A2B2O7 Pyrochlore Research Articles

High-pressure behavior of high-entropy A2B2O7 pyrochlore

A single high-entropy pyrochlore-type compound (A2B2O7) was successfully synthesized, incorporating 8 different rare-earth cations at the A site and 2 different metal cations at the B site in equiatomic amounts [(8A1/8)2(2B1/2)2O7]. Powders with a nominal composition of (La1/8Sm1/8Nd1/8Pr1/8Y1/8Gd1/8Dy1/8Yb1/8)2(Hf1/2Zr1/2)2O7 were fabricated by glycine nitrate procedure (GNP). The GNP process yielded powders with low crystallinity and after subsequent calcination, a well crystalline ceramic powders were formed. The phase evolution was initially analyzed using X-ray diffraction (XRD). In situ high-pressure X-ray diffraction was employed to investigate the structural stability and high-pressure behavior of the A2B2O7 pyrochlore up to 25.8 GPa. Theoretical investigations of the high-entropy pyrochlore systems were performed and showed good agreement with the experimentally obtained results.

(Invited) Modification of Dielectric Properties of Film Dielectric Materials by Solid Solution System

Compound semiconductor based power devices such as SiC and GaN require high temperature operating passive devices such as a capacitor. SiC-based active devices can operate above 250 °C. The development of high-temperature operational passives is urgently needed to advance to the module and system level. In the case of capacitors, currently available capacitors can only operate efficiently up to 175 °C. Therefore, a thin film capacitor that can operate at high temperatures and be monolithically integrated near the active device should be developed. In this talk, I will present our achievements in the development of high-temperature operating dielectric film materials using the solid solution system. One topic is A2B2O7 pyrochlore ferroelectrics. The ferroelectric materials with the pyrochlore structure such as Sr2Nb2O7 exhibited the high dielectric constant above 100 at high and low temperature ranges. However, due to the crystallographic direction dependence of the Curie-Weiss temperature, the temperature stability of the dielectric constant from room temperature to 300 °C was not available. In order to obtain the high temperature stability of dielectric constant from room temperature to 300 °C with high dielectric constant over 100, the pyrochlore ferroelectric solid solution thin films were investigated by employing the combinatorial synthesis. Composition spread thin films of solid solution of two or three kinds of materials were deposited on substrates by physical vapor deposition technique, which provided the systematic physical properties. High-throughput combinatorial synthesis method is effective in the rapid optimization of the correlation between the crystallization and the dielectric property change behavior. For the composition spread solid solution film, the Sr- or La-containing A2B2O7 pyrochlore ferroelectrics were selected from the viewpoints of ion radios and crystallographic direction dependence of the Curie-Weiss temperature. The composition spread films were formed on a Pt/Ti/SiO2/Si substrate by combinatorial RF sputtering with stoichiometric composition targets. The X-ray fluorescence spectrum of the composition-spread film showed a linear variation of the metal composition. After deposition, the samples were annealed at different temperatures under oxygen atmosphere to reduce oxygen vacancies and improve crystallinity. The crystallization of the composition-spread solid solution thin film was confirmed by X-ray diffraction patterns, indicating the composition dependence of the crystallographic orientation. Some compositions showed high dielectric constant over 100 and high temperature stability from room temperature to 300 °C. In the presentation, the correlation between film composition, orientation and dielectric constant will be discussed in detail. In the presentation, we also introduce our achievement of the BaTiO3 based relaxor ferroelectrics and other dielectric materials development using the solid solution system. For the relaxor ferroelectrics, among the BaTiO3 based relaxor ferroelectrics, we have selected x[BaTiO3]-(1-x)[Bi(Mg2/3Nb1/3)O3] . The permittivity of 400 and the stability <8% from room temperature to 400 °C were obtained, which are promising as high-temperature dielectric medium.

Surface Oxygen Defect Engineering of A2B2O7 Pyrochlore Semiconductors Boosts the Electrocatalytic Reduction of CO2-to-HCOOH.

The electrocatalytic conversion of inert CO2 to value-added chemical fuels powered by renewable energy is one of the benchmark approaches to address excessive carbon emissions and achieve carbon-neutral energy restructuring. However, the adsorption/activation of supersymmetric CO2 is facing insurmountable challenges that constrain its industrial-scale applications. Here, this theory-guided study confronts these challenges by leveraging the synergies of bimetallic sites and defect engineering, where pyrochlore-type semiconductor A2B2O7 is employed as research platform and the conversion of CO2-to-HCOOH as the model reaction. Specifically, defect engineering intensified greatly the chemisorption-induced CO2 polarization via the bimetallic coordination, thermodynamically beneficial to the HCOOH production via the *HCO2 intermediate. The optimal V-BSO-430 electrocatalyst with abundant surface oxygen vacancies achieved a superior HCOOH yield of 116.7mmol h-1 cm-2 at -1.2 VRHE, rivalling the incumbent similar reaction systems. Furthermore, the unique catalytic unit featured with a Bi1-Sn-Bi2 triangular structure, which is reconstructed by defect engineering, and altered the pathway of CO2 adsorption and activation to allow the preferential affinity of the suspended O atom in *HCO2 to H. As a result, V-BSO-430 gave an impressive FEHCOOH of 93% at -1.0 VRHE. This study held promises for inspiring the exploration of bimetallic materials from the massive semiconductor database.

Heavily doped high-entropy A2B2O7 pyrochlore

A novel class of high-entropy pyrochlore compounds with multiple elements at the A and B site positions (A2B2O7) was successfully obtained. Powders with (La1/7Sm1/7Nd1/7Pr1/7Y1/7Gd1/7Yb1/7)2(Sn1/3Hf1/3Zr1/3)2O7 nominal composition were fabricated from pure metal oxides obtained through a reaction of metal nitrates (for site A) and metal chlorides (for site B) with sodium hydroxide during the solid-state displacement reaction (SSDR). The phase evolution was analyzed using XRD method. During the thermal treatment of ten individual metal oxides, the single pyrochlore phase was created. The present study showed that the highdensity (98%TD) ceramics with a hardness of 8.1GPa was successfully obtained after pressureless sintering at 1650 ?C for 4 h. Results of the Raman study and the Rietveld structural refinement showed that sintered highentropy ceramics is characterized by a single-phase pyrochlore structure, which was investigated in detail.

Synthesis and characterization of high-entropy A2B2O7 pyrochlore with multiple elements at A and B sites

Single nano high-entropy pyrochlore-type compound (A2B2O7) with 7 different rare-earth cations at site A and 3 different metal cations at site B with equiatomic amounts (7A1/7)2(3B1/3)2O7 is successfully obtained. The powder with nominal composition (La1/7Sm1/7Nd1/7Pr1/7Y1/7Gd1/7Yb1/7)2(Sn1/3Hf1/3Zr1/3)2O7 was fabricated by reacting metal nitrates (site A) and metal chlorides (site B) with glycine during the combustion reaction. The XRD analysis revealed that the powder attained during synthesis is in an amorphous state. To induce crystallization of the obtained pyrochlore structure, the post-calcination process at 600-1500?C was conducted and studied. Results of this study showed that the monophase pyrochlore (A2B2O7) structure is obtained during the calcination at 900?C. The high-density ceramic pellet with 97% of theoretical density and free of any additives was obtained through pressureless sintering at 1650?C for 4 h in the air using the powder calcined at 900?C.

Preparation and irradiation stability of A2B2O7 pyrochlore high-entropy ceramic for immobilization of high-level nuclear waste

The high-entropy pyrochlores have attracted increasing attention due to their ability to immobilize multiple nuclides simultaneously, compared with baseline single-component pyrochlores. In this work, nine kinds of multicomponent oxide ceramics are synthesized and eight of them exhibit a single pyrochlore phase. In addition, the ratio of rA to rB of high-entropy pyrochlore Gd2(Ti0.25Zr0.25Hf0.25Ce0.25)2O7 is 1.4499, this one observation suggests that for high-entropy pyrochlore, the rA/rB limit of pyrochlore phase is different from that of single-component pyrochlore (1.46 < rA/rB < 1.78). In order to evaluate the radiation resistance of high-entropy pyrochlores, four typical samples were irradiated by 800 keV Kr2+ ions beam. The GIXRD results show that all the high-entropy pyrochlores only undergo a phase transition from an ordered pyrochlore structure to a disordered fluorite structure (O-D transition) at the highest irradiation dose (∼ 40 dpa) in this study, instead of being amorphous. For (Sm0.5Gd0.5)2(Ti0.25Zr0.25Hf0.25Ce0.25)2O7 and (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2(Ti0.25Zr0.25Hf0.25Ce0.25)2O7, phase separation appears after irradiation. The results from the first principles calculations confirm that Gd2(Ti0.25Zr0.25Hf0.25Ce0.25)2O7 exhibits stronger radiation resistance than the corresponding single pyrochlores, which agrees with the irradiation experiment result. This study demonstrates that the high-entropy pyrochlores exhibit excellent irradiation stability and can be considered as the potential matrix material for immobilization of high-level nuclear waste.

Machine learning-enabled prediction of chemical durability of A2B2O7 pyrochlore and fluorite

Pyrochlore-structure type and its derivative in a general formula A2B2O7 (A = rare earth elements and actinides; B = Ti, Sn, Zr, Hf, Pb, Si, etc.) display excellent structural flexibility and rich crystal chemistry as promising nuclear waste form materials capable of immobilizing actinides and fission products. It is essential to understand these materials’ chemical durability and element release of radionuclides in order to evaluate their performance in near-field environment. However, it is a formidable grand technological challenge to experimentally perform durability testing across hundreds of thousands of possibilities resulting from their extreme compositional complexities due to cation substitutions at both A and B-sites. In this work, we demonstrate a machine learning approach to determine the key materials parameters and structural characteristics governing the leaching behaviors from a small set of selected compositions as model systems, enabling a science-based prediction of their chemical durability that can be extended to a wide range of chemical compositions. The combination of four key structural characteristics and materials parameters, including ionic radius size difference rA-B, ionic potential difference EB-A, electronegativity difference χB-A, and lattice parameter α, creates features an optimized prediction of the chemical durability. Two machine learning models, linear regression and Kernel ridge regression models, are trained on the randomly-split training dataset derived from the experimentally-determined elemental release rates, and subsequently tested on the testing dataset. The predicted leaching rates from both machine learning models show an excellent agreement with the experimental data, demonstrating the feasibility of rapidly evaluating the material properties of new compositions. These results highlight the immense potential of synergizing informatics through machine learning-based models and well-controlled experiments of selected model systems to accelerate materials design and discovery with optimized compositions and performance of promising materials for effective nuclear waste management.

Atomic order-disorder engineering in the La2Zr2O7 pyrochlore under low energy ion irradiation

Cation and anion disordering affect the structural and electronic properties of the isometric A2B2O7 pyrochlore materials. Here, we report a study on the structural response of La2Zr2O7 at two different temperatures (300 K and ~88 K) as a function of ion fluence (1 × 1013, 5 × 1013, and 1 × 1014 ions/cm2). The effect of ion fluence and irradiation temperature on the structural properties have been investigated using the grazing angle x-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy. GIXRD results confirmed that the weakening/broadening of the diffraction peaks and lattice volume expansion increases monotonically as a function of ion fluence at both the temperatures and are more pronounced at ~88 K. The cation and anion disordering appear to be ion fluence and irradiation temperature-dependent. Raman spectroscopy shows that the atomic disordering is more pronounced with enhanced ion fluence and revealed the involvement of the X48f parameter in the enhancement of disordering in the system. The HRTEM analysis revealed that the deterioration in the atomic ordering (amorphization) is significantly more pronounced at ~88 K. The qualitative analysis of cation/anion disordering and structural deformation revealed that irradiation parameters play a crucial role in developing and altering the properties of the pyrochlore materials for the technological applications.

Investigation of mechanical and thermal properties of rare earth pyrochlore oxides by first‐principles calculations

AbstractThe pyrochlore‐type rare‐earth oxides attract considerable attentions due to their outstanding properties and extensive applications. In this work, contour maps of mechanical/thermal properties as a function of A and B cation radii across a wide variety of A2B2O7 (A = La‐Lu and Y; B = Ti, Sn, Hf, Zr, Pr and Ce) pyrochlore oxides are studied using the first‐principles calculations. The mechanical/thermal properties vary dramatically with increasing of the B cation sizes but do not show a strong systematic dependence on the A cation sizes. Furthermore, the machine learning algorithm is performed for the large family of pyrochlores and the parameters playing key role on mechanical/thermal properties are clarified. Besides, the expressions of focused mechanical and thermal properties are constructed. These results are expected to guide the future material design through composition tailoring.

Probing the reactivity and structure relationship of Ln2Sn2O7 (Ln=La, Pr, Sm and Y) pyrochlore catalysts for CO oxidation

To elucidate the structure and reactivity relationship of A2B2O7 pyrochlore compounds, a series of Ln2Sn2O7 catalysts (Ln = La, Pr, Sm and Y) with varied rare earth cation A sites but with a fixed Sn cation B site have been synthesized and probed by CO oxidation. It is revealed that all the samples are composed of cubic pyrochlore phases. By replacing the A site with different rare earth cations, the rA/rB ratios decrease in the sequence of La2Sn2O7, Pr2Sn2O7, Sm2Sn2O7, Y2Sn2O7, which transforms the crystalline structures of the samples from well-ordered to less-ordered pyrochlore phases. As a consequence, the mobility of the lattice oxygen related to the intrinsic 8a oxygen vacancies improved evidently, thus inducing the formation of more abundant surface mobile oxygen species. Therefore, the intrinsic and overall activity of Ln2Sn2O7 catalysts can be strengthened. Due to the least ordered structure and the presence of the largest amount of surface active oxygen species, Y2Sn2O7 displays the highest activity among all the catalysts in this study.

High-pressure behavior of A2B2O7 pyrochlore (A=Eu, Dy; B=Ti, Zr)

In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A2B2O7 pyrochlore (A = Eu, Dy; B = Ti, Zr) up to ∼50 GPa. Based on X-ray diffraction results, all compositions transformed to the high-pressure cotunnite structure. The B-site cation species had a larger effect on the transition pressure than the A-site cation species, with the onset of the phase transformation occurring at ∼41 GPa for B = Ti and ∼16 GPa B = Zr. However, the A-site cation affected the kinetics of the phase transformation, with the transformation for compositions with the smaller ionic radii, i.e., A = Dy, proceeding faster than those with a larger ionic radii, i.e., A = Eu. These results were consistent with previous work in which the radius-ratio of the A- and B-site cations determined the energetics of disordering, and compositions with more similarly sized A- and B-site cations had a lower defect formation energy. Raman spectra revealed differences in the degree of short-range order of the different compositions. Due to the large phase fraction of cotunnite at high pressure for B = Zr compositions, Raman modes for cotunnite could be observed, with more modes recorded for A = Eu than A = Dy. These additional modes are attributed to increased short-to-medium range ordering in the initially pyrochlore structured Eu2Zr2O7 as compared with the initially defect-fluorite structured Dy2Zr2O7.

Rotating Disk Electrode Voltammetry of Thin Films of Novel Oxide Materials

A custom-built apparatus for performing rotating disk electrode voltammetry (RDE) using vapor-deposited, epitaxial, thin film samples as rotating electrodes is described. This method allows for quantitative electrochemical characterization using thin film samples, including those deposited on insulating substrates, and allows for very facile exchange of samples. Cyclic voltammetry and RDE voltammetry of iron ferricyanide in aqueous media were explored to examine if the system causes unusual deviations from ideal hydrodynamic voltammetric behavior. The set-up was also used to examine the ORR activity of a platinum thin film as a model system for a higher-current reaction, relevant to fuel-cell research. Both studies indicated that the set-up did not cause any deviations from anticipated RDE behavior, demonstrating that this is a viable method for performing rotating disk electrode voltammetry of vapor-deposited thin films, with emphasis on those relevant to fuel cell research. The set-up was used to investigate/test the ORR and OER activity of a series of A2B2O7 pyrochlore samples.

Effect of composition on thermal conductivity of MgO–Nd2Zr2O7 composites for inert matrix materials

Inert matrix fuels based on magnesium oxide (MgO) as the inert phase have been of historic interest due to its high thermal conductivity. Minor actinide-bearing phases possessing an A2B2O7 pyrochlore structure are also believed to contain a range of favorable attributes, suggesting a possible pairing with MgO to yield a high performance inert matrix fuel. The thermal diffusivity, heat capacity, and thermal expansion of MgO–Nd2Zr2O7 composites were measured from room temperature to 1273K, where the MgO phase content was varied from 40 to 70 volume percent. The thermal conductivity of each composition was calculated using these results and then compared to widely employed methods to approximate the thermal conductivity of composite materials based upon the properties of the constituent phases. Results suggest that use of either a rule of mixtures or geometric mean approximation for the thermal conductivity of composite systems such as this one would be subject to significant uncertainties when the constituent properties widely differ. A sigmoidal average of the upper and lower Hashin–Shtrikman bounds was found to be in good agreement with the thermal conductivity of the composites as determined experimentally.

Accommodation of niobium and cerium variable valence cations in the crystal lattice of the Ln 2 3+ Ti 2 4+ O7 (Ln = Dy, Yb) pyrochlores

We have studied the accommodation of niobium and cerium variable valence cations in the Ln 2 3+ Ti 2 4+ O7 (3+/4+) (Ln = Dy, Yb) pyrochlores in air and in a 5% H2 + 95% Ar reducing atmosphere. The (Dy0.8Ca0.1Ce0.1)2Ti2O7 − δ and (Yb0.8Tb0.1Ca0.1)2[Ti1−x Nb x ]2O7 − δ (x = 0–0.3) oxygen ion conducting solid solutions were synthesized through coprecipitation followed by heat treatment in air for 4 h at temperatures of 1400 and 1550°C, respectively. According to X-ray photoelectron spectroscopy (XPS) data, the valence state of the niobium in the (Yb0.8Tb0.1Ca0.1)2[Ti1−x Nb x ]2O7 − δ (x = 0.05, 0.1) solid solutions is 4+ and, hence, isovalent niobium substitution for titanium takes place. During synthesis in air, the titanium sublattice of the (Yb0.8Tb0.1Ca0.1)2[Ti1−x Nb x ]2O7 − δ pyrochlores is stable when the Nb4+ content does not exceed 10% (x = 0.1). According to conductivity data for the high-conductivity material (Yb0.8Tb0.1Ca0.1)2[Ti0.95Nb0.05]2O6.9 at low oxygen partial pressures, there is no reduction of pentavalent niobium and no increase in electronic conductivity, which is supported by XPS results that only Nb4+ is present in the solid solution. In pyrochlore-like (Dy0.8Ca0.1Ce0.1)2Ti2O7 − δ synthesized in air at 1400°C, the dysprosium site contains both tri- and tetravalent cerium, as supported by conductivity measurements under reducing conditions. The Ce4+ + e → Ce3+ reduction process led to a change in the color and disintegration of the sample when the oxygen partial pressure was reduced. In the 3+/4+ A2B2O7 pyrochlores, variable valence cations on the A site (eightfold coordination) more readily participate in redox processes than do those on the B site (sixfold coordination).

Glycine–nitrate synthesis of Sr doped La2Zr2O7 pyrochlore powder

Materials with A2B2O7 (pyrochlore) structure have received significant attention for their applications as new protonic conductors and materials used in electronic devices. One of the unique synthesis routes for La2Zr2O7 (pyrochlore) powders is the glycine–nitrate combustion method, which shows superior properties of the synthesised powder using glycine as a complexing agent. The Sr doped La2Zr2O7 powders in pure pyrochlore structure were produced using this approach. Selected characteristics of the synthesised powders, such as crystal structure, lattice parameters, crystallite size, the vibrational properties, the morphology of the particles, along with the specific surface area and particle size, have been investigated. The dependence of some properties on annealing temperatures of the powders has been studied.

Plutonium and uranium accommodation in pyrochlore oxides

Fluorite-related ceramics have attracted considerable attention as potential host materials for the immobilisation of radionuclides. Here we have used computer simulation to investigate the relative solution energies and mechanisms for Pu3+, Pu4+ and U4+ accommodation in an extensive range of A2B2O7 pyrochlore compounds. Solution is considered from simple oxides and via co-solution mechanisms. Results are discussed in terms of their implications for actinide retention and materials fabrication.

Nonstoichiometry in A2B2O7 Pyrochlores

Energies associated with deviations from stoichiometry for an extensive series of A2B2O7 pyrochlores have been predicted. A3+ cations range in size from Lu to La and B4+ cations from Ti to Pb. Results are presented in the form of contour maps as a means of conveying large quantities of data as well as predicting characteristics for pyrochlore compounds not explicitly modeled. These contour maps indicate that the BO2 excess nonstoichiometry in the pyrochlore structure is distinct from solid‐solution fluorite. Within the limitations of this methodology, the contour maps provide a means to understand and predict distinct compositional variations. Defect cluster formation is discussed.

Predictions of strontium accommodation in A2B2O7 pyrochlores

A2B2O7 pyrochlore oxides are being considered as potential host materials for the immobilization of fission products. It is therefore important to establish the relative ability of these compounds to accommodate fission product ions. We address this issue by using computer simulations to predict the structures and relative equilibrium energies associated with solution of Sr2+ over an extensive compositional range. Results indicate that strontium is accommodated via substitution for A host cations with oxygen vacancy compensation. This results in a nonstoichiometric composition. Optimum compositions and defect clusters structures are identified by constructing contour energy maps.

The oxygen positional parameter in pyrochlores and its dependence on disorder

Abstract Atomistic simulation calculations based on energy minimization techniques have been used to predict the O positional parameter of a wide range of A2B2O7 pyrochlore oxides. Cations studied range from Lu3+ to La3+ on the A site and Ti4+ to Pb4+ on the B site. In all cases the model included a specific predicted degree of disorder without which it was not possible to reproduce experimental values accurately. However, the extent of disorder invoked was dependent on the specific A and B cations involved, with Gd2Ti2O7 exhibiting the lowest and Gd2Pb2O7 the highest degree of disorder.

Disorder in Pyrochlore Oxides

Contour plots of disorder reaction energies across a wide variety of A2B2O7 pyrochlore oxides have been produced, using atomistic simulation calculations based on energy‐minimization techniques. The cations studied range from Lu3+ to La3+ on the A site and from Ti4+ to Ce4+ on the B site. The present results relate the experimentally observed stability range of the pyrochlore structure to A and B cation size and disorder. Disorder enthalpies decrease dramatically with increasing B cation size, while the rate of increase with A cation size depends markedly on the particular B cation. The association of anion Frenkel and cation antisite disorders is found to be of importance.