Fluorite-related Structure Research Articles

Refined structural studies on the fluorite-related polymorphs of sol-gel undoped and Eu3+-doped yttrium tantalates.

Compounds with the general formula RE3MO7 (RE = rare earth ions; M = Ta, Nb, Sb, Ru, Ir, Os, Re, etc.), crystallize as a fluorite-related structure, forming polymorphs with different space groups. The space group strongly depends on the RE3+ and M5+ ionic radii and processing conditions. Structural characterization is well-established for the lanthanide series, but literature studies have divergent views about how to attribute yttrium tantalate (Y3TaO7) space groups-some authors have described the Y3TaO7 structure as orthorhombic and belonging to space group C2221 or Ccmm, whereas others have assigned a cubic Fm3̄m structure to it. Here, we have characterized the structure of undoped and Eu3+-doped Y3TaO7 (0.1 to 50 mol% of Eu3+) samples synthesized by the sol-gel method that crystallized as a cubic disordered fluorite-type structure, space group Fm3̄m. Their powder X-ray diffraction measurements, Rietveld analyzes and Raman spectra were used as a conclusive technique of the structural properties. We have also investigated whether a secondary phase (M'-YTaO4) emerged in the samples and compared the phase composition of each sample to their Raman spectra. Low-temperature photoluminescence measurements (∼15 K) using Eu3+ as a structural probe helped us analyze the inhomogeneous broadening observed in the emission spectra. These measurements can be used as an important tool to attribute the crystalline phases of rare earth tantalates and niobates.

Extremely Shallow Valence Band in Lanthanum Trihydride.

Hydride ions (H-) in solvents are chemically active anions with strong electron-donating ability and are used as reducing agents in organic chemistry. Here, we evaluate the energy level of 1s-electrons in H- accommodated in solid lanthanum hydrides, LaHx (2 ≤ x ≤ 3), by photoemission (ultraviolet photoelectron and photoelectron yield spectroscopies) measurements and density functional theory calculations. We show that a very shallow valance band maximum with an ionization potential of 3.8 eV is attained in LaH3 and that the primary cause is attributed to the small electronegativity of hydrogen and the significant bonding-antibonding interaction between neighboring H-s with a close separation originating from the H-stuffed fluorite-related structure. These results encourage the challenge for p-type conduction in hydride semiconductors and provide a clue to the chemical understanding of polyhydride superconductors.

Structural, magnetic and magnetocaloric properties in the rare earth ruthenate RE3RuO7 (RE = Pr, Nd, Gd and Tb) oxides with fluorite related structure

The magnetic refrigeration (MR) based on magnetocaloric effect (MCE) is a key technique, since it possesses ideally higher performance than the traditional gas compression technology. However, the lack of MR materials with excellent MCE performance is a bottle-neck for practical MR applications. Herein, we systematically report the structural properties using powder X-ray diffraction and scanning electron microscope as well as the magnetic and magnetocaloric properties through the magnetization measurements of four rare-earth (RE) ruthenate RE3RuO7 (RE = Pr, Nd, Gd and Tb) compounds. Light RE-based Pr3RuO7 and Nd3RuO7 compounds form an orthorhombic Cmcm structure, while heavy RE based Gd3RuO7 and Tb3RuO7 compounds crystallize in an orthorhombic P21nb structure. The magnetic phase transition of all the present compounds are confirmed to be of the first-order type with TN of 52, 15, 11 and 12 K for Pr3RuO7, Nd3RuO7, Gd3RuO7 and Tb3RuO7 compounds, respectively. The related MCE parameters including the maximum isothermal magnetic entropy change, temperature-averaged entropy change (5 K lift), and refrigerant capacity with the magnetic field change of 0–5 T were computed to be 1.35 J/kgK, 1.31 J/kgK, and 27.4 J/kg for Pr3RuO7, to be 2.18 J/kgK, 2.04 J/kgK, and 25.6 J/kg for Nd3RuO7, to be 8.12 J/kgK, 7.90 J/kgK, and 124.6 J/kg for Gd3RuO7, and to be 7.78 J/kgK, 7.49 J/kgK, and 128.8 J/kg for Tb3RuO7, respectively.

Reversible Charge/Discharge Reaction of a Ternary Metal Fluoride, Pb2CuF6: A Highly Conductive Cathode Material for Fluoride-Ion Batteries

Fluoride-ion all-solid-state batteries with high energy densities are promising replacements for lithium-ion batteries. Herein, we report Pb2CuF6, which has a fluorite-related structure, as a cathode material for use in fluoride-ion batteries. The Pb2CuF6 cathode composite reacted to charging and discharging without the addition of a solid electrolyte. It exhibited a capacity of 319 mA h cm–3 per electrode volume during the first discharge, exceeding that of the conventional CuF2 electrode. The discharge capacity of Pb2CuF6 remained at 24% of the theoretical capacity, even at 1 mA cm–2 (corresponding to a rate of 0.77C), and the Cu utilization ratio was approximately four times higher than that of a CuF2 cathode. β-PbF2, a fluoride-ion conductor, and metallic Cu were formed during the discharging process of Pb2CuF6; during the subsequent charging process, Pb2CuF6 was formed again. All products of the charge–discharge reaction were present in a nanoparticle form. Pb2CuF6 exhibited an ionic conductivity of 3.0 × 10–5 S cm–1 at 140 °C. The high-rate performance of Pb2CuF6 is attributed to both the high ionic conductivity of the reaction products and the formation of nanoparticles. These findings are beneficial for the material design to improve the charge–discharge performance of electrode materials for fluoride-ion batteries.

Microstructural origin of peculiar spectra and excellent luminescence properties of Y10Ta4O25:Eu3+ with a fluorite-related structure

Y10Ta4O25:Eu3+ presents peculiar spectra and excellent red luminescence properties due to its special fluorite-related structure. The microstructure was characterized by the site-selective luminescence of Eu3+ activators.

Charge Localization and Magnetic Correlations in the Refined Structure of U3O7.

Atomic arrangements in the mixed-valence oxide U3O7 are refined from high-resolution neutron scattering data. The crystallographic model describes a long-range structural order in a U60O140 primitive cell (space group P42/n) containing distorted cuboctahedral oxygen clusters. By combining experimental data and electronic structure calculations accounting for spin-orbit interactions, we provide robust evidence of an interplay between charge localization and the magnetic moments carried by the uranium atoms. The calculations predict U3O7 to be a semiconducting solid with a band gap of close to 0.32 eV, and a more pronounced charge-transfer insulator behavior as compared to the well-known Mott insulator UO2. Most uranium ions (56 out of 60) occur in 9-fold and 10-fold coordinated environments, surrounding the oxygen clusters, and have a tetravalent (24 out of 60) or pentavalent (32 out of 60) state. The remaining uranium ions (4 out of 60) are not contiguous to the oxygen cuboctahedra and have a very compact, 8-fold coordinated environment with two short (2 × 1.93(3) Å) "oxo-type" bonds. The higher Hirshfeld charge and the diamagnetic character point to a hexavalent state for these four uranium ions. Hence, the valence state distribution corresponds to 24/60 × U(IV) + 32/60 U(V) + 4/60 U(VI). The tetravalent and pentavalent uranium ions are predicted to carry noncollinear magnetic moments (with amplitudes of 1.6 and 0.8 μB, respectively), resulting in canted ferromagnetic order in characteristic layers within the overall fluorite-related structure.

Effect of oxygen vacancies on direct N2O decomposition over ZrO2-Y2O3 catalysts

ABSTRACTThe effect of oxygen vacancies on catalytic activity for direct N2O decomposition was demonstrated in Zr1−xYxO2−δ solid solutions with a fluorite-related structure. The fluorite-type structure was formed as samples with a small amount of Y3+ (0.05 ≤ x ≤ 0.19) and a large amount of Y3+ (x ≥ 0.80), leading to the generation of a C-type structure derived from the fluorite-type structure by removing a quarter of the oxide anions. The oxygen vacancies worked as N2O adsorption sites and catalytic active sites, with the highest activity realized by the YO1.5 catalyst with a C-type structure, which contained the largest number of oxygen vacancies in the ZrO2-Y2O3 system.

Magnetic Studies on Eu3MO7 (M = Nb, Ta, Ir) with Fluorite-related Structure by 151Eu Mössbauer Spectroscopy and Magnetic Susceptibility Measurements

Magnetic properties of europium-containing compounds Eu3MO7 (M = Nb, Ta, Ir) with fluorite-related structure have been investigated. Magnetic susceptibility measurements show that all these compounds are paramagnetic and have no magnetic ordering down to 1.8 K. At very low temperatures (T<50 K), the susceptibilities of each compound attain constant values, which is characteristic of the ground state for Eu3+ ions. The results of 151Eu Mössbauer spectroscopic measurements show that the asymmetric parameter for Eu(2) in seven-coordination is much larger than that for Eu(1) in cubic environment, which in accordance with the crystallographic result.

Studies on phase transition temperature of rare earth niobates Ln3NbO7 (Ln = Pr, Sm, Eu) with orthorhombic fluorite-related structure

The phase transition of ternary rare earth niobates Ln3NbO7 (Ln = Pr, Sm, Eu) was investigated by the measurements of high-temperature and low-temperature X-ray diffraction, differential scanning calorimetry (DSC) and differential thermal analysis (DTA). These compounds crystallize in an orthorhombic superstructure derived from the structure of cubic fluorite (space group Pnma for Ln = Pr; C2221 for Ln = Sm, Eu). Sm3NbO7 undergoes the phase transition when the temperature is increased through ca. 1080 K and above the transition temperature, its structure is well described with space group Pnma. For Eu3NbO7, the phase transition was not observed up to 1273 K Pr3NbO7 indicates the phase transition when the temperature is increased through ca. 370 K. The change of the phase transition temperature against the Ln ionic radius for Ln3NbO7 is quite different from those for Ln3MO7 (M = Mo, Ru, Re, Os, or Ir), i.e., no systematic relationship between the phase transition temperature and the Ln ionic radius has been observed for Ln3NbO7 compounds.

Magnetic interactions in praseodymium ruthenate Pr3RuO7 with fluorite-related structure

Solid solutions Pr3(Ru1-xTax)O7 (0≤x≤1.0) and (Pr1-xYx)3RuO7 (0≤x≤0.7) were obtained as a single phase compound. They crystallize in an orthorhombic superstructure derived from that of the cubic fluorite with space group Cmcm. The results of the Rietveld analysis for X-ray diffraction profiles of Pr3(Ru1-xTax)O7 showed that Ru and Ta atoms are randomly situated at the six-coordinate 4b site. For (Pr1-xYx)3RuO7, with increasing the concentration of Y ions (x value), the smaller Y ions occupy selectively the seven-coordinate 8g site rather than the eight-coordinate 4a site.Through magnetic susceptibility measurements for Pr3(Ru1-xTax)O7, the antiferromagnetic transition temperatures decrease linearly with increasing x value, and at x=0.75 no magnetic ordering was found down to 1.8K, indicating the magnetic interaction is not one-dimensional, but three-dimensional. On the other hand, the antiferromagnetic transition temperature for (Pr1-xYx)3RuO7 decreases with increasing x value, but above x≥0.50 it becomes constant (~12K). This result indicates that Pr3+ ions at the seven-coordinate site greatly contribute to the antiferromagnetic interactions observed in (Pr1-xYx)3RuO7.Density functional calculations of Pr3RuO7 demonstrate that the electronic structure gives insulating character and that oxygen 2p orbitals hybridize strongly with Ru 4d orbitals in the valence band (VB). Near the top of VB, the Pr 4f orbitals at the seven-coordinated site also show a weak hybridization with the O(1) 2p orbitals. The Ru-O(1)-Pr superexchange pathway take part in three-dimensional magnetic interaction and play an important role in an enhancement of long-range magnetic ordering.

Fast oxygen diffusion in bismuth oxide probed by quasielastic neutron scattering

We present the first, to our knowledge, study of solid state oxygen translational diffusion by quasielastic neutron scattering. Such studies in the past might have been precluded by relatively low diffusivities of oxygen anions in the temperature range amenable to neutron scattering experiments. To explore the potential of the quasielastic scattering technique, which can deduce atomic diffusion jump length of oxygen anions through the momentum transfer dependence of the scattering signal, we have selected the fastest known oxygen conductor, bismuth oxide. We have found the oxygen anion jump length in excellent agreement with the nearest oxygen-vacancy distance in the anion sublattice of the fluorite-related structure of bismuth oxide.

Isomorphous substitutions and conductivity in molybdates Nd5−xLnxMo3O16+y (y∼0.5), where Ln = La, Ce, Pr

Neodymium substitution in rare earth elements (REE) was studied in the systems Nd5−xLnxMo3O16+y, where Ln = La, Ce, Pr with y∼0.5 by means of X-ray powder diffraction (including the Rietveld algorithm), scanning electron microscopy (SEM) and by measurements of the electrical conductivity. At x values of 0.5, 0.3, and 5.0 the mixtures of REE oxides with MoO3 transform to single-phase solid solutions with fluorite-related structure and doubled fluorite unit cell parameters as a result of solid state reactions occurring at 1050 °C. In the systems with La and Pr the unit cell parameter of solid solutions increases with x. While in the system with Ce this parameters decrease. Conductivity of ceramic solid electrolytes increases up to 17 times compared with the unmodified Nd5Mo3O16+y.

Synthesis and photoluminescence properties of fluorite-related (Y1−xEux)10W2O21 phosphor

Eu3+-doped fluorite-related yttrium–tungsten oxide Y10W2O21 (5Y2O3·2WO3) was prepared using vibrating-mill-mixed powder and solid-state reactions. The synthesis temperature was investigated by thermogravimetric/differential thermal analysis and X-ray diffraction. The morphology of the phosphor was confirmed by scanning electron microscope (SEM) measurement. The photoluminescence (PL) properties, including the excitation and emission spectra, decay curves (lifetime), thermal stability and quantum efficiency, were investigated. The PL emission intensity was found to increase with Eu3+ content up to about 22.5mol% and then decrease due to the concentration quenching effect. There were two kinds of coordinating site (seven-fold coordination and eight-fold coordination) of Y3+ ions in the fluorite-related structure of Y10W2O21. Eu3+ in different coordinating sites of Y3+ in the host could lead to energy-level splitting of the 5D0→7F2 transition, resulting in an emission peak red shift. The major intensity emission wavelengths, located at 612nm and 627nm for excitation at 392nm, were attributed to replacement of the Eu3+ ions with 7 or 8 coordination sites of Y3+ ions.

In situ high temperature powder neutron diffraction study of undoped and Ca-doped La28−xW4+xO54+3x/2(x = 0.85)

In situ neutron diffraction experiments of 2% Ca-doped and nominally undoped lanthanum tungstate (La28−xW4+xO54+3x/2, with x = 0.85) have been carried out under controlled pD2O and pO2 at elevated temperatures. All the diffraction patterns could be refined using an average cubic fluorite-related structure, in accordance with recent reports. The material exhibits disorder of the oxygen and the cation sublattices. Splitting of the oxygen sites around tungsten from the 32f to 96k Wyckoff position in the Fmm space group improves the model and can better represent the oxygen disorder. No phase transition was detected from room temperature up to 800 °C under any of the studied conditions. Expansion of the unit cell constants in the presence of water at intermediate and low temperatures was correlated with the formation of protonic defects. The thermal expansion coefficient for lanthanum tungstate is rather linear under all studied conditions (∼11 × 10−6 K−1). The in situ diffraction studies are correlated with dilatometry investigations and conductivity measurements.

Complex ceramic structures. I. Weberites

The weberite structure (A2B2X7) is an anion-deficient fluorite-related superstructure. Compared with fluorites, the reduction in the number of anions leads to a decrease in the coordination number of the B cations (VI coordination) with respect to the A cations (VIII coordination), thus allowing the accommodation of diverse cations. As a result, weberite compounds have a broad range of chemical and physical properties and great technological potential. This article summarizes the structural features of weberite and describes the structure in several different ways. This is the first time that the stacking vector and stacking angle are used to represent the weberite structure. This paper also discusses the crystallographic relationship between weberite, fluorite and pyrochlore (another fluorite-related structure). The cation sublattices of weberite and pyrochlore are correlated by an axial transformation. It has been shown that the different coordination environment of anions is due to the alternating layering of the AB3 and A3B close-packed cation layers. A stability field of weberite oxides is proposed in terms of the ratio of ionic radius of cations and relative bond ionicity. In addition, a selection of weberite compounds with interesting properties is discussed.

Electrical transport properties of In-doped Ce1−xInxO2−δ (x = 0.1; 0.2)

We report the synthesis and electrical conductivity of fluorite-type Ce(1-x)In(x)O(2-delta) (x = 0.1; 0.2) in air, dry Ar, N(2) and H(2) in the temperature range of 300-800 degrees C. We have employed a new CO(2) capture technique to prepare the "metastable" fluorite-related structure Ce(1-x)In(x)O(2-delta) from the corresponding In-doped Ba-containing perovskite of the nominal chemical formula BaCe(1-x)In(x)O(3-delta) at 800 degrees C. The amount of CO(2) gained per ceramic gram was found to be consistent with the formation of BaCO(3), confirming the complete leaching of Ba in BaCe(1-x)In(x)O(3-delta). The CO(2) capture efficiency was found to be in the range of 90-99% at 800 degrees C, which is significantly higher than those of well-known low-temperature CO(2) absorbing materials, including Li(2)O, Li(6)Zr(2)O(7), Li(1.8)Na(0.2)ZrO(3) and LiNaZrO(3). Powder X-ray diffraction (PXRD) and energy dispersive X-ray analysis (EDAX) confirmed the perovskite into fluorite structural transformation reaction. The AC impedance study showed a clear intercept to the real axis at the low-frequency over the investigated temperatures in all the atmospheres, indicating a non-blocking nature of electrode (Pt) and electrolyte interface. The constant phase element (CPE) value was found to be in the order of 10(-10) F in air, N(2) and Ar for high-frequency part of the semicircle due to bulk contribution, and about two orders of magnitude lower values were observed for the low-frequency semicircle which may correspond to grain-boundary effect. The 20 mol% In-doped CeO(2) exhibits a total electrical conductivity of about 4 x 10(-6) S/cm at 600 degrees C in Ar, while in H(2) an about four orders of magnitude higher electrical conductivity of 3 x 10(-2) S/cm was observed. The activation energy for electrical conductivity was found to be 1.36 eV in Ar, 1.43 eV in N(2), 1.34 eV in H(2), and 1.1 eV in air for Ce(0.8)In(0.2)O(1.9).

Iron-57 Mössbauer spectroscopy study of phases in the CaZrTi2−2x Nb x Fe x O7 zirconolite system

Materials of composition CaZrTi2−2x NbxFexO7 with the fluorite-related zirconolite structure have been prepared. The 57Fe Mossbauer spectra show that iron is initially located in the five co-ordinate cation sites. As the iron content increases the iron enters the octahedral sites until, at a composition CaZrTi0.4Nb0.8Fe0.8O7, ca. 50% of the iron is five co-ordinate and the remainder is located in the octahedral sites.

Synthesis of non-stoichiometric Bi 2O 4− x by oxidative precipitation

Bi 2O 4− x , a Bi mixed-valence phase was prepared at 95 °C, by a precipitation process, in a basic medium with a highly oxidizing K 2S 2O 8/Na 2S 2O 8. This phase has a low thermal stability as it decomposes below 400 °C in a multiple step process by some O 2 losses prior to finally transforming into γ-Bi 2O 3. The as-prepared powders are 50–60 nm in size with a narrow size distribution. Optical spectra of Bi 2O 4− x exhibit a broad absorption band with a band gap of ∼1.4 eV as compared to 2.61 eV for Bi 2O 3. The composition of this non-stoichiometric phase, which crystallizes in cubic fluorite related structure with a cell parameter of 5.538(3) Å, is Bi 2O 3.65 ± 0.10 .

Preparation, structure and oxide ion conductivity in Ce6−xYxMoO15−δ (x=0.1–1.4) solid solutions

Abstract The Ce 6− x Y x MoO 15− δ solid solution with fluorite-related structure have been characterized by differential thermal analysis/thermogravimetry (DTA/TG), X-ray diffraction (XRD), IR, Raman, scanning electric microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) methods. The electric conductivity of samples is investigated by Ac impedance spectroscopy. An essentially pure oxide-ion conductivity of the oxygen-deficiency was observed in pure argon, oxygen and air. The highest oxygen-ion conductivity was found in Ce 5.5 Y 0.5 MoO 15− δ ranging from 5.9×10 −5 (S cm −1 ) at 300 °C to 1.3×10 −2 (S cm −1 ) at 650 °C, respectively. The oxide-ion conductivities remained stable over 80 h-long test at 800 °C. These properties suggested that significant oxide-ionic conductivity exists in these materials at moderately elevated temperatures.

Synthesis of novel cation-ordered compounds with fluorite-related structure prepared by oxidation of Sn–Ta–O pyrochlore

Pyrochlore-type oxide, Sn II 1.64(Ta 1.88Sn IV 0.12)O 6.58 was oxidized for exploring novel metastable phases. Two novel Sn IV 0.82(Ta 0.94Sn IV 0.06)O 4.11 phases with a fluorite-related structure, which was obtained as a pure phase, and with a rutile-related structure appearing as an impurity for the fluorite-related phase were successfully obtained similar to the previous Sn–Nb–O system. The ordered arrangement of respective cations of Sn and ( 0.94 Ta + 0.06 Sn ) in the precursor Sn II 1.64(Ta 1.88Sn IV 0.12)O 6.58 was left in the both oxidized Sn IV 0.82(Ta 0.94Sn IV 0.06)O 4.11 phases. In contrast to the previous Sn–Nb–O system, the cation-ordered α -PbO 2-related Sn IV 0.82(Ta 0.94Sn IV 0.06)O 4.11 could not be obtained in the present conditions. Such a difference between the Sn–Ta–O and Sn–Nb–O systems was interpreted by larger energy barrier of the transformation from the fluorite-related phase to α -PbO 2-related phase in Sn–Ta–O system than in Sn–Nb–O system.