1 range, the phase composition becomes monophasic disordered defect fluorite. Variation of the average rA/rB ratio value leads to the increase of the xO48f coordinate parameter from 0.3192 (0) to 0.3309 (0) when the phase structure of the sample present ordered pyrochlore. X-ray diffraction, Raman spectroscopy, and Transmission electron microscopy results confirm that the ordered-disordered transformation of (Nd1-xYx)2(Zr1-xCex)2O7 pyrochlore is related to the xO48f coordinate parameter.

Synthesis of high-entropy La2B2O7 ceramics with non-equivalent principal elements in B-sites and their CMAS resistance performance

A new method is proposed to predict the formation of high-entropy single-phase oxide with a pyrochlore structure synthesis from non-equivalent principal elements. Based on this concepte, the La2B2O7 (B = Zn2+, Mg2+/Ni2+, Ce4+/Zr4+/Hf4+/Sn4+/Ti4+, W6+, Mo6+) were designed, in which high-entropy La2(ZnMgZrWMo)2O7, La2(ZnMgHfWMo)2O7 and La2(ZnMgSnWMo)2O7 pyrochlore ceramics were successfully prepared by solid-phase sintering. The microstructure of the entropy-stabilized phase was demonstrated using X-ray diffraction and elemental mapping, and the thermal properties including the thermal expansion coefficient and thermal conductivity were analyzed. The results show that La2(ZnMgZrWMo)2O7 has a higher thermal expansion coefficient (13.9 × 10–6 K–1) and lower thermal conductivity (0.8 W/m·K) than its corresponding single-components compound. In addition, La2B2O7 (B = Zn2+, Mg2+, Zr4+/Hf4+/Sn4+, W6+, Mo6+) pyrochlore high-entropy ceramics exhibit good calcium – magnesium – aluminosilicate (CMAS) resistance, the apatite protective layer generated after the reaction of La with CMAS effectively prevents further corrosion. This study suggests a novel method for predicting the formation of high-entropy single-phase oxides of pyrochlore structures and provides a new design concept for anti-CMAS corrosion materials.

Order–disorder structural tailoring and its effects on the chemical stability of (Gd, Nd)2(Zr, Ce)2O7 pyrochlore ceramic for nuclear waste forms

Series of unequal quantity Nd/Ce co-doped ceramic nuclear waste forms, (Gd, Nd)2(Zr, Ce)2O7, were prepared to tailor its ordered pyrochlore or disordered fluorite structure. The phase transition, microtopography, and elemental composition of the ceramic samples were systematically investigated, especially the effect of order-disorder structure on the chemical stability. It was confirmed that unequal quantity of Nd/Ce could synchronously replace the Gd/Zr-sites of Gd2Zr2O7. And the phase transition of order-disorder structure could be successfully tailored by regulating the average cationic radius ratio of (Gd, Nd)2(Zr, Ce)2O7 series. The elements of Gd, Nd, Zr, and Ce are uniformly distributed in the ordered or disordered structures. The MCC-1 leaching results showed that (Gd, Nd)2(Zr, Ce)2O7 pyrochlore ceramic nuclear waste forms had excellent chemical stability, whose elements' normalized leaching rates were as low as 10−4 -10−7 g‧m−2‧d−1 after 7 days. In particular, the chemical stability of disordered structure was superior to that of ordered structure. It was proposed that the force constant and the closest packing were changed with the structure transformation resulting the chemical stability difference.

The effect of (Ni2+ + Sr2+) co-doping on structure and dielectric properties of Bi1.5MgNb1.5O7 pyrochlores

The dielectric properties of bismuth-based cubic pyrochlore largely depend on the A-site cation. The effect of (Ni2+ + Sr2+) co-doping on the structure and dielectric properties of Bi1.5MgNb1.5O7 (BMN) was investigated. It reveals that the Ni2+ and Sr2+ in the co-doped Bi1.5Mg0.5NixSryNb1.5O7 (BNSN, x = 0.0–0.3 mol%, y = 0.0–0.2 mol%) ceramics preferentially replaced the Mg2+ on the A-site. For the samples with x = 0.3 and y < 0.2, it showed a very low dielectric loss (∼0.0003, 1 MHz). The dielectric loss of BNSN gradually increased with the increase of Sr2+ doping. However, the dielectric constant increased with increasing Sr2+ content. The BNSN exhibited a large dielectric constant (∼166, 1 MHz) at x < 0.3 and y = 0.2. Ni-doping would reduce the oxygen vacancy, which in turn decrease the dielectric loss. While, the increasing dielectric constant is attributed to the high ion polarizability of Sr2+ ions.

Insight Into Disorder, Stress and Strain of Radiation Damaged Pyrochlores: A Possible Mechanism for the Appearance of Defect Fluorite.

We have examined the irradiation response of a titanate and zirconate pyrochlore—both of which are well studied in the literature individually—in an attempt to define the appearance of defect fluorite in zirconate pyrochlores. To our knowledge this study is unique in that it attempts to discover the mechanism of formation by a comparison of the different systems exposed to the same conditions and then examined via a range of techniques that cover a wide length scale. The conditions of approximately 1 displacement per atom via He2+ ions were used to simulate long term waste storage conditions as outlined by previous results from Ewing in a large enough sample volume to allow for neutron diffraction, as not attempted previously. The titanate sample, used as a baseline comparison since it readily becomes amorphous under these conditions behaved as expected. In contrast, the zirconate sample accumulates tensile stress in the absence of detectable strain. We propose this is analogous to the lanthanide zirconate pyrochlores examined by Simeone et al. where they reported the appearance of defect fluorite diffraction patterns due to a reduction in grain size. Radiation damage and stress results in the grains breaking into even smaller crystallites, thus creating even smaller coherent diffraction domains. An (ErNd)2(ZrTi)2O7 pyrochlore was synthesized to examine which mechanism might dominate, amorphization or stress/strain build up. Although strain was detected in the pristine sample via Synchrotron X-ray diffraction it was not of sufficient quality to perform a full analysis on.

Local disorder and structure relation induced by magnetic exchange interactions in A2(Mo1−yMny)2O7 pyrochlores

We present an extended X-ray absorption fine structure study at the Mo K-edge of A2(Mo1−yMny)2O7 (A = Gd, Ho; y = 0.05 and 0.10) pyrochlores, as a function of temperature and composition, coupled to diffractometric and magnetic characterizations. Extending the study reported in our previous paper on the pristine A2Mo2O7 compounds to these Mo/Mn partially substituted samples, where we hypothesize a competition between double-exchange and superexchange couplings, we aim to check which structure parameters are related to the nature of the nearest-neighbor magnetic interactions, looking for the presence and evolution of order and structure anomalies. Two Ho samples Ho2(Mo1−yMny)2O7 (y = 0.05 and 0.10) keep the spin-glass nature of the parent compositions with a strong distortion of the MoO(1)6 octahedron and of the Mo‒Ho and Mo‒Mo second shell. On the other hand, two Gd2(Mo1−yMny)2O7 samples, ferromagnetic at low temperature in the undoped case, show the appearance on a local scale of a spin-glass transition mirrored by a quite high structure disorder and by a distortion coherent with a lattice frustration after Mn-doping. Therefore, the Gd and Ho Mn-doped samples display a similar frustrated behavior, differently from the corresponding undoped ones.

Effects of Ni2+ substitution on the structure and dielectric properties of Bi1.5MgNb1.5O7 cubic pyrochlores

The dielectric properties of bismuth-based cubic pyrochlores strongly depend on the environment of the A-site ions, e.g. the Ni2+ ions doped into Bi1.5MgNb1.5O7 (BMN) pyrochlores for tailoring dielectric properties. Both BMN and Bi1.5NiNb1.5O7 (BNN) ceramics exhibit a cubic pyrochlore structure with preferential (222) planes. However, {442} reflections are observed in BNN pyrochlores, revealing an off-center displacement of A and O' ions. The dielectric constant of BNN pyrochlores is lower than that of BMN pyrochlores, besides BNN pyrochlores have a larger dielectric loss (0.002) than BMN pyrochlores (0.0007). Ni-doping results in a loose and flexible structure contributing positively to the dielectric tunability, besides creating a large amount of oxygen vacancies. The higher amount of oxygen vacancies increases the dielectric loss of BNN pyrochlores. However, BNN pyrochlores exhibit enhanced temperature stability, with a temperature coefficient of –57 ppm/oC, which is significantly better than that of BMN pyrochlores (–362 ppm/oC).

Ensemble-Based Modeling of the NMR Spectra of Solid Solutions: Cation Disorder in Y2(Sn,Ti)2O7.

The sensitivity of NMR to the local environment, without the need for any long-range order, makes it an ideal tool for the characterization of disordered materials. Computational prediction of NMR parameters can be of considerable help in the interpretation and assignment of NMR spectra of solids, but the statistical representation of all possible chemical environments for a solid solution is challenging. Here, we illustrate the use of a symmetry-adapted configurational ensemble in the simulation of NMR spectra, in combination with solid-state NMR experiments. We show that for interpretation of the complex and overlapped lineshapes that are typically observed, it is important to go beyond a single-configuration representation or a simple enumeration of local environments. The ensemble method leads to excellent agreement between simulated and experimental spectra for Y2(Sn,Ti)2O7 pyrochlore ceramics, where the overlap of signals from different local environments prevents a simple decomposition of the experimental spectral lineshapes. The inclusion of a Boltzmann weighting confirms that the best agreement with experiment is obtained at higher temperatures, in the limit of full disorder. We also show that to improve agreement with experiment, in particular at low dopant concentrations, larger supercells are needed, which might require alternative simulation approaches as the complexity of the system increases. It is clear that ensemble-based modeling approaches in conjunction with NMR spectroscopy offer great potential for understanding configurational disorder, ultimately aiding the future design of functional materials.

Structure and dielectric properties of Ru doped Bi1.5ZnNb1.5O7 ceramics

The structure and dielectrics properties of (Bi1.5Zn0.5)(Zn0.5Nb1.5-xRux)O7 (BZNR) pyrochlore ceramics were investigated. The pyrochlore ceramics were prepared by solid-state methods. The XRD results indicate that all ceramics exhibit single cubic pyrochlore structure. The crystal chemistry parameters, bond valence sum AV(O′)[A4] of BZN pyrochlore, slightly decreases with the amount of Ru4+ increasing. The dielectric relaxation behavior of BZNR ceramics was observed. The activation energy involved in the relaxation processes could be approximated by the Arrhenius-type equation. With Ru4+ content increasing, the Ea increased and Tm shifted towards higher temperature. The degree of relaxation behavior was relevant to the reorientation of defect pairs with the substitution of Ru4+ on B sites of α-BZN pyrochlore structure and crystal chemistry parameters.

Self-propagating plus quick pressing synthesis and characterizations of Gd2-xNdxTi1.3Zr0.7O7 (0 ≤ x ≤ 1.4) pyrochlores

Synroc is recognized as an ideal matrice for the immobilization of high-level radioactive waste (HLW). In this study, the Synroc mineral of pyrochlore was employed as host phase for the immobilization of Nd2O3, which was selected as surrogate of trivalent actinide nuclides. Gd2-xNdxTi1.3Zr0.7O7/Cu composites were rapidly synthesized by self-propagating high-temperature synthesis plus quick pressing (SHS/QP) using CuO as the oxidant and Ti as the reductant. The result shows that the Nd2O3 doped reactions could be ignited as x ≤ 1.4 and Gd2-xNdxTi1.3Zr0.7O7 pyrochlores were successfully prepared with Cu as the secondary phase. The synthesized pyrochlore-based waste form exhibits density of 4.93 g/cm3 and Vickers hardness of 14.90 GPa, as well as promising aqueous durability. The LRGd and LRNd value of the x = 1.4 sample are as low as 3.28 × 10−5 and 2.27 × 10−5 g m−2·d−1 after 42 days leaching.

Flux synthesis and chemical stability of Nd and Ce co-doped (Gd1−xNdx)2(Zr1−xCex)2O7 (0 ≤ x ≤ 1) pyrochlore ceramics for nuclear waste forms

A series of Nd and Ce co-doped (Gd1−xNdx)2(Zr1−xCex)2O7 (0 ≤ x ≤ 1) pyrochlores were synthesized by flux method at 900–1000°C using nitrates as starting materials and a small amount of NaF as fluxing agent. The influences of contents of Nd/Ce, fluxing agent and reaction temperature on the phase structure of the pyrochlores were systematically investigated by X-ray diffraction (XRD) and Raman spectroscopy (Raman). The chemical stability of Nd and Ce co-doped pyrochlore bulk ceramics was also evaluated. XRD and Raman results showed that the crystalline phase of (Gd1−xNdx)2(Zr1−xCex)2O7 (0 ≤ x ≤ 1) pyrochlores always remained a single disordered defect-fluorite structure for any x value. XRD, Raman spectrum and SEM-EDX results revealed that the Nd and Ce could be co-doped at the Gd-site and Zr-site of the Gd2Zr2O7 pyrochlore structure, respectively. The normalized release rates of Gd, Zr, Nd and Ce in (Gd1−xNdx)2(Zr1−xCex)2O7 pyrochlore bulk ceramics (x = 0.25, 0.5 and 0.75) remained relatively constant in a low value below 10−4gm−2d−1 at 90°C for 7 days, exhibiting superior chemical stability.

Crystal structure and dielectric properties of Mn-substituted Bi1.5Zn1.0Nb1.5O7 pyrochlore ceramics as temperature stable LTCC material

As a potentially attractive candidate of LTCC material, (Bi1.5Zn0.5−x Mn x )(Zn0.5Nb1.5)O7 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) ceramics with improved temperature coefficient of the dielectric constant were synthesized by a conventional mixed oxide route. The relations between the microstructure and dielectric properties of the specimens were investigated systematically. Microscopic analysis showed that all the samples maintained the (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 phase. The variation of the unit cell volumes contributed to the shift of the peaks caused by the chemical substitution of ions. The temperature coefficient of the dielectric constant of (Bi1.5Zn0.5)(Zn0.5Nb1.5)O7 increased remarkably by Mn2+ substitution, but dielectric constant varied slightly while the dielectric loss increased steadily. The (Bi1.5Zn0.5−x Mn x )(Zn0.5Nb1.5)O7 ceramic with x = 0.5 satisfied the EIA specification NP0 and exhibited excellent dielectric properties of er = 132, tanδ = 2.45 × 10−3, τe = −17 ppm/°C with the low-firing temperature of 950 °C.