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Sm2−xCaxZr2O7−x/2 (x = 0, 0.05, 0.1) and Gd2−xCaxZr2O7−x/2 (x = 0.05, 0.1) mixed oxides in a pyrochlore–fluorite morphotropic phase region were prepared via the mechanical activation of oxide mixtures, followed by annealing at 1600 °C. The structure of the solid solutions was studied by X-ray diffraction and refined by the Rietveld method, water content was determined by thermogravimetry (TG), their bulk and grain-boundary conductivity was determined by impedance spectroscopy in dry and wet air (100–900 °C), and their total conductivity was measured as a function of oxygen partial pressure in the temperature range: 700–950 °C. The Sm2−xCaxZr2O7−x/2 (x = 0.05, 0.1) pyrochlore solid solutions, lying near the morphotropic phase boundary, have proton conductivity contribution both in the grain bulk and on grain boundaries below 600 °C, and pure oxygen–ion conductivity above 700 °C. The 500 °C proton conductivity contribution of Sm2−xCaxZr2O7−x/2 (x = 0.05, 0.1) is ~ 1 × 10−4 S/cm. The fluorite-like Gd2−xCaxZr2O7−x/2 (x = 0.1) solid solution has oxygen-ion bulk conductivity in entire temperature range studied, whereas proton transport contributes to its grain-boundary conductivity below 700 °C. As a result, of the morphotropic phase transition from pyrochlore Sm2−xCaxZr2O7−x/2 (x = 0.05, 0.1) to fluorite-like Gd2−xCaxZr2O7−x/2 (x = 0.05, 0.1), the bulk proton conductivity disappears and oxygen-ion conductivity decreases. The loss of bulk proton conductivity of Gd2−xCaxZr2O7−x/2 (x = 0.05, 0.1) can be associated with the fluorite structure formation. It is important to note that the degree of Ca substitution in such solid solutions (Ln2−xCax)Zr2O7−δ (Ln = Sm, Gd) is low, x < 0.1. In both series, grain-boundary conductivity usually exceeds bulk conductivity. The high grain-boundary proton conductivity of Ln2−xCaxZr2O7−x/2 (Ln = Sm, Gd; x = 0.1) is attributable to the formation of an intergranular CaZrO3-based cubic perovskite phase doped with Sm or Gd in Zr sublattice.

High-entropy pyrochlore-type structures based on rare-earth zirconates are successfully produced by conventional solid-state reaction method. Six rare-earth oxides (La 2 O 3 , Nd 2 O 3 , Sm 2 O 3 , Eu 2 O 3 , Gd 2 O 3 , and Y 2 O 3 ) and ZrO 2 are used as the raw powders. Five out of the six rare-earth oxides with equimolar ratio and ZrO 2 are mixed and sintered at different temperatures for investigating the reaction process. The results demonstrate that the high-entropy pyrochlores (5RE 1/5 ) 2 Zr 2 O 7 have been formed after heated at 1000°C. The (5RE 1/5 ) 2 Zr 2 O 7 are highly sintering resistant and possess excellent thermal stability. The thermal conductivities of the (5RE 1/5 ) 2 Zr 2 O 7 high-entropy ceramics are below 1 W·m –1 ·K –1 in the temperature range of 300–1200°C. The (5RE 1/5 ) 2 Zr 2 O 7 can be potential thermal barrier coating materials.

High-entropy ceramics (HECs) have quickly gained attention since 2015. To date, nearly all work has focused on five-component, equimolar compositions. This perspective article briefly reviews different families of HECs and selected properties. Following a couple of our most recent studies, we propose a step forward to expand HECs to compositionally complex ceramics (CCCs) to include medium-entropy and non-equimolar compositions. Using defective fluorite and ordered pyrochlore oxides as two primary examples, we further consider the complexities of aliovalent cations and anion vacancies as well as ordered structures with two cation sublattices. Better thermally insulating yet stiff CCCs have been found in non-equimolar compositions with optimal amounts of oxygen vacancies and in ordered pyrochlores with substantial size disorder. It is demonstrated that medium-entropy ceramics can prevail over their high-entropy counterparts. The diversifying classes of CCCs provide even more possibilities than HECs to tailor the composition, defects, disorder/order, and, consequently, various properties.

High-performance dielectrics are widely used in high-power systems, electric vehicles, and aerospace, as key materials for capacitor devices. Such application scenarios under these extreme conditions require ultra-high stability and reliability of the dielectrics. Herein, a novel pyrochlore component with high-entropy design of Bi 1.5 Zn 0.75 Mg 0.25 Nb 0.75 Ta 0.75 O 7 (BZMNT) bulk endows an excellent energy storage performance of W rec ≈ 2.72 J/cm 3 together with an ultra-high energy efficiency of 91% at a significant enhanced electric field E b of 650 kV/cm. Meanwhile, the temperature coefficient (TCC) of BZMNT (∼ −220 ppm/°C) is also found to be greatly improved compared with that of the pure Bi 1.5 ZnNb 1.5 O 7 (BZN) (∼ −300 ppm/°C), demonstrating its potential application in temperature-reliable conditions. The high-entropy design results in lattice distortion that contributes to the polarization, while the retardation effect results in a reduction of grain size to submicron scale which enhances the E b . The high-entropy design provides a new strategy for improving the high energy storage performance of ceramic materials.

While acidic oxygen evolution reaction plays a critical role in electrochemical energy conversion devices, the sluggish reaction kinetics and poor stability in acidic electrolyte challenges materials development. Unlike traditional nano-structuring approaches, this work focuses on the structural symmetry breaking to rearrange spin electron occupation and optimize spin-dependent orbital interaction to alter charge transfer between catalysts and reactants. Herein, we propose an atomic half-disordering strategy in multistage-hybridized Bi x Er 2-x Ru 2 O 7 pyrochlores to reconfigure orbital degeneracy and spin-related electron occupation. This strategy involves controlling the bonding interaction of Bi-6 s lone pair electrons, in which partial atom rearrangement makes the active sites transform into asymmetric high-spin states from symmetric low-spin states. As a result, the half-disordered Bi x Er 2-x Ru 2 O 7 pyrochlores demonstrate an overpotential of ~0.18 V at 10 mA cm −2 accompanied with excellent stability of 100 h in acidic electrolyte. Our findings not only provide a strategy for designing atom-disorder-related catalysts, but also provides a deeper understanding of the spin-related acidic oxygen evolution reaction kinetics. While water electrolysis offers a potential path for renewable hydrogen fuel, water oxidation electrocatalysts typically suffer from poor stabilities in acid. Here, authors prepare ruthenium-based pyrochlores and demonstrate promising activities and durabilities for acidic water electro-oxidation.

The production of green hydrogen in water electrolyzers is limited by the oxygen evolution reaction (OER). State-of-the-art electrocatalysts are based on Ir. Ru electrocatalysts are a suitable alternative provided their performance is improved. Here we show that low-Ru-content pyrochlores (R 2 MnRuO 7 , R = Y, Tb and Dy) display high activity and durability for the OER in acidic media. Y 2 MnRuO 7 is the most stable catalyst, displaying 1.5 V at 10 mA cm −2 for 40 h, or 5000 cycles up to 1.7 V. Computational and experimental results show that the high performance is owed to Ru sites embedded in RuMnO x surface layers. A water electrolyser with Y 2 MnRuO 7 (with only 0.2 mg Ru cm −2 ) reaches 1 A cm −2 at 1.75 V, remaining stable at 200 mA cm −2 for more than 24 h. These results encourage further investigation on Ru catalysts in which a partial replacement of Ru by inexpensive cations can enhance the OER performance. Ru-pyrochlores find their way as alternative anodes of PEM water electrolyzers, and their high performance is owing to Ru sites embedded in RuMnO x surface layers. Here, a water electrolyser with Y 2 MnRuO 7 and only 0.2 mgRu cm −2 has been tested with significant durability.

Atomic scale study has been carried out to probe the pyrochlore phase formation in local scale and compare it with the long range ordering. Lanthanum zirconate (La 2 Zr 2 O 7 ) pyrochlore was synthesized by wet chemistry and characterized by X-ray diffraction to study the long range ordering. Time Differential Perturbed γ-γ Angular Correlation (TDPAC) Spectroscopy was performed to identify the pyrochlore phase formation in local scale. Electron paramagnetic resonance and Photo-luminescence spectroscopic studies were used to identify the defects in local scale. Role of annealing and defects in nucleation of pyrochlore phase has been explored in the present study. An early identification of pyrochlore phase was successfully done by TDPAC spectroscopy.

Electronic correlation effects are manifested in quantum materials when either the on-site Coulomb repulsion is large or the electron kinetic energy is small. The former is the dominant effect in cuprate superconductors and heavy-fermion systems whereas it is the latter in twisted bilayer graphene and geometrically frustrated metals. However, the simultaneous cooperation of both effects in the same quantum material remains rare. The design aim is to produce correlated topological flat bands pinned at the Fermi level. Here, we observe a flat band at the Fermi level in a 3 d pyrochlore metal CuV 2 S 4 . Our angle-resolved photoemission spectroscopy data reveal that destructive quantum interference associated with the V pyrochlore sublattice and further renormalization to the Fermi level by electron interactions induce this flat band. Consequently, we discover transport signatures that evidence a deviation from Fermi liquid behaviour as well as an enhanced Sommerfeld coefficient. Our work illustrates the combined cooperation of local Coulomb interactions and geometric frustration in a pyrochlore lattice system to induce correlated topology by constructing and pinning correlated flat bands near the Fermi level. Observations of strong electron correlation effects have been mostly confined to compounds containing f orbital electrons. Now, the study of the 3 d pyrochlore metal CuV 2 S 4 reveals that similar effects can be induced by flat-band engineering.

In frustrated quantum magnets, long-range magnetic order fails to develop despite a large exchange coupling between the spins. In contrast to the magnons in conventional magnets, their spin excitations are poorly understood. Here, we show that the thermal Hall conductivity κxy provides a powerful probe of spin excitations in the \"quantum spin ice\" pyrochlore Tb2Ti2O7. The thermal Hall response is large, even though the material is transparent. The Hall response arises from spin excitations with specific characteristics that distinguish them from magnons. At low temperature (<1 kelvin), the thermal conductivity resembles that of a dirty metal. Using the Hall angle, we construct a phase diagram showing how the excitations are suppressed by a magnetic field.