Pyrochlore Oxides Research Articles

Regulation of oxygen vacancy within oxide pyrochlores by F-doping to boost oxygen-evolution activity

The development of cost-effective and durable oxygen evolution reaction (OER) catalysts is an essential prerequisite for hydrogen production through proton exchange membrane water electrolysis. Herein, a series of F-doped pyrochlore yttrium ruthenate Y2Ru2O7−δ-xFx (YRuOF) are designed via low-temperature fluorination of precursor oxides with polyvinylidene fluoride. Both X-ray photoelectron spectroscopy and transmission electron microscopy results confirm that F is doped into the pyrochlore crystal lattice, inducing the oxygen vacancies (VO). Importantly, the OER catalytic activity is positively correlated with VO concentration. The density functional calculation (DFT) reveals that the existence of VO enhances the metal-oxygen covalency of pyrochlore, which is conducive to the lattice oxygen evolution mechanism, hence greatly accelerating the OER kinetic. Consequently, the optimized YRuOF catalyst exhibits a greatly enhanced OER activity with an overpotential of ~235 mV at 10 mA cm−2 in comparison with that on undoped pyrochlore (~296 mV). Furthermore, such a F-doped YRuOF catalyst has a very good OER stability. This study provides an ideal perspective to develop highly efficient and durable YRuOF for the oxygen evolution reaction.

Comparison of short-range order in irradiated dysprosium titanates

The structural response of Dy2TiO5 oxide under swift heavy ion irradiation (2.2 GeV Au ions) was studied over a range of structural length scales utilizing neutron total scattering experiments. Refinement of diffraction data confirms that the long-range orthorhombic structure is susceptible to ion beam-induced amorphization with limited crystalline fraction remaining after irradiation to 8 × 1012 ions/cm2. In contrast, the local atomic arrangement, examined through pair distribution function analysis, shows only subtle changes after irradiation and is still described best by the original orthorhombic structural model. A comparison to Dy2Ti2O7 pyrochlore oxide under the same irradiation conditions reveals a different behavior: while the dysprosium titanate pyrochlore is more radiation resistant over the long-range with smaller degree of amorphization as compared to Dy2TiO5, the former involves more local atomic rearrangements, best described by a pyrochlore-to-weberite-type transformation. These results highlight the importance of short-range and medium-range order analysis for a comprehensive description of radiation behavior.

Sizeable bandgap modulation in Y2Hf2O7 pyrochlore oxide thin films through B-site substitution

The high-quality (111)-oriented pyrochlore oxide thin films Y2(Hf1-xIrx)2O7 (YHIO) were prepared by pulsed laser deposition. The bandgap values of Ir-substituted YHIO thin films for x = 0–0.5 range from 4.6 to 1.9 eV. Taking account of spin orbital coupling and correlation effect (U), the first-principles calculations based on density functional theory were used to intercept this variation. The theoretical results illustrate that the variation is attributed to the lower Ir 5d orbital in energy and stronger orbital hybridization between the oxygen 2p orbital and the Ir 5d orbital near the Fermi surface. These results suggest that we can widely modulate the bandgap through B-site chemical substitution based on the Y2Hf2O7 thin film.

Frustrated magnetism in fluoride and chalcogenide pyrochlore lattice materials

The pyrochlore lattice is the arrangement of corner sharing tetrahedra, the well-known host of a wide range of exotic phenomena driven by magnetic frustration. Until recently, much of the work in this domain was confined to the oxide pyrochlore family of materials. Recent advancements are bringing the same research focus to several new families of materials where halide or chalcogenide anions replace oxygen. Each of these new families opens up new avenues of research such as allowing new magnetic ions, modifications to the local spin anisotropies, and a re-organization of the magnetic energy scales. In this research update, the authors highlight how these effects have opened new pathways for frustrated magnetism research, with an emphasis on future prospects.

Monopole matter from magnetoelastic coupling in the Ising pyrochlore

Ising models on a pyrochlore oxide lattice have become associated with spin ice materials and magnetic monopoles. Ever more often, effects connecting magnetic and elastic degrees of freedom are reported on these and other related frustrated materials. Here we extend a spin-ice Hamiltonian to include coupling between spins and the O−2 ions mediating superexchange; we call it the magnetoelastic spin ice model (MeSI). There has been a long search for a model in which monopoles would spontaneously become the building blocks of new ground-states: the MeSI Hamiltonian is such a model. In spite of its simplicity and classical approach, it describes the double-layered monopole crystal observed in Tb2Ti2O7. Additionally, the dipolar electric moment of single monopoles emerges as a probe for magnetism. As an example we show that some Coulomb phases could, in principle, be detected through pinch points associated with O−2-ion displacements.

Evidence of improved tolerance to electronic excitation in nanostructured Nd2Zr2O7

Grain size driven effects on electronic excitation-induced structural modifications have been investigated in nanocrystalline (NC) Nd2Zr2O7 on irradiation with 100 MeV iodine ions. Characterizations have been performed with in situ x-ray diffraction, Raman spectroscopy, and plane-view high-resolution transmission electron microscopy techniques. NC-powders of Nd2Zr2O7 were synthesized by auto gel-combustion and sintered at different temperatures to obtain different grain-sized samples. XRD analysis of the smallest grain-sized sample reveals the highest order–disorder transition (from pyrochlore to a more radiation-resistant phase; anion-deficient fluorite) rate at initial ion fluences followed by least amorphization at higher ion fluences. A strong correlation of the transformation build-up with the double ion impact model confirms the two step amorphization process in NC-Nd2Zr2O7 with the disordered anion-deficient fluorite structure as an intermediate phase. TEM result supports the formation of circular ion track consisting of randomly distributed regions (anion-deficient fluorite structure and amorphous regions), surrounded by a microstrain induced defect-rich pyrochlore superstructure. Lesser ordering at cationic sites and a relatively larger number of grain boundaries are responsible for the highest radiation tolerance exhibited by the smallest grain-sized sample. The present study reports a relatively higher radiation stability of NC-ternary pyrochlore oxide, Nd2Zr2O7, with a grain size of a few tens of nm, which establishes its application as a potential inert matrix for nuclear applications.

Lattice Disorder and Oxygen Migration Pathways in Pyrochlore and Defect-Fluorite Oxides

Atomic-scale disorder plays an important role in the chemical and physical properties of oxide materials. The structural flexibility of pyrochlore-type oxides allows for crystal-chemical engineerin...

Characterizing oxygen atoms in perovskite and pyrochlore oxides using ADF-STEM at a resolution of a few tens of picometers

We present an aberration corrected scanning transmission electron microscopy (ac-STEM) analysis of perovskite (LaFeO3) and pyrochlore (Yb2Ti2O7 and Pr2Zr2O7) oxides and demonstrate that both the shape and contrast of visible atomic columns in annular dark-field (ADF) images are sensitive to the presence of nearby atoms of low atomic number (e.g. oxygen). We show that point defects (e.g. oxygen vacancies), which are invisible – or difficult to observe due to limited sensitivity – in x-ray and neutron diffraction measurements, are the origin of the complex magnetic ground state of pyrochlore oxides. In addition, we present a method by which light atoms can be resolved in the quantitative ADF-STEM images. Using this method, we resolved oxygen atoms in perovskite and pyrochlore oxides and propose this method to be suitable for other materials containing both light and heavy elements.

Synthesis and Characterization of Pyrochlore-Type Praseodymium Stannate Nanoparticles: An Effective Electrocatalyst for Detection of Nitrofurazone Drug in Biological Samples.

Apart from perovskites, the development of different types of pyrochlore oxides is highly focused on various electrochemical applications in recent times. Based on this, we have synthesized pyrochlore-type praseodymium stannate nanoparticles (Pr2Sn2O7 NPs) by using a coprecipitation method and further investigated by different analytical and spectroscopic techniques such as X-ray diffraction, Raman spectroscopy, field emission-scanning electron microscopy, high resolution-transmission electron microscopy, and X-ray photoelectron spectroscopy analysis. Followed by this, we have designed a unique and novel electrochemical sensor for nitrofurazone detection, by modifying the glassy carbon electrode (GCE) with the prepared Pr2Sn2O7 NPs. For that, the electrochemical experiments were performed by using cyclic voltammetry and differential pulse voltammetry techniques. The Pr2Sn2O7 NPs modified GCE exhibits high sensitivity (2.11 μA μM-1 cm-2), selectivity, dynamic linear ranges (0.01-24 μM and 32-332 μM), and lower detection limit (4 nM). Furthermore, the Pr2Sn2O7 NPs demonstrated promising real sample analysis with good recovery results in biological samples (human urine and blood serum) which showed better results than the noble metal catalysts. Based on these results, the present work gives clear evidence that the pyrochlore oxides are highly suitable electrode materials for performing outstanding catalytic activity toward electrochemical sensors.

Epitaxial Ferroelectric Hf0.5 Zr0.5 O2 with Metallic Pyrochlore Oxide Electrodes.

The synthesis of fully epitaxial ferroelectric Hf0.5 Zr0.5 O2 (HZO) thin films through the use of a conducting pyrochlore oxide electrode that acts as a structural and chemical template is reported. Such pyrochlores, exemplified by Pb2 Ir2 O7 (PIO) and Bi2 Ru2 O7 (BRO), exhibit metallic conductivity with room-temperature resistivity of <1 mΩ cm and are closely lattice matched to yttria-stabilized zirconia substrates as well as the HZO layers grown on top of them. Evidence for epitaxy and domain formation is established with X-ray diffraction and scanning transmission electron microscopy, which show that the c-axis of the HZO film is normal to the substrate surface. The emergence of the non-polar-monoclinic phase from the polar-orthorhombic phase is observed when the HZO film thickness is ≥≈30nm. Thermodynamic analyses reveal the role of epitaxial strain and surface energy in stabilizing the polar phase as well as its coexistence with the non-polar-monoclinic phase as a function of film thickness.

Local Ordering in Disordered Nd &lt;sub&gt;x&lt;/sub&gt;Zr &lt;sub&gt;1-x&lt;/sub&gt;O &lt;sub&gt;2-0.5x&lt;/sub&gt; Pyrochlore as Observed Using Neutron Total Scattering

Pyrochlore complex oxides (A2B2O7) are an important class of fluorite-derivative ceramics with exceptional chemical and structural versatility which make them ideal model systems for studying disordering mechanisms over a range of spatial scales. Neutron total scattering methods were used to analyze the structural behavior in the non-stoichiometric series NdxZr1-xO2-0.5x (0.5 ≤ x ≤ 0.23) as a function of Nd content, x. Characterization of the structure functions using Rietveld refinement and the pair distribution functions with small-box refinement reveal complex disordering pathways; the average, long-range phase changes over a very narrow compositional range from pyrochlore (Fd m) to defect fluorite (Fm m) through full randomization of the cation and anion sublattices at x ~ 0.31, while the local, short-range structure continuously adopts a weberite-type atomic arrangement (C2221). Comparison to a previously studied Ho2Ti2-xZrxO7 solid solution series reveals how changes in chemical composition and stoichiometry modify defect formation and determine how disordering progresses across different length scales in pyrochlore oxides.

Understanding synergistic metal–oxide interactions of in situ exsolved metal nanoparticles on a pyrochlore oxide support for enhanced water splitting

Structure-controlled hybrid catalysts are prepared by carefully controlling the in situ exsolution process of metallic pyrochlore oxide for efficient oxygen evolution reaction and hydrogen evolution reaction under alkaline conditions.

Pyrochlore oxides as visible light-responsive photocatalysts

This perspective describes the use of pyrochlore oxides in photocatalysis with focus on the strategies to enhance their activity.

Study on thermal, mechanical, microstructural properties and failure analyses of lanthanum zirconate based thermal barrier coatings: A review

This research study summarizes the recent information about the characteristics of multi layer and composite lanthanum zirconate based thermal barrier coatings (TBCs) that provide high-temperature thermal protection for the jet engines parts. Thus the outcome of lightweight gas turbine components able to work at 3000° F nearly 1650 °C needs to be built. Such difficulties can be solved using coatings with optimized microstructure and chemical composition. Recently, Lanthanum zirconate (La2Zr2O7-LZ) based pyrochlore oxides (Ra2Zr2O7, Re = rare earth) are now developing TBC material, owing to their fewer thermal conductivity with enhanced stability at elevated operating temperatures. Even though LZ cannot be directly replaced for standard YSZ because of its mismatch between the grew of thermal oxide (TGO) film. When LZ compared to Zirconia partially stabilized with yttria (YSZ), LZ possesses a poor thermal conductivity, lesser coefficient of thermal expansion, lower sintering ability, and lack of toughness than YSZ. Hence LZ comes to be combined with standard TBC YSZ in order to integrate advantages in multilayer architecture. In this study describes the thermal, mechanical and physical properties and failure analysis of Multilayer LZ/YSZ were described. It also reviews the characterization of coatings according to different criteria and conditions. At last, suggestions for investigation on layered LZ based TBCs applications for next-generation was indicated.

Electrical and Thermal Transport Properties of the β-Pyrochlore Oxide CsW2O6

We report the electrical resistivity, thermoelectric power, and thermal conductivity of single-crystalline and sintered samples of the 5d pyrochlore oxide CsW2O6. The electrical resistivity of the single crystal is 3 mΩ cm at 295 K and gradually increases with decreasing temperature above 215 K (Phase I). The thermoelectric power of the single-crystalline and sintered samples shows a constant value of approximately −60 µV K−1 in Phase I. These results reflect that the electron conduction by W 5d electrons in Phase I is incoherent and in the hopping regime, although a band gap does not open at the Fermi level. The thermal conductivity in Phase I of both samples is considerably low, which might be due to the rattling of Cs+ ions. In Phase II below 215 K, the electrical resistivity and the absolute value of thermoelectric power of both samples strongly increase with decreasing temperature, corresponding to a transition to a semiconducting state with a band gap open at the Fermi level, while the thermal conductivity in Phase II is smaller than that in Phase I.

Engineering Catalysts for Anion Exchange Membrane Electrolyzer Anodes and Cathodes

The development of catalyst for both Oxygen Evolution and Hydrogen Evolution reactions (OER and HER, respectively) in Anion-Exchange Membrane (AEM) electrolyzers is proceeding along with the development of better solids electrolytes, as well as electrode and system designs.AEM-based electrolyzers are now achieving operating lives exceeding 1000s of hours and 5.0 > A/cm2 at 1.8V [1] with increasing evidence that catalyst/ionomer interactions greatly effect both catalyst performance and ionomer stability[2]. Further, the promise of PGM-free electrolysis continues even though PGM-based catalysts seem to be most stable.Pajarito Powder is developing catalysts explicitly designed for AEM electrolysis and has been working with AEM materials and systems developers to measure and understand key performance issues for electrolysis catalysts. Together with partners like LANL, Nel, GT, and WUSTL Pajarito catalysts are now showing that for stable HER available PGM-free catalysts lag PGM catalyst by about 100 mV, compared to a near 250 mV lag for OER.The best OER catalysts demonstrated however by Nel, GT, and PP are based on PbRuOx [3], and have outperformed commercialized IrOx by >140 mV for up to 1000 h has been shown and outperform PGM-free catalysts by 250+ mV. However, the combination of Pb and the rarity of Ru have motivated alternative formulations. Leading stable PGM-free catalysts in turn include multi-site electrocatalysts such as barium and lanthanum oxide systems doped with strontium, samarium, and niobium are combined with Cr, Mn, Fe, Co, and Ni in perovskite, mullite, spinel, and delaffosite crystal structures. The activity and electronic conductivity of these systems have been achieved simultaneously through balancing the structural strain of the La and Ba doped with Sr, Sm, and Nb while the doping and exchange of the transition metal component refines the electronic structure and therefore bonding of the catalysts. These documented materials have some limitation in terms of conductivity and stability, but there are indications that improvements are possible through refining these types of catalysts to including use of mesoporous and conductive supports that both bolster catalyst system conductivity and increase the active surface area through dispersion of the catalyst phase.For stable HER the performance of Ni-alloys are rapidly approaching Pt-alloys for the HER, where the in-cell performance only lags by about 50-100 mV and they are stable for >1000 h. At the same time, improvements in PGM-free OER catalysts continue to be made, though challenging.The current status of electrocatalysts for AEM, prospects for further catalysts improvements, and challenges will be presented and discussed.[1] Li, D., Park, E.J., Zhu, W. et al. Highly quaternized polystyrene ionomers for high performance anion exchange membrane water electrolysers. Nat Energy 5, 378–385 (2020). https://doi.org/10.1038/s41560-020-0577-x and[2] Maurya S., Lee, AS., Li, D., Park, EJ., Leonard, DP., Noh, S. On the origin of permanent performance loss of anion exchange membrane fuel cells: Electrochemical oxidation of phenyl group Journal of Power Sources, Volume 436, 1 October 2019, 226866[3] Horowitz, H. S., Longo, J.M., and Horowitz, H. H. Oxygen Electrocatalysis on Some Oxide Pyrochlores J. Electrochem. Soc. 1983 volume 130, issue 9, 1851-1859 and Pyrochlore electrocatalysts for efficient alkaline water electrolysis, Parrondo, J., George, M., Capuano, C., Ayers, C., Ramani, V. J. Mater. Chem. A, 2015,3, 10819-10828.

Influence of Mn substitution on crystal structure and magnetic properties of Y2Zr1-xTi1-xMn2xO7 (x = 0.0, 0.05, 0.10) family of pyrochlore oxides

The present paper deals with the effect of Mn doping on structural and magnetic properties of Y2Zr1-xTi1-xMn2xO7 (x = 0.0, 0.05, 0.10) compounds. All the samples are single phase and crystallize in the cubic system with Fd-3m space group. Both lattice parameter a and cell volume V decrease with Mn substitution because of the smaller ionic radius of Mn4+ than Zr4+ and Ti4+. No significant differences between zero field-cooled (ZFC) and field-cooled (FC) curves are evident, demonstrating that the samples exhibit no magnetic ordering or spin glass-like transition. Both the Mn-doped phases exhibit antiferromagnetic behavior, which may possibly be due to the presence of super-exchange (SE) Mn4+–O2−–Mn4+ interactions. Small magnetic hysteresis loops are observed for the Mn-doped phases suggesting the presence of weak ferromagnetic interactions.

Domain Control by Adjusting Anisotropic Stress in Pyrochlore Oxide Cd2Re2O7

The 5d pyrochlore oxide Cd2Re2O7 exhibits successive phase transitions from a cubic pyrochlore structure (phase I) to a tetragonal structure without inversion symmetry below Ts1 of ~200 K (phase II) and further to another noncentrosymmetric tetragonal structure below Ts2 of ~120 K (phase III). The two low-temperature phases may be characterized by odd-parity multipolar orders induced by the Fermi liquid instability of the spin-orbit-coupled metal. To control the tetragonal domains generated by the transitions and to obtain a single-domain crystal for the measurements of anisotropic properties, we prepared single crystals with the (0 0 1) surface and applied biaxial and uniaxial stresses along the plane. Polarizing optical microscopy observations revealed that inducing a small strain of approximately 0.05% could flip the twin domains ferroelastically in a reversible fashion at low temperatures, which evidences that the tetragonal deformation switches at Ts2 between c > a for phase II and c < a for phase III. Resistivity measurements using single-domain crystals under uniaxial stress showed that the anisotropy was maximum at around Ts2 and turned over across Ts2: resistivity along the c axis is larger (smaller) than that along the a axis by ~25% for phase II (III) at around Ts2. These large anisotropies probably originate from spin-dependent scattering in the spin-split Fermi surfaces of the cluster electric toroidal quadrupolar phases of Cd2Re2O7.

Non-adiabatic small polaron hopping transport above metal-like to insulator transition in the vacant 3d-orbital Tb2Ti2O7 pyrochlore oxide

We report the validity of Mott variable range hopping (VRH) conduction mechanism and small polaron hopping in Tb2Ti2O7 in the temperature range 603–803 K. The temperature-dependent resistivity data are in good agreement with Mott 3d VRH. The other parameters estimated such as hopping range, hopping energy and density of states near Fermi level are in good agreement with those of many semiconductor oxides. The Holstein’s condition for non-adiabatic conduction mechanism is also satisfied. The ac conductivity is governed by Jonscher’s power law as $$ {\sigma}_{a.c}^{\prime}\left(\upsilon, T\right)={\sigma}_{d.c}(T)+\alpha (T){\upsilon}^n $$ and the exponent n values in the range of 1.02–1.23 suggest that conduction takes place due to the small polaron hopping mechanism. The possible polaron formation and their mechanism are discussed systematically.

Synergistic effects in oxygen evolution activity of mixed iridium-ruthenium pyrochlores

Pyrochlore oxides (A2B2O7) simultaneously containing iridium and ruthenium in the B-site are promising catalysts for oxygen evolution reaction (OER) in acid media. The catalytic activity of the pyrochlore based catalysts is increased by the coexistence of Ir and Ru in the B-site of the pyrochlore structure. Lanthanide (Yb, Gd, or Nd) stabilized mixed pyrochlores with a fraction of Ru in the B-site of xRu =0.2, 0.4, 0.6, 0.8 were synthesized by the spray-freeze freeze-dry approach. All prepared mixed pyrochlore catalysts are surpassing the OER activity of the corresponding iridium and ruthenium analogues featuring no cation mixing as well as that of the benchmark IrO2 catalyst. The synergy of Ir and Ru in the B-site of the pyrochlore structure suppresses the effect of the A-site cation radius on the OER activity. The observed OER activity scales with the Ir-Ru bond distance which represents the local structure of the prepared materials. The most active ytterbium catalyst also shows a significant stability improvement under OER operando conditions over the benchmark IrO2