Abstract: The family of magnetic rare-earth pyrochlore oxides R2M2O7 plays host to a diverse array of exotic phenomena, driven by the interplay between geometrical frustration and spin-orbit interaction, which leads to anisotropy in both magnetic moments and their interactions. In this article we establish a general, symmetry-based theory of pyrochlore magnets with anisotropic exchange interactions. Starting from a very general model of nearest-neighbor exchange between Kramers ions, we find four distinct classical ordered states, all with q=0, competing with a variety of spin liquids and unconventional forms of magnetic order. The finite-temperature phase diagram of this model is determined by Monte Carlo simulation, supported by classical spin-wave calculations. We pay particular attention to the region of parameter space relevant to the widely studied materials Er2Ti2O7, Yb2Ti2O7, and Er2Sn2O7. We find that many of the most interesting properties of these materials can be traced back to the “accidental” degeneracies where phases with different symmetries meet. These include the ordered ground-state selection by fluctuations in Er2Ti2O7, the dimensional reduction observed in Yb2Ti2O7, and the lack of reported magnetic order in Er2Sn2O7. We also discuss the application of this theory to other pyrochlore oxides.

Abstract: Combining density functional theory (DFT) and embedded dynamical mean-field theory (DMFT) methods, we study the metal-insulator transition in R_{2}Ir_{2}O_{7} (R=Y, Eu, Sm, Nd, Pr, and Bi) and the topological nature of the insulating compounds. Accurate free energies evaluated using the charge self-consistent DFT+DMFT method reveal that the metal-insulator transition occurs for an A-cation radius between that of Nd and Pr, in agreement with experiments. The all-in-all-out magnetic phase, which is stable in the Nd compound but not the Pr one, gives rise to a small Ir^{4+} magnetic moment of ≈0.4 μ_{B} and opens a sizable correlated gap. We demonstrate that within this state-of-the-art theoretical method, the insulating bulk pyrochlore iridates are topologically trivial.

Abstract: Zn–air batteries suffer from the slow kinetics of oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER). Thus, the bifunctional electrocatalysts are required for the practical application of rechargeable Zn–air batteries. In terms of the catalytic activity and structural stability, pyrochlore oxides (A2[B2–xAx]O7–y) have emerged as promising candidates. However, a limited use of A-site cations (e.g., lead or bismuth cations) of reported pyrochlore catalysts have hampered broad understanding of their catalytic effect and structure. More seriously, the catalytic origin of the pyrochlore structure was not clearly revealed yet. Here, we report the new nanocrystalline yttrium ruthenate (Y2[Ru2–xYx]O7–y) with pyrochlore structure. The prepared pyrochlore oxide demonstrates comparable catalytic activities in both ORR and OER, compared to that of previously reported metal oxide-based catalysts such as perovskite oxides. Notably, we first find that the catalytic activity of the Y2[Ru2–xYx]O7–y is ...

Abstract: 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 dif...

Abstract: A novel CeO2–xSnO2/Ce2Sn2O7 pyrochlore stoichiometric redox cycle with superior H2 production capacities is identified and corroborated for two-step solar thermochemical water splitting (STWS). During the first thermal reduction step (1400°C), a reaction between CeO2 and SnO2 occurred for all the CeO2–xSnO2 (x = 0.05–0.20) solid compounds, forming thermodynamically stable Ce2Sn2O7 pyrochlore rather than metastable CeO2-δ. Consequently, substantially higher reduction extents were achieved owing to the reduction of CeIV to CeIII. Moreover, in the subsequent reoxidation with H2O (800°C), H2 production capacities increased by a factor of 3.8 as compared to the current benchmark material ceria when x = 0.15, with the regeneration of CeO2 and SnO2 and the concomitant reoxidation of CeIII to CeIV. The H2O-splitting performance for CeO2–0.15SnO2 was reproducible over seven consecutive redox cycles, indicating the material was also robust. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3450–3462, 2017

Abstract: Glass-Ln 2 Ti 2 O 7 pyrochlore (Ln = Gd, Tb, Er, Yb) was fabricated by sintering the pelletized mixture of glass precursor and Ln-Ti composite at 1200 °C. The phase pure pyrochlore was in - situ crystallized in the amorphous glass matrix. The Ln-Ti composite was prepared by a simple soft chemistry route in an aqueous solution to ensure the homogeneity of the product. Thermal analysis, Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, and scanning electron microscopy techniques were employed to investigate the glass-Ln 2 Ti 2 O 7 pyrochlore structure formation. The cell parameters for the pyrochlore structures by Le Bail fitting are in good agreement with the published data. The presence of the melting glass matrix facilitates the pyrochlore formation, with particle sizes in the range of 1–3 μm. This new aqueous synthetic method provides a simple pathway to produce glass-Ln 2 Ti 2 O 7 pyrochlore without using organic solvent and/or milling procedures, making it an attractive potential method for scale-up production.

Abstract: The electronic structure of ternary tin oxide with pyrochlore structure, Sn2Nb2O7, which has the contribution of the Sn 5s orbitals in the valence band, is examined by synchrotron-radiation-excited photoelectron spectroscopy and X-ray absorption spectroscopy together with ab initio calculations. The empirical spectra are qualitatively consistent with the calculated density of states with the exception of a striking discrepancy in the electronic structure around the valence band maximum (VBM). The band calculation suggests the presence of a sharp peak around the VBM, mainly due to dispersionless bands that are derived from hybridization of Sn 5s orbitals and O 2p orbitals close to the Sn atom. However, the photoelectron spectra show no such characteristic spectral feature. From the theoretical prediction about the orbital origin of the localized VBM and the experimental estimation of the elemental composition by chemical analysis, it seems reasonable to conclude that the discrepancy of the electronic struc...

Abstract: The excellent ion conductivities of pyrochlore-type materials are believed to be based on oxygen anion transportations caused by the intrinsic defects, in which the anion Frenkel (a-Fr) pair (VO+IO) defect is the most stable one that lacks detailed study. The partially disordered pyrochlore with formation of the a-Fr pair defect will result in more disorder in local pyrochlore structure and increase number of possible migration paths for oxygen anions, which could further improve the ion conductivities of materials. Hence, we studied the formation of a-Fr defect pairs in La2Hf2O7 as a representative pyrochlore structure by density functional theory (DFT) calculations. Three types of defect migration sites were discovered with the ability to incorporate interstitial oxygen atoms from 48f sites and form a-Fr defect pairs (IO+VO (48f)). Besides the most stable vacant 8a sites with lowest defect formation energy of 3.49 eV/pair, two other novel migration sites have been first reported with ability to form a-F...

Abstract: Radiation damage in minerals is caused by the a-decay of incorporated radionuclides, such as U and Th and their decay products. The effect of thermal annealing (400-1000 K) on radiation-damaged pyro-chlores has been investigated by Raman scattering, X-ray powder diffraction (XRD), and combined differential scanning calorimetry/thermogravimetry (DSC/TG). The analysis of three natural radiation-damaged pyrochlore samples from Miass/Russia [6.4 wt% Th, 23.1 10(18) alpha-decay events per gram (dpg)], Panda Hill/Tanzania (1.6 wt% Th, 1.6 10(18) dpg), and Blue River/Canada (10.5 wt% U, 115.4 10(18) dpg), are compared with a crystalline refer-ence pyrochlore from Schelingen (Germany). The type of structural recovery depends on the initial degree of radiation damage (Panda Hill 28 %, Blue River 85 % and Miass 100 % according to XRD), as the recrystallization tem-perature increases with increasing degree of amorphization. Raman spectra indicate reordering on the local scale during annealing-induced recrystallization. As Raman modes around 800 cm(-1) are sensitive to radiation damageM. T. Vandenborre, E. Husson, Comparison of the force field in various pyrochlore families. I. The A(2)B O-2 (7) oxides. J. Solid State Chem. 1983, 50, 362, S. Moll, G. Sattonnay, L. Thome, J. Jagielski, C. Decorse, P. Simon, I. Monnet, W. J. Weber, Irradiation damage in Gd-2 Ti-2 O-7 single crystals: Ballistic versus ionization processes. Phys. Rev. 2011, 84, 64115.), the degree of local order was deduced from the ratio of the integrated intensities of the sum of the Raman bands between 605 and 680 cm (1) divided by the sum of the integrated intensities of the bands between 810 and 860 cm(-1). The most radiation damaged pyrochlore (Miass) shows an abrupt recovery of both, its short-(Raman) and long-range order (X-ray) between 800 and 850 K, while the weakly damaged pyrochlore (Panda Hill) begins to recover at considerably lower temperatures (near 500 K), extending over a temperature range of ca. 300 K, up to 800 K (Raman). The pyrochlore from Blue River shows in its initial state an amorphous X-ray diffraction pattern superimposed by weak Bragg-maxima that indicates the existence of ordered regions in a damaged matrix. In con-trast to the other studied pyrochlores, Raman spectra of the Blue River sample show the appearance of local modes above 560 K between 700 and 800 cm(-1) resulting from its high content of U and Ta impurities. DSC measurements confirmed the observed structural recovery upon anneal-ing. While the annealing-induced ordering of Panda Hill begins at a lower temperature (ca. 500 K) the recovery of the highly-damaged pyrochlore from Miass occurs at 800 K. The Blue-River pyrochlore shows a multi-step recovery which is similarly seen by XRD. Thermogravi-metry showed a continuous mass loss on heating for all radiation-damaged pyrochlores (Panda Hill ca. 1 %, Blue River ca. 1.5 %, Miass ca. 2.9 %).

Abstract: Temporary storage of oxygen in a solid catalyst is imperative for many important industrial oxidation reactions in the gas phase, for instance the post-treatment of automotive exhaust gas. A peculiar mixed Ce–Zr (1:1) oxide, the ordered κ-Ce2Zr2O8 phase, is a promising catalytic material exhibiting an extraordinarily high oxygen storage capacity (OSC) and high thermal and chemical stability. We elucidate the temperature-dependent transformation between the pyrochlore pyr-Ce2Zr2O7 and κ-Ce2Zr2O8 phase upon oxygen uptake by in situ X-ray diffraction, X-ray absorption, and in situ Raman spectroscopy, providing insights into the electronic and structural changes on the atomic level, which are at the heart of the extraordinarily high OSC. We demonstrate that the Ce3+ concentration can be followed during transformation in situ by Raman spectroscopy of the electronic spin flip in the f-shell of Ce3+. The catalytic activity of the κ-Ce2Zr2O8 phase was investigated without an additional active component such as no...

Abstract: The substituted pyrochlore La 1.9 Ba 0.1 Sn 2 O 7 was prepared by nitrate route and the physical and electrochemical characterizations were investigated. The optical gap is found to be 3.01 eV and the conductivity is characteristic of a semiconducting behavior with activation energy of 0.42 eV. The conduction band of La 1.9 Ba 0.1 Sn 2 O 7 (−2.64 V SCE ), predicted from electro negativities of the constituent atoms, is close to that determined experimentally (−2.99 V SCE ). The capacitance measurement (C −2 –V) exhibits a linear behavior, characteristic of p type conductivity, from which a flat band potential of −0.05 V SCE and hole density N D of 8.42 × 10 14 cm −3 were determined, in agreement with the small electrical conductivity (σ 300K ∼3 × 10 −6 Ω −1 cm −1 ). The Nyquist plot showed a semicircle which is a contribution of capacitance and resistance in parallel. A bulk resistance contribution (R b = 71 kΩ cm 2 ) and a low density of surface states are observed. The center of the arc is localized below the real axis with a depletion angle of 14° ascribed to a constant phase element (CPE). As application, chromate was successfully converted to Cr 3+ on the pyrochlore under sunlight. ∼1 h was necessary to reach the adsorption equilibrium for an initial chromate concentration of 10 −4 M. The conduction band is more cathodic than the potential of HCrO 4 − which is spontaneously reduced under solar light. The best performance is obtained at pH ∼ 3 with a catalyst dose of 1 mg/mL. 40% of chromate disappears with a quantum yield of 0.53% in less than 2 h. The process obeys to a first order kinetic with an apparent rate constant of 2.3 × 10 −3 min −1 .

Abstract: The superconducting pyrochlore oxide Cd2Re2O7 is revisited with a particular emphasis on the sample-quality issue. The compound has drawn attention as the only superconductor (Tc = 1.0 K) that has been found in the family of {\alpha}-pyrochlore oxides since its discovery in 2001. Moreover, it exhibits two characteristic structural transitions from the cubic pyrochlore structure, with the inversion symmetry broken at the first one at 200 K. Recently, it has attracted increasing attention as a candidate spin-orbit coupled metal (SOCM), in which specific Fermi liquid instability is expected to lead to an odd-parity order with spontaneous inversion-symmetry breaking [L. Fu, Phys. Rev. Lett. 115, 026401 (2015)] and parity-mixing superconductivity [V. Kozii and L. Fu: Phys. Rev. Lett. 115 (2015) 207002; Y. Wang et al., Phys. Rev. B 93 (2016) 134512]. We review our previous experimental results in comparison with those of other groups in the light of the theoretical prediction of the SOCM, which we consider meaningful and helpful for future progress in understanding this unique compound.

Abstract: This paper reports on new study of the niobium mineralization of the Chuktukon carbonatites and weathering crust. The Chuktukon massif is located within the Chadobets upland in the southwestern part of the Siberian Platform. Primary Nb-minerals of carbonatites are pyrochlore group minerals (mainly fluorcalciopyrochlore), newly discovered mineral - rippite (K 2 (Nb,Ti) 2 (Si 4 O 12 )O(O,F)), and Nb-rutile. Fluorcalciopyrochlore has oscillatory zonation with variations in content of Nb 2 O 5 (59-67 wt.%), TiO 2 (2.8-5 wt.%), SiO 2 (up to 3.3 wt.%), CaO (15.9-17.7 wt.%), Na 2 O (7-8.2 wt.%) and F (4.5-5.13 wt.%). Rippite is Nb-rich silicate (~44 wt.% of Nb 2 O 5 ) and contains inclusions of primary fluorcalciopyrochlore. Nb-rutile form oscillatory and sectorial hourglass-like zoned crystals with variable content of Nb 2 O 5 (9-14.7 wt.%). The carbonatites underwent strong hydrothermal alteration and subsequent weathering, and, as a result, in the hydrothermally altered carbonatites and weathering crust the primary Nb-minerals become unstable, dissolve and decompose, and Nb-bearing goethite (up to 4.3 wt.% of Nb 2 O 5 ) formed. Primary fluorcalciopyrochlore is replaced by the secondary Sr-, Pb-, LREE- and Ba-rich pyrochlore group minerals.

Abstract: Lanthanide stannate pyrochlores (Ln2Sn2O7; Ln = Nd, Gd, and Er) were investigated in situ to 50 GPa in order to determine their structural response to compression and compare their response to that of lanthanide titanate, zirconate, and hafnate pyrochlores. The cation radius ratio of A3+/B4+ in pyrochlore oxides (A2B2O7) is thought to be the dominant feature that influences their response on compression. The ionic radius of Sn4+ is intermediate to that of Ti4+, Zr4+, and Hf4+, but the 〈Sn-O〉 bond in stannate pyrochlore is more covalent than the 〈B-O〉 bonds in titanates, zirconate, and hafnates. In stannates, based on in situ Raman spectroscopy, pyrochlore cation and anion sublattices begin to disorder with the onset of compression, first measured at 0.3 GPa. The extent of sublattice disorder versus pressure is greater in stannates with a smaller Ln3+ cation. Stannate pyrochlores (Fd-3m) begin a sluggish transformation to an orthorhombic, cotunnite-like structure at ~28 GPa; similar transitions have been observed in titanate, zirconate, and hafnate pyrochlores at varying pressures (18-40 GPa) with cation radius ratio. The extent of the phase transition versus pressure varies directly with the size of the Ln3+ cation. Post-decompression from ~50 GPa, Er2Sn2O7 and Gd2Sn2O7 adopt a pyrochlore structure, rather than the multi-scale defect-fluorite + weberite-type structure adopted by Nd2Sn2O7 that is characteristic of titanate, zirconate, and hafnate pyrochlores under similar conditions. Like pyrochlore titanates, zirconates, and hafnates, the bulk modulus, B 0, of stannates varies linearly and inversely with cation radius ratio from 1 1 1 GPa (Nd2Sn2O7) to 251 GPa (Er2Sn2O7). The trends of bulk moduli in stannates in this study are in excellent agreement with previous experimental studies on stannates and suggest that the size of the Ln3+ cation is the primary determining factor of B 0. Additionally, when normalized to r A/r B, the bulk moduli of stannates are comparable to those of zirconates and hafnates, which vary from titanates. Our results suggest that the cation radius ratio strongly influences the bulk moduli of stannates, as well as their overall compression response.

Abstract: R2Sn2O7 (R = Pr–Lu) rare-earth stannates with the pyrochlore structure have been synthesized by solid-state reactions, by firing stoichiometric mixtures of SnO2 and R2O3 in air at 1473 K. The high-temperature heat capacity of the rare-earth stannates has been determined by differential scanning calorimetry in the temperature range 350 to 1000 K, and the Raman spectra of polycrystalline Tb2Sn2O7 and Dy2Sn2O7 samples have been measured.

Abstract: The formation of the spin-ice pyrochlore Ho2Ge2O7 by two different high temperature, high pressure routes has been explored using in situ neutron diffraction. The first route involves the solid-state reaction of Ho2O3 and GeO2, and formation of the pyrochlore phase is observed at 994(27) °C and 3.81(2) GPa, which are significantly milder conditions than those previously reported. The second route involves the hydrothermal synthesis of the tetragonal Ho2Ge2O7 pyrogermanate from Ho(NO3)3·5H2O and GeO2 and its subsequent transformation to the pyrochlore phase, which is observed at 683(23) °C and 3.89(3) GPa. The lowering of the formation temperature of high pressure phases by employment of a precursor of appropriate stoichiometry may also have applications in the wider field of solid-state chemistry.