Pyrochlore-related Structure Research Articles

Charge ordering and magnetic frustration in CsFe2F6

The structural, electronic and magnetic properties of a charge-ordered iron fluoride material CsFe2+Fe3+F6 have been explored by density functional theory calculations based on the generalized gradient approximation + U approach, which was implemented in the VASP code. The material exhibits a 3D pyrochlore-related structure which consists of corner-shared Fe2+F6 and Fe3+F6 octahedra. Our results confirm that CsFe2F6 is a Mott–Hubbard insulator, and bears a magnetically frustrated ground state in which the localized 3d electrons are antiferromagnetically coupled between the homogeneous Fe ions (Fe3+–Fe3+ along the b axis, and Fe2+–Fe2+ along the a axis), while interactions between the heterogeneous Fe ions (Fe3+–Fe2+ along the c axis) are frustrated, consistent with Goodenough–Kanamori superexchange interactions. Although the disproportionation of the total 3d charge is extremely low, explicit evidence is provided on the charge ordering by an order parameter, which is defined as the difference in minority dyz orbital (in the local coordinates) occupations between the Fe3+ and Fe2+ cations. In addition, spin ordering and the spin–orbit coupling effect play an insignificant role in the charge ordering and the preferential occupation of the dyz orbital scenario in CsFe2F6.

Design of Medium Band Gap Ag–Bi–Nb–O and Ag–Bi–Ta–O Semiconductors for Driving Direct Water Splitting with Visible Light

Two new metal oxide semiconductors belonging to the Ag-Bi-M-O (M = Nb, Ta) chemical systems have been synthesized as candidate compounds for driving overall water splitting with visible light on the basis of cosubstitution of Ag and Bi on the A-site position of known Ca2M2O7 pyrochlores. The low-valence band edge energies of typical oxide semiconductors prevents direct water splitting in compounds with band gaps below 3.0 eV, a limitation which these compounds are designed to overcome through the incorporation of low-lying Ag 4d(10) and Bi 6s(2) states into compounds of nominal composition "AgBiM2O7". It was found that the "AgBiTa2O7" pyrochlores are in fact a solid solution with an approximate range of Ag(x)Bi(5/6)Ta2O(6.25+x/2) with 0.5 < x < 1. The structure of Ag4/5Bi5/6Ta2O6.65 was determined from the refinement of time-of-flight neutron diffraction data and was found to be a cubic pyrochlore with a = 10.52268(2) Å and a volume of 1165.143(6) Å(3). The closely related compound, AgBiNb2O7, appears to have an integer stoichiometry and to adopt an orthorhombically distorted pyrochlore-related structure with a subcell of a = 7.50102(8) Å, b = 7.44739(7) Å, c = 10.5788(1) Å, and V = 590.93(2) Å(3). Density functional theory-based calculations predict this distortion should result from A-site cation ordering. Fits to UV-vis diffuse reflectance data suggest that AgBiNb2O7 and "AgBiTa2O7" are both visible-light-absorbing semiconductors with the onset of strong direct absorption at 2.72 and 2.96 eV, respectively. Electronic structure calculations for an ordered AgBiNb2O7 structure show that the band gap reduction and the elevation of the valence band primarily result from hybridized Ag d(10)-O 2p orbitals that lie at higher energy than the normal O 2p states in typical pyrochlore oxides. While the minimum energy gap is direct in the band structure, the lowest energy dipole allowed optical transitions start about 0.2 eV higher in energy than the minimum energy transition and involve different bands. This suggests that the minimum electronic band gap in these materials is slightly smaller than the onset energy for strong absorption in the optical measurements. The elevated valence band energies of the niobate and tantalate compounds are experimentally confirmed by the ability of these compounds to reduce 2 H(+) to H2 gas when illuminated after functionalization with a Pt cocatalyst.

Hydrogen evolution from pure water over a new advanced photocatalyst Sm2GaTaO7

In this work, a new oxide Sm2GaTaO7 with pyrochlore-related structure was successfully synthesized by conventional solid state reaction. RuO2 nanoparticles were loaded as cocatalyst onto Sm2GaTaO7 surface. X-ray powder diffraction (XRD) and Rietveld refinement characterization results revealed that Sm2GaTaO7 crystallized into a monoclinic system with space group C2/c. The band gap (Eg) was calculated by Kubelka–Munk formula, obtaining a value in the order of 4.1 eV. The Sm2GaTaO7 oxide showed a particle size around 2–3 μm and a specific surface area of 0.5 m2/g. The photocatalytic results revealed that Sm2GaTaO7 was able to produce hydrogen from pure water. The hydrogen evolution activity was enhanced by using 0.2 wt.% of RuO2 amount, which exceeded 2.4 times compared with the base photocatalyst, Sm2GaTaO7.

Tazzoliite: a new mineral with a pyrochlore-related structure from the Euganei Hills, Padova, Italy

AbstractTazzoliite, ideally Ba2CaSr0.5Na0.5Ti2Nb3SiO17[PO2(OH)2]0.5, is a new mineral (IMA 2011-018) from Monte delle Basse, Euganei Hills, Galzignano Terme, Padova, Italy. It occurs as lamellar pale orange crystals, which are typically a few m m thick and up to 0.4 mm long, closely associated with a diopsidic pyroxene and titanite. Tazzoliite is transparent. It has a white streak, a pearly lustre, is not fluorescent and has a hardness of 6 (Mohs' scale). The tenacity is brittle and the crystals have a perfect cleavage along {010}. The calculated density is 4.517 g cm–3. Tazzoliite is biaxial (–) with 2Vmeas of ~50º, it is not pleochroic and the average refractive index is 2.04. No twinning was observed. Electronmicroprobe analyses gave the following chemical formula: (Ba1.93Ca1.20Sr0.52Na0.25Fe0.102+)Σ4 (Nb2.88Ti2.05Ta0.07Zr0.01V0.015+)Σ5.02SiO17[(P0.13Si0.12S0.07)Σ0.32O0.66(OH)0.66][F0.09(OH)0.23]Σ0.32.Tazzoliite is orthorhombic, space group Fmmm, with unit-cell parameters a = 7.4116(3), b = 20.0632(8), c = 21.4402(8) Å, V = 3188.2(2) Å3 and Z = 8. The crystal structure, obtained from single-crystal X-ray diffraction data, was refined to R1(F2) = 0.063. It consists of a framework of Nb(Ti) octahedra and BaO7 polyhedra sharing apexes or edges, and Si tetrahedra sharing apexes with Nb(Ti) octahedra and BaO7 polyhedra. The structure, which is related to the pyrochlore structure, contains three Nb(Ti) octahedra: two are Nb dominant and one is Ti dominant. Chains of A2O8 polyhedra [A2 being occupied by Sr(Ca, Fe)] extend along [100] and are surrounded by Nb octahedra. Channels formed by six Nb(Ti) octahedra and two tetrahedra, or four A1O8(OH) polyhedra (A1 being occupied by Ba), alternate along [100]. The channels are partially occupied by [PO2(OH)2] in two possible mutually exclusive positions, alternating with fully occupied A3O7 polyhedral pairs [A3 being occupied by Ca(Na)]. The seven strongest X-ray powder diffraction lines [d in Å (I/I0) (hkl)] are: 3.66 (60) (044), 3.16 (30) (153), 3.05 (100) (204), 2.98 (25) (240), 2.84 (50) (064), 1.85 (25) (400) and 1.82 (25) (268). Raman spectra of tazzoliite were collected in the range 150–3700 cm–1 and confirm the presence of OH groups. Tazzoliite is named in honour of Vittorio Tazzoli in recognition of his contributions to the fields of mineralogy and crystallography.

RbFe2+Fe3+F6: Synthesis, structure, and characterization of a new charge-ordered magnetically frustrated pyrochlore-related mixed-metal fluoride

A new charge-ordered magnetically frustrated mixed-metal fluoride with a pyrochlore-related structure has been synthesized and characterized. The material, RbFe2F6 (RbFe2+Fe3+F6) was synthesized through mild hydrothermal conditions. The material exhibits a three-dimensional pyrochlore-related structure consisting of corner-shared Fe2+F6 and Fe3+F6 octahedra. In addition to single crystal diffraction data, neutron powder diffraction and magnetometry measurements were carried out. Magnetic data clearly reveal strong antiferromagnetic interactions (a Curie-Weiss temperature of -270 K) but sufficient frustration to prevent ordering until 16 K. No structural phase transformation is detected from the variable temperature neutron diffraction data. Infrared, UV -vis, thermogravimetric, and differential thermal analysis measurements were also performed. First-principles density functional theory (DFT) electronic structure calculations were also done. Crystal data: RbFe2F6, orthorhombic, space group Pnma (No. 62), a = 7.0177(6) {\AA}, b = 7.4499(6) {\AA}, c = 10.1765(8) {\AA}, V = 532.04(8) {\AA}3, Z = 4.

Compositionally controlled rhombohedral to cubic phase transition in CsTe 2− xW xO 6 related to pyrochlore structure

CsTe 2O 6 adopts a rhombohedrally distorted pyrochlore related structure due to the 1:3 ordering of Te 4+ and Te 6+ in the octahedral sites respectively. Phases of the type CsTe 2− x W x O 6 were found to have the cubic pyrochlore structure from x = 0.2 to 0.5. These phases all contain Te 4+ and Te 6+ (mixed with W 6+) and are disordered in octahedral sites of the pyrochlore structure. This mixed valence situation results in strong optical absorption in the visible region of the spectrum but does not produce a measurable electrical conductivity.

The crystal structure of ingersonite, Ca3Mn2+Sb45+O14, and its relationships with pyrochlore

The crystal structure of ingersonite, (Ca2.93Mn 2+ 1.06Fe 2+ 0.01)(Sb 5+ 3.95Mg0.05)F0.15O13.85, has been solved and refi ned in the space group P3121 (a = 7.282(2), c = 17.604(4) A, V = 808.4(3) A 3 , Z = 3) to R = 2.32% for 2219 Fo > 4σ(Fo) using MoKα X-ray data. The structure of ingersonite is isostructural with the synthetic weberite-3T polytype and related to the pyrochlore structure type. Both ingersonite and pyrochlore structures can be described as a sequence of pairs of polyhedral layers (named M and N), stacked along (111) and (001), respectively. In terms of the cation sites, M and N layers have general formula AB3 and A3B, respectively, where B are the octahedral cations forming the B2X6 framework of the pyrochlore structure and A are the larger, interstitial cations forming eightfold polyhedra in pyrochlore. In ingersonite, the M layers occur at z ~ 1/6, 1/2, and 5/6: The B octahedra are occupied by Sb 5+ and share corners to form a pseudohexagonal tungsten bronze (HTB) motif with the A position occupied by octahedral Mn 2+ at the center of the pseudohexagonal rings. N layers occur at z ~ 0, 1/3, and 2/3, with A = Ca and B = Sb 5+ : Isolated B octahedra share 6 edges with 6 eightfold A polyhedra, to form a continuous sheet similar to the analogous layer in pyrochlore. The stacking of successive pairs of M and N layers in ingersonite is the same as in pyrochlore. Nonetheless, a difference in the relative position between M and N layers in ingersonite and pyrochlore is observed. The crystal-chemical relationships with other pyrochlore-related minerals are outlined. Keywords: Ingersonite, crystal structure, chemical data, weberite-3T polytype, pyrochlore-related structure, X-ray diffraction data

Structural Studies of Pyrochlore-Related Spin-1/2 System Ag2Cu2O3

For Ag 2 Cu 2 O 3 , the close coupling of the crystal distortion to the magnetic ordering which takes place at 70 K has been found by X-ray and neutron diffraction analyses and magnetic measurements. The system has a pyrochlore-related structure with corner-sharing tetrahedra of Cu 2+ ions, in which the geometrical frustration is inherent. We show how the structural distortion releases the frustration, stabilizing the ordered state of Cu spins ( S =1/2). The magnetic susceptibility and specific heat of Ag 2 Cu 2 O 4 have also been measured. The results indicate that the system is nonmagnetic.

Crystal structure of a new compound Er 2MnW 0.8O 6.4 phase

A new compound of Er 2MnW 0.8O 6.4 system was synthesized by solid-state reaction method under low oxygen partial pressure. A single phase with a composition Er 2MnW 0.8O 6.4 was obtained after trying various composition ratios and its crystal structure was refined by powder X-ray diffraction data using the Rietveld analysis. The space group was P3 121 belonging to trigonal system with the lattice parameters of a = 0.725 and c = 0.936 nm. The electron diffraction data also showed this size of the unit cell. The derived structure was not a pyrochlore-related structure because no systematic stacking layers by hexagonal tungsten bronze type appeared.

Structural Analysis of a Complex Oxide Eu&lt;sub&gt;2&lt;/sub&gt;Mn&lt;sub&gt;2/3&lt;/sub&gt;Nb&lt;sub&gt;4/3&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; with a Pyrochlore-Related Structure

The complex oxide Eu2Mn2/3Nb4/3O7 structurally characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results of XRD structure analysis shown that the X-ray diffraction profile calculated with monoclinic C2/c model is in a good agreement with the observed X-ray diffraction patterns. In addition to fundamental reflection peaks, super lattice lines could be also assigned with C2/c symmetry. Therefore, it is derived that Eu2Mn2/3Nb4/3O7 phase has not a pyrochlore structure but has a zirconolite-type structure (pyrochlore-related structure) with a C2/c space group. Eu2Mn2/3Nb4/3O7 has two kinds of distorted MO6 octahedra and HTB layers, which deviates from the regular forms. The microstructure of Eu2Mn2/3Nb4/3O7 investigated by TEM also proved the results of XRD structural analysis. It was viewed that HRTEM image of a characteristic twin structure in Eu2Mn2/3Nb4/3O7 which a zirconolite-type structure has.

Structural analysis of complex oxides Ln 2MnTa 1+ xO 7+ δ (Ln = rare earth and yttrium) with pyrochlore-related structures

The crystal structures of complex oxides Ln 2MnTa 1+ x O 7+ δ (Ln=rare earth and Y) with a pyrochlore-related structure were refined by Rietveld method using powder X-ray diffraction data. These compounds include extra small peaks of superlattice lines and Ln 2MnTa 1+ x O 7+ δ phase has a zirkelite-type structure (pyrochlore-related structure) with P3 121 space group. Ln 2MnTa 1+ x O 7+ δ has two kinds of distorted (Mn, Ta)O 6 octahedra and hexagonal tungsten bronze (HTB) layers also deform from the regular hexagon. Ln atom is coordinated by eight or seven oxygen atoms, forming polyhedra of DyO 8 or DyO 7.

Structure of Ln2MnxM2-xO7(Ln = rare earth or Y, M = Mo, Ta, Nb or W) Studied by Transmission Electron Microscopy

Ln2MnxM2-xO7 (Ln=rare earth or Y, M=Mo, Ta, Nb or W) was synthesized under ordinary pressure and various atmospheres with the expectation of achieving various symmetries and properties. These compounds are analyzed as similar structures of rhombohedrally distorted pyrochlore structures by X-ray diffraction (XRD). However, transmission electron microscope (TEM) images reveal differences between structures: Ho2Mn2/3Nb4/3O7 and Er2Mn2/3Mo4/3O7 possessed a monoclinic structure, while Ho2Mn2/3Ta4/3O7 and Ho2MnTaO7 possessed a trigonal structure with an incommesurate nature. The differences in the structures were caused by the stacking sequence along the c-direction of a pair of A cation and B cation layers, where B cations formed a hexagonal tungsten bronze (HTB)-type layer that is a characteristic frame-layer in the pyrochlore-related structure. In the case of Er2MnWO7, however, different types of structures with a trigonal cell show that no pair of A-cation and B-cation layers existed along the c-direction in the crystal.

Study of the pyrochlore-related structure of α-Cs 2U 4O 12 by powder neutron and X-ray diffraction

The crystal structure of the low temperature α variety of the cesium uranate Cs 2U 4O 12 has been determined by a simultaneous refinement of X-ray and neutron diffraction data obtained on a powder sample. The structure is rhombohedral (space group R 3 and not R3 m as previously determined for the cation framework only) with hexagonal cell parameters a=15.4232(1) Å and c=19.1816(2) Å. Cs 2U 4O 12 contains a three-dimensional pyrochlore-like framework of UO 6 octahedra and UO 7 distorted pentagonal bipyramids sharing corners or edges. Bond valence calculations show that, of four U sites, two are only occupied by U 5+, one by U 6+ and the fourth by a mixture of U 6+ and U 5+. This new structure type derives from the so-called “modified pyrochlore” RbNiCrF 6 type (or CsNbWO 6) by the transformation of some corner-sharing octahedra to edge-sharing pentagonal bipyramids.

Crystalline Host Phases for Actinides, Obtained by Self-Propagating High-Temperature Synthesis

Host matrices for actinides prepared by self-propagating high-temperature synthesis are studied. The matrices consist of a pyrochlore or fluorite phase and metallic molybdenum. The factor determining the structural type of the crystal lattice of the target phase is the ionic radius ratio. When the difference in the ionic radii is insignificant, as in the case of Y3+ (r 0.102 nm) and Zr4+ (r 0.084 nm), the oxide Zr1-xYxO2-0.5x with a fluorite structure is formed, in which the cations occupy the eight-coordinate sites. This structure permits incorporation of heavy lanthanides and tetravalent actinides: U4+ (r 0.10 nm), Np4+ (r 0.098 nm), and Pu4+ (r 0.096 nm). When the difference in the ionic radii is more considerable, as in the case of Y3+ and Ti4+ (r 0.061 nm), a pyrochlore-related structure is realized. In this case the cations occupy different (eight- or six-coordinate) sites. The pyrochlore structure is preserved if the radii of ions occupying different structural sites change in parallel. This structure is typical of zirconates of trivalent actinides and light REEs. The decision on the major host phase for actinides is determined by the waste composition. At low content of light REEs and americium an oxide with a fluorite-related structure shows promise. At high content of these elements, zirconates and titanates with the pyrochlore structure are more stable.

Microstructure of an oxide Ho 2Mn 2/3Ta 4/3O 7 with a pyrochlore-related structure

A complex oxide Ho 2Mn 2/3Ta 4/3O 7 was synthesized in an Ar atmosphere containing 1% H 2 at 1523 K. The compound was structurally characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Its structure consisted of a trigonal basic cell and a super cell with an incommensurate nature. The incommensurate nature of the superstructure was confirmed to have orientation anomalies appearing along the c∗- and b∗-directions in the super-cell reflections. Wavy moiré-like images were observed in the dark-field images from reflections exhibiting the orientation anomalies.

Microstructural investigation of a new complex oxide Ho 2Mn 2/3Nb 4/3O 7 with a pyrochlore-related structure

A complex oxide Ho 2Mn 2/3Nb 4/3O 7 with a pyrochlore-related structure was synthesized from Ho 2O 3, MnO, and Nb 2O 5 in Ar containing 1% H 2 atmosphere. Its structure was investigated by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The oxide, which was determined to be Ho 2.0Mn 0.6Nb 1.4O 7.1by an electron probe microanalyzer (EPMA), had a monoclinic symmetry (space group C2/ c) with lattice parameters a = 1.2813(3), b = 0.7397(1), c = 1.1625(2) nm, and β = 100.575(8)°. The oxide had a large number of lamellar domains bounded by planar defects on (00 l), and such domains were composed of twinned structures on the twin axis <1̄1̄0>.

Synthesis and characterisation of the new cubic phase CsVTeO5 and re-investigation of the pyrochlore CsVTeO6

The compound CsVTeO5 has been synthesized by solid state reactions and studied by X-ray powder diffraction. CsVTeO5 is polymorphic with a cubic high temperature variety, space group Fd3m and a=10.482(2). The observed X-ray powder pattern of this cubic phase is similar to the AB2O6 pyrochlore-type structure and also to those reported for ATiXO5 phases (A=Rb, Cs, Tl and X=P, As). These phases correspond to the high temperature cubic polymorph of KTiOPO4 (KTP). The framework of these cubic phases consists of randomly disordered TiO6 octahedra and XO4 (X=P, As) tetrahedra with the A+ cation occupying large cavities within this framework. The cubic phase 6' [a=10.483(1)], previously reported as a pyrochlore-type structure, corresponds in fact to CsVTeO5. The cubic CsVTeO6 phase, with a pyrochlore related structure, has also been synthesized and the structure was refined by Rietveld profiling of the X-ray data. The space group is Fd3m with a=10.077(1). The value of the unit cell parameter is in good agreement with those reported for the ABTeO6 pyrochlore series (A=K, Rb, Cs and B=Nb, Ta). The thermal stabilities of CsVTeO5 and CsVTeO6 under argon and oxygen atmospheres are discussed.

Structure, Magnetism and Colossal Magnetoresistance Behavior in A2Mn2O7 Pyrochlores (A = Dy-Lu, Y, Sc, In or Tl)

This paper presents a bird's eye view of the structural, electrical and magnetic properties of A 2 Mn 2 O 7 (A = rare earth, Y, Sc, In, Tl) phases crystallizing in pyrochlore-related structure. Electrical measurements indicate the compounds containing rare earth, Y, Sc or In are insulators whereas Tl 2 Mn 2 O 7 is highly conducting. All the compounds show spontaneous magnetization at low temperatures indicating ferromagnetic transitions. Detailed ac and dc susceptibility as well as neutron diffraction studies do not provide any evidence for long-range order in A 2 Mn 2 O 7 phases where A is a rare earth, Y or Sc, but suggest a spin-glass like behavior. However, powder neutron diffraction and small-angle neutron scattering investigations substantiate the long-range magnetic ordering in Tl 2 Mn 2 O 7 , which is also now known to exhibit colossal magnetoresistance (CMR) behavior. Structural analysis of this phase shows no deviations from ideal stoichiometry and gives an Mn-O distance of ∼1.90 A, significantly shorter than the Mn-O distances in the range 1.94 to 2.00 A observed in phases based on LaMnO 3 perovskites which show CMR. The above observation indicates that oxidation state of Mn in TI 2 Mn 2 O 7 should be very close to 4+ and has neither mixed valency for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn 3+ . Both were thought to play an essential role in CMR materials. We propose that CMR in Tl 2 Mn 2 O 7 is based on magnetic ordering driven by superexchange and strong spin fluctuation scattering above T c .

Stoichiometry, Defects, and Ordering in Lead Tantalum Oxides with Pyrochlore-Related Structures

The phase diagram in the PbO–Ta2O5system has been reinvestigated in the Pb1.5Ta2O6.5–Pb2.44Ta2O7.44composition range. The existence of the defect-pyrochlore cubic phase Pb1.5Ta2O6.5is confirmed but the apparent solid solution previously reported in the Pb2.31Ta2O7.31–Pb2.44Ta2O7.44composition range is shown to be due to the coherent intergrowth of defined rhombohedral phases with slightly different compositions.

Synthesis and phase stability of Ln 2MnTa 1 + xO 7 + δ (Ln = rare earth and yttrium) compounds

The complex oxides Ln 2MnTa 1 + x O 7 + δ containing pentavalent tantalum with a pyrochlore-related structure have been obtained at 1523 K in air for Ln = Eu to Er, Yb and Y as a single phase with the Ta/Mn composition ratio being 1 + x (0.16 ≤ x ≤ 0.35). The excess amount of tantalum ( x) varies for Ln 2MnTa 1 + x O 7 + δ phases with different rare earth elements and was found to increase with increasing atomic weight of the rare earth elements. The results of the investigation of phase stability of these phases under various oxygen partial pressures indicated that the Ln 2MnTa 1 + x O 7 + δ phases with Ln = Gd to Er, Yb and Y are stable over the range p(O 2) = 1 − 10 −21 atm, but Eu 2MnTa 1 + x O 7 + δ decomposes when p(O 2) is below 10 −19 atm. The oxidation states of the Ln, Mn and Ta ions in Ln 2MnTa 1 + x O 7 + δ were established by measurements of the magnetic susceptibility.