Fluorite Type Research Articles

Upconversion luminescence of Er3 +/Yb3 + and their role in the stabilization of cubic NaLaF4 nanocrystals in transparent oxyfluoride glass ceramics

Novel Er3+ and Yb3+ doped transparent oxyfluoride glass ceramics containing NaLaF4 nanocrystals were prepared from oxyfluoride glasses with general composition of Na2O-NaF-LaF3-Al2O3-SiO2. The crystallization processes, changes in morphology and upconversion luminescence were analyzed. The heat treatment of the precursor glasses promoted the formation of hexagonal NaLaF4 and an unidentified fluorite type compound. An enhancement of upconversion luminescence with the introduction of YbF3 in the glass ceramics was detected and indicated an efficient energy transfer from Yb3+ to Er3+. The local environment of Er3+ in hexagonal NaLaF4 and the fluorite phase was analyzed using site-selective spectroscopy and compared with polycrystalline ceramics. The incorporation of Er3+ ions in three non-equivalent cationic sites in the crystalline lattice of hexagonal NaLaF4 nanocrystals was observed. The local environment of erbium ions in the fluorite phase was found to be similar to α-NaYF4 suggesting the formation of Er3+ and Yb3+ stabilized α-NaLaF4.

Structure, densification and electrical properties of Gd3+ and Cu2+ co-doped ceria solid electrolytes for SOFC applications: Effects of Gd2O3 content

Ceria-based solid electrolytes exhibit superior electrical conductivity compared to traditional yttria-stabilized zirconia ceramics. However, they require high sintering temperatures to achieve full densification. Transition metal oxides exhibiting low melting points, such as CuO, have been used as additives to lower the sintering temperature of these materials. In this context, the present work is focused on the evaluation of the effects of gadolinium oxide (Gd2O3) content on the structure, densification and electrical properties of ceria co-doped with CuO. Nominal compositions of Ce0.99−xGdxCu0.01O2-δ (0 ≤ x ≤ 0.3) were synthesized by the polymeric precursor method. The precursor powders were characterized by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) and the calcined powders were studied by X-ray diffraction (XRD) and Rietveld refinement to obtain crystallographic parameters. The sinterability of green bodies was evaluated by dilatometry up to 1200°C. The relative density was determined in samples sintered between 950 and 1050°C and the microstructural characterization was performed by scanning electron microscopy (SEM). The electrical properties were investigated by impedance spectroscopy (IS). The XRD results confirms the formation of a cubic fluorite type structure in the entire composition range. The lattice parameters obtained by Rietveld refinement showed a reduction in the crystallite size with increasing gadolinium content. Densification was improved with increasing Gd-content up to x = 0.15. The electrical conductivity was enhanced by gadolinium addition, reaching a maximum of 7.81mScm−1 at 600°C for the composition x = 0.15 sintered at a temperature as low as 1050°C.

Gd\u2013Bi\u2013Ce\u2013O materials as catalysts in CO oxidation

Ceria-based solid solutions doped by Gd and/or Bi were synthesized by co-precipitation method from acid aqueous solution of cerium, gadolinium, and bismuth salts, followed by calcination at the temperature of 500 °C. Characterization of the synthesized samples by many methods was carried out. It is shown that all obtained powders of solid solutions crystallized into a cubic structure of the fluorite type, with an average particle size of 10–17 nm, and for the sample obtained through intermediate acetylacetonate complexes was 5–8 nm. The samples had a mesoporous structure with a pore diameter of 2–5 nm. These systems were tested as the catalysts in CO oxidation in the model gas mixture by the flow method. Gd0.05Bi0.15Ce0.8O2 system obtained through intermediate acetylacetonates has the highest activity, so this sample can be used as a catalyst.

High pressure studies on Thorium-Praseodymium mixed oxides

Angle dispersive x-ray diffraction measurements on thorium-praseodymium mixed oxides, (Th1-yPry)O2-x [y = 0, 0.15, 0.4 and 1], have been carried out up to a pressure of around 36 GPa at ambient temperature. A structural transformation from cubic fluorite type to an orthorhombic α-PbCl2 type structure has been observed for y = 0, 0.4 and 1, at a pressure of Pt = 28.8 GPa, 28.8 GPa and 30.1 GPa respectively. No such phase transition could be observed for y = 0.15 oxide. This is attributed to the nano-crystallinity of the sample. In support of this, studies on nano-crystalline ThO2 shows no phase transition up to 42 GPa, indicating the increased structural stability with a decrease in crystallite size. The observed discontinuity in the pressure volume curve of nano ThO2 indicates a possible isostructural transition around 15 GPa. The choice of the equation of state to describe the compression data and derive the bulk moduli of these oxides is discussed. An increase in the bulk modulus with ThO2 substitution was observed for the bulk oxides. However, the nano-crystalline mixed oxides (y = 0.15) showed a reduction in the bulk modulus compared to their bulk counterpart.

Volume and bond length dependences of the electronic structure of 6-fold and 8-fold coordinated Co2+ in pressure transformed CoF2

This work reports an optical absorption study on CoF2 under high-pressure conditions in the 0-80 GPa range. A recent structural study1 performed on the pressure-induced phase-transition sequence of CoF2 reveals that this compound undergoes a structural transformation associated with a change of Co2+ coordination from 6 (rutile type) to 8 (fluorite type) at 15 GPa, what implies a profound change in the Co2+ electronic ground state associated with the inversion of d orbitals from (t2g5eg2) to (eg4t2g3). This allows us to explore the Co2+ electronic structure, and hence 10Dq, as a function of the density and the Co-F bond length in both CoF6 (rutile phase) and CoF8 (fluorite phase) coordinations. The results are compared with those obtained in KCoF3 (perovskite) in a wide range of Co-F distances (from 2.04 to 1.83 Å).

Preparation and characterization of nanostructured Gd doped cerium oxide thin films by pulsed laser deposition for acetone sensor application

10mol% Gd doped cerium oxide nanostructured films were deposited on quartz substrate by pulsed laser deposition technique at different substrate temperatures. X-ray diffraction studies indicated polycrystalline nature of the thin films with fluorite type structure. Atomic force microscopy results indicated the presence of nanocrystallites over the thin film surface. The root mean square (rms) roughness of the thin films increased with increasing substrate temperature. Morphological studies were carried out with scanning electron microscope. Raman Spectra was recorded using micro Raman spectrometer. Energy band gaps of Gd doped cerium oxide thin films at various substrate temperatures were determined by using Tauc’s relationship. The sensing properties of the nanostructured thin films towards acetone vapor were investigated for various concentrations of acetone in air at different operating temperatures in the range 273–523K.

Direct Evidence for the Role of Oxygen Vacancy Configuration for Oxide-Based Resistive Switching Devices

The miniaturization of standard silicon-based memory technologies reaches physical limits in terms of size and power dissipation. Novel memory and computing architectures based on resistive switches are promising for future electronics beyond Von Neumann computing architecture. In oxide-based resistive switches, the memristance is associated with the movement of ionic carriers through oxygen vacancies in an oxide film at high electric field strength via structural defect modifications that are still poorly understood. Here, we newly employ an oxide solid solution by systematic extrinsic doping to probe the role of oxygen vacancy configuration either set as free, immobile or clustered to directly answer the fundamental question how the mobility of oxygen vacancies affects the switching characteristics of the devices at high electric fields. Till date strategies to tailor systematically oxygen vacancy concentration and configuration for oxide-based resistive switches are still to be explored. For this, we design resistive switching devices of Ce1-xGdxO2-y as a novel resistive switching oxide constituent. The active oxide layer of the devices, consisting of a 500 nm thick CeO2 film with Gd3+ doping concentrations of 3 to 30 mol% deposited by Pulsed Laser Deposition (PLD), is sandwiched between micro-fabricated Pt-crosspoint electrodes. Cyclovoltammetry measurements show a strong dependence of the ROFF/RON -resistance ratio on the effective gadolinia concentration per bit. At low gadolinia doping concentration < 10 mol%, a purely capacitive behaviour prevails. By increasing up to 20 mol% a significant hystereis in the current-voltage profile is measured with resistance ROFF/RON -ratios up to 100. Turning to high gadolinia concentrations from 20 to 30 mol%, decreases the hysteretic current-voltage profile. To analyse the structural changes related to the oxygen vacancy concentration and configuration, changes opposed by the extrinsic gadolinia dopant in the ceria lattice of the resistive switching devices, we turn to Raman Spectroscopy. From these results we conclude that for moderate gadolinium concentrations up to 20 mol% the concentration of mobile oxygen vacancies steadily increases and is available as "free" carriers for the resistive switches; in this state the oxide is in a fluorite structure type. Further increase in doping, for which we see a reduction of switching characteristics, leads to the formation of oxygen vacancy clusters in the structure, which are simply as a defect configuration "immobile". For this state we confirm a phase change to a more bixbyite structure. Unequivocally the experiments together with the defect chemical model, show that when the vacancies are set as "free", a maximum in memristance is found for 20 mol% Gd3+ doping, which clearly coincides with the maximum in ionic conductivity. In contrast, for higher gadolinia concentration the oxide exhibits only minor memristance, which originates from the decrease in structural symmetry leading to the formation of "immobile" oxygen defect clusters, thereby reducing the density of mobile ionic carriers available for resistive switching. A clear correlation between memristance and oxygen vacancies’ mobility was seen in this defect chemical model experiment on gadolinium doped ceria device structures. Extrinsic doping allows to tune the configuration, concentration and mobility of the oxygen vacancies in a much more systematic and controlled manner than what can be achieved through oxygen-deficient deposition or annealing steps as being the state-of-the-art in resistive switches. The research demonstrates guidelines for engineering of the oxide`s solid solution series to set the configuration of its oxygen vacancy defects and their mobility to tune the memristance for future nonvolatile memory and logic applications.

Stability and range of the type II Bi1 − xWxO1.5 + 1.5x solid solution

We have established the stability and range of the cubic type II phase of Bi1−xWxO1.5+1.5x using a combination of X-ray diffraction, neutron diffraction and X-ray absorption spectroscopy. Type II is a high temperature modification that can be obtained by quenching/rapid cooling of samples with compositions between x=0.148 to x=0.185. Slower cooling rates yield the stable low temperature polymorph, the tetragonal type Ib phase (Bi rich samples), and mixtures of type Ib and Aurivillius phase (W-rich samples). Throughout the entire solid solution range, type II exhibits a (3+3) dimensional incommensurate modulation with modulation vectors slightly smaller than 1/3 based on a cubic fluorite type subcell (δ-Bi2O3). The main structural motifs are well-defined tetrahedra of WO6 octahedra in a δ-Bi2O3-matrix, with additional W being incorporated on corners and face centers of the approximate commensurate 3×3×3 supercell in octahedral coordination, confirmed by XANES analysis of the W L3-edge. Impedance measurements reveal oxide ionic conductivities comparable to those of yttria-stabilised zirconia even after a decrease in ionic conductivity of about half an order of magnitude on thermal cycling due to transition to the tetragonal type Ib phase.

High pressure Raman spectroscopic studies on nanocrystalline ThO2

Nanocrystalline thoria with average diameters of 9, 15, and 24 nm have been synthesised and their Raman spectra are compared with bulk crystal data in order to establish size-dependent changes to the phonon spectrum at ambient conditions. Micro-Raman spectroscopy has been used to investigate the structural stability of these samples up to pressures of ∼50 GPa in a diamond anvil cell at room temperature. The results are compared with the pressure dependence of Raman spectra of bulk thoria that undergoes a structural transition from fluorite to cotunnite structure around 38 GPa. We observed a linear dependence of the first order Raman band on pressure up to 20 GPa. The pressure dependence of this most prominent single Raman band is similar for nano and bulk phases suggesting that they have similar compressibilities. This band exhibits normal stiffening behaviour under compression. Phase transition pressures of different sized fluorite type nanocrystalline ThO2 are found to be less than that reported for bulk ThO2 except for the one with 9 nm average diameter. Under hydrostatic conditions, the compression and decompression cycles show large hysteresis. Moreover, the reduction of crystallite size results in a sluggish dynamic process in the pressure-induced phase transformation of nano-ThO2.

Solubility limits, crystal structure and lattice thermal expansion of Ln2O3 (Ln=Sm, Eu, Gd) doped CeO2

0.5xLn2O3-(1−x) CeO2 (Ln = Sm, Eu, Gd, x = 0–1.0) powders were prepared by reverse-coprecipitation-calcination method and characterized by XRD. The limited substitution solid solution was formed while the Ln2O3 (Ln = Sm, Eu, Gd) introduce in the fluorite type structure CeO2, the maximum solid solubility is 40%. The lattice parameters of the fluorite structure solid solutions increased with extensive adoption of Ln3+ and temperature going up. The lattice thermal expansion coefficients (25–1200 °C) of fluorite structure Eu0.1Ce0.9O1.95 and Gd0.1Ce0.9O1.95 reached to 14.96 × 10−6 °C−1 and 14.75 × 10−6 °C−1, respectively.

Ternary crystals Sr1−y Eu y−x 2+Eu x 3+F2+x of fluorite phases with a variable europium valence and their thermal conductivity (50–300 K)

Crystals of new fluorite materials: ternary nonstoichiometric Sr0.9Eu0.086Eu0.014F2.014 and Sr0.5Eu0.432Eu0.068F2.068 phases with a structure of fluorite (CaF2) type, containing europium ions in different oxidation states (Eu2+, Eu3+), have been grown for the first time from a melt by the Bridgeman method. The variation in their thermal conductivity k(T) in a temperature range of 50–300 K has been experimentally studied. The equivalent cation positions with a large difference in cation masses (m Sr = 87.62 amu and m Eu = 151.96 amu) lead to low k(T) values of ternary (Sr1-yEuy-x 2+Eux 3+F2+x) and binary (Eu0.864 2+Eu0.136 3+F2.136) crystals. One can obtain crystals with thermal characteristics specified in a wide range by adjusting the ratio of strontium and europium (regardless of the valence). Since the thermal conductivity of Sr0.5Eu0.432Eu0.068F2.068 and Eu0.864Eu0.136F2.136 is independent of temperature in the range of 200–300 K, they can be recommended as reference materials with k(T) = const.

Synthesis, Crystal Structure, and Magnetic Properties of Pyrochlore‐Type Eu2Ta2(O,N)7+δ

Pyrochlore‐type Eu2Ta2(O,N)7+δ phases were prepared by reaction of ammonia with an amorphous europium tantalum oxide precursor. 151Eu Mössbauer and EPR spectroscopy as well as magnetic susceptibility measurements point to the presence of exclusively Eu3+. For phase‐pure samples (X‐ray powder diffraction), the nitrogen content varies between 1.0 and 1.8 wt %, leading to compositions in the range Eu2Ta2O7.1N0.6 – Eu2Ta2O6.5N1.0. Pyrochlore‐type phases are structurally derived from the fluorite type with 1/8 of the anions missing, resulting in an ideal composition A2B2X7. In Eu2Ta2(O,N)7+δ the excess anions partly occupy these vacancies. The prepared phases are colorless with a direct optical bandgap of 4.3 eV and they show the typical Van Vleck paramagnetic behavior known for trivalent Eu atoms.

Crystal structure and chemical bonding in the mixed anion compound BaSF.

BaSF was synthesised by a solid state reaction at high temperature and its crystal structure was determined thanks to X-ray diffraction on a single crystal. This transparent yellow fluorochalcogenide has an intergrowth structure built from the stacking of fluorite type layers and sulfur layers. In BaSF sulfur atoms form dimers with interatomic distances as short as 2.1074(10) Å. DFT calculations confirm that this compound is a band insulator with the Fermi level lying in between the antibonding π* and σ* molecular orbitals of the sulfur dimers. Reflectance measurements show that the optical band gap of BaSF is about 2.7 eV in good agreement with the value found from DFT calculations.

Multicomponent equiatomic rare earth oxides with a narrow band gap and associated praseodymium multivalency.

New multicomponent equiatomic rare earth oxides (ME-REOs) containing 3-7 rare earth elements (Ce, Gd, La, Nd, Pr, Sm and Y) in equiatomic proportions are synthesized using nebulized spray pyrolysis. All the systems crystallized as a phase pure fluorite type (Fm3[combining macron]m) structure in spite of the high chemical complexity. A nominal increase in the lattice parameter compared to CeO2 is observed in all ME-REOs. X-ray photoelectron spectroscopy performed on the ME-REOs confirmed that all the constituent rare earth elements are present in the 3+ oxidation state, except for Ce and Pr which are present in 4+ and in a mixed (3+/4+) oxidation state, respectively. The presence of Ce4+ contributes substantially to the observed stability of the single phase structure. These new oxide systems have narrow direct band gaps in the range of 1.95-2.14 eV and indirect band gaps in the range of 1.40-1.64 eV, enabling light absorption over the entire visible spectral range. Furthermore, the oxygen vacancy concentration rapidly increases and then saturates with the number of rare earth elements that are incorporated into the ME-REOs. The lowering of the band gap is found to be closely related to the presence of multivalent Pr. Interestingly, the band gap values are relatively invariant with respect to the composition or thermal treatments. Considering the high level of oxygen vacancies present and the observed low band gap values, these new material systems can be of importance where the presence of oxygen vacancies is essential or in applications where a narrow band gap is desirable.

Geochemical, microthermometric, and isotopic constraints on the origin of fluorite deposits in central Anatolia, Turkey

We investigate rare earth element geochemistry, microthermometric characteristics, and radiogenic isotope systematics of fluorites and stable isotope compositions of gangue minerals from several fluorite deposits in central Turkey. In the deposits, fluorite is the main ore mineral and it is accompanied by quartz, calcite, and minor pyrite and barite. Veins are represented by three different fluorite types based on their color. Total REY contents of fluorites are highly variable, ranging from 24 to 693 ppm. LREE concentrations of fluorites of all colors are similar but medium and heavy REE abundances of green fluorites are nearly an order of magnitude greater than in both host rocks and purple and yellow fluorites, indicating multiple sources for crystallization. REEs show significant fractionation and purple fluorites with relatively low HREE contents were likely precipitated at an earlier stage. As crystallization continued, green fluorites were nucleated because of ion exchange of LREEs with the host rock/minerals. Fluid inclusions yielded a wide range of homogenization temperatures from 86 °C to 292 °C and salinities from 0 to 20 wt.% NaCl equiv. The 87Sr/86Sr ratios of fluorites, varying from 0.707627 to 0.709380, overlap with the range of host rocks. 143Nd/144Nd values suggest two populations: purple fluorites with less radiogenic and green fluorites with more radiogenic Nd isotope ratios. The Sr-Nd isotope systematics of Bayindir fluorites are consistent with that of the Bayindir syenite, indicating that hydrothermal solutions progressively reacted with the host rock until equilibrium was established. δ18O values of quartz are slightly higher than the magmatic range. δ13C and δ18O of calcites fall into the range of marine carbonates. δ34S values of barites indicate derivation from diverse reservoirs changing from marine to terrestrial sources. In contrast, sulfur in pyrites points to a magmatic origin. Therefore, is it suggested that magmatic fluids to some extent contributed to the precipitation of fluorite veins.

Oxygen ion conductivity in samarium and gadolinium stabilized cerium oxide heterostructures

Gadolinium (GDC) and samarium (SDC) doped ceria were investigated in terms of multilayer systems, evaporated by e-beam technique on optical quartz, Alloy600 and sapphire substrate. GDC-SDC heterostructures of 1.3μm thicknesses, composed of 1, 2, 3, 5 and 7 layers and they were investigated by structural and ionic conductivity techniques. Bragg peaks show nanocrystalline state of Gd and Sm doped ceria thin films. XRD patterns show fluorite type structure with space group Fm3m. The XRD analysis of thin films, deposited on quartz substrate, reveals the increase of (220) peak with increasing number of layers. The decrease of (111) peak is slightly notable, also. Thin film heterostructures have a face-centered cubic cell with the following lattice parameters, such as 5.4180nm for GDC of and of 5.4245nm for SDC. The scanning electron microscopy cross sectional analysis of three-layered structure clearly indicates the interfaces of different material. There are no visually distinct discontinuities in higher layer structures (5–7 layers). Total conductivity increases linearly with increasing of temperature, but decreases with the increase of number of layers. The highest total ionic conductivity at 1214K temperature for SDC and GDC thin monolayers was 1.62S/m and 1.02S/m, respectively. The activation energy increases with the increase of number of layer as well.

Atomistic simulation of a superionic transition in fluorite type structures UO2, UN2, TiH2

The results of the atomistic simulation of a superionic transition and melting of stoichiometric UN2, UO2 and TiH2 have been presented. Simulation shows that superionic transitions of UO2 and TiH2 take place at temperature below melting temperature, while UN2 did not show such feature. This difference in properties of studied structures is caused by various gap between formation energies of Frenkel pair defects for sublattices. The possibility of describing the superionic transition within the theory of second-order phase transitions has been discussed, the conditions of the existence of superionic transition have been discussed.

Catalytic properties of Ce x Zr1–x O2 prepared using a template in the oxidation of CO

The catalytic activity in CO oxidation of Ce x Zr1–x O2 double oxides prepared using pine sawdust and cetyltrimethylammonium bromide (CTAB) as templates is compared. It is found by means of SEM and the low-temperature adsorption of N2 that biomorphic oxides reproduce the macropore structure of the template. It is shown via XRD and Raman spectroscopy that all samples contained mixed ceria-zirconia oxide. The double oxides form a cubic phase with a lattice of the fluorite type at a ratio of Ce: Zr = 4, regardless of the nature of the template; when Ce: Zr = 1, the biomorphic mixed oxide forms a tetragonal phase. According to Raman spectroscopy and XRD it was shown that the distortion of the oxygen sublattice is higher in biomorphic samples. Energy dispersive analysis shows that Ca impurities were present in the biomorphic samples, introducing additional distortions in the lattice of double oxide and leading to the formation of anionic vacancies. It is found that when Ce: Zr = 4, the conversion of CO on biomorphic oxide in the range of 100–350°C is higher than that observed for Ce x Zr1–x O2 (CTAB); reducing the Ce: Zr ratio in the biomorphic sample to 1 results in a marked decrease in CO conversion at 100–200°C. It is concluded that these differences are due to changes in the mobility of the lattice oxygen.

Upconversion luminescence of a transparent glass ceramics with hexagonal Na(Gd,Lu)F4 nanocrystals

Novel Er3+ doped transparent glass ceramics containing hexagonal Na(Gd,Lu)F4 nanocrystals were prepared from oxyfluoride glasses. The distribution of rare earth ions in the crystalline and glassy phase has been analyzed by X-ray diffraction and erbium luminescence decay kinetics measurement. A strong deviation of rare earth ion content in fluoride nanocrystals in comparison to the base glass has been observed. Preferential incorporation of Gd3+ over Lu3+ ions in the fluoride lattice leads to the stabilization of hexagonal Na(Gd,Lu)F4 structure and prevents the formation of cubic fluorite type solid solutions. A considerable enhancement of upconversion luminescence correlates with the formation of hexagonal solid solutions and is attributed to an efficient energy transfer between Er3+ ions.

Structural, thermal, electrical and morphological characterization of (Bi2O3)1−x−y(Sm2O3)x(Yb2O3)y nanostructures prepared by solid state synthesis

In the present work, the ternary system of (Bi2O3)1−x−y(Sm2O3)x(Yb2O3)y was investigated. For the production and stabilization of the fcc-type solid solution, nano-Sm2O3 and nano-Yb2O3 were doped into nano-Bi2O3 by solid-state synthesis techniques. The XRD results showed that the crystallographic structure of the samples had displayed a fluorite type face-centered cubic δ-Bi2O3 phase. The phase stability was also checked by the DTA measurements. The temperature dependent electrical conductivity results revealed that the maximum electrical conductivity observed for the sample of the nanostructure-(Bi2O3)0.8(Sm2O3)0.1(Yb2O3)0.1 system was 5.39 × 10−2 (ohm.cm)−1 at 650 °C. The results also show that the lowest activation energy was 0.7062 eV and the lowest crystallite size was 31.62 nm for the nanostructure-(Bi2O3)0.75(Sm2O3)0.1(Yb2O3)0.15 system. Consequently, the face-centered cubic stable δ-phase-(Bi2O3)0.8(Sm2O3)0.1(Yb2O3)0.1 is the optimal dopant amount due to the relatively good stability and oxygen ionic conductivity obtained, which are two of our major concerns for the electrolyte layer of solid oxide fuel cells (SOFCs).