Fluorite-type Solid Solutions Research Articles

X-ray diffraction and Raman spectroscopic studies on ThO2−UO2 solid solutions

In view of the vast resources of thorium in India, thorium-based fuel cycle has been envisaged as one of the most promising options to meet sustainable energy requirements. In this direction, ThO2−233UO2 and ThO2−UO2 (low-enriched) mixed oxide fuels with < 30% UO2 have been proposed for the Advanced Heavy Water Reactor. In the present work, phase relations in the Th1-xUxO2+δ (x ≤ 0.3) system under reducing and oxidizing conditions have been investigated by means of X-ray diffraction and Raman spectroscopic studies. Diffraction studies confirmed the formation of single-phase fluorite-type solid solutions; however, the samples synthesized in air exhibited slightly lower lattice parameters than those of their reduced counterparts, attributed to oxidation of U4+. Raman spectroscopic studies indicate the formation of various defects upon uranium substitution in parent ThO2, depending on synthesis conditions. It also highlights the existence of more disorder in the air-synthesized samples as compared to their reduced counterparts, plausibly due to the presence of a variety of oxidation states of uranium and associated oxygen anions interstitials. Thermal annealing of the solid solutions under air at T ≥ 1873 K results in heavy uranium evaporation loss, which has been established by XRD and XRF studies.

Phase equilibria in the ZrO2–HfO2–Nd2O3 system at 1500 °C and 1700 °C

The phase equilibria in the ZrO2–HfO2–Nd2O3 ternary system at 1500 °C and 1700 °C were studied over the whole concentration range by X-ray diffraction and microstructural analyses. Corresponding isothermal sections were constructed from the data obtained. It was found that solid solutions in this system are derived from a tetragonal (T) modification of ZrO2, a monoclinic (M) modification of HfO2, a hexagonal (A) modification of Nd2O3, a cubic phase with a fluorite (F) structure of ZrO2 (HfO2), and an ordered phase with a pyrochlore (Py) structure of Nd2Zr2O7 (Nd2Hf2O7). The phase boundaries and unit cell lattice parameters were determined. The solubility of Nd2O3 in M − HfO2 is pretty low and about less than 1 mol%, as confirmed by XRD and microstructural analyses. The ordered pyrochlore-type (Py) phase of Nd2Zr2O7 (Nd2Hf2O7) forms continuous series of solid solutions at 1500 °C and 1700 °C. The homogeneous region of these continuous series of solid solutions changes insignificantly with increasing temperature. The changes in the construction of the isothermal section of the ZrO2-HfO2-Nd2O3 phase diagram at 1500 °C compared to 1700 °C are associated with the thermal stability of cubic fluorite-type (F) solid solutions. Other phases in the ZrO2-HfO2-Nd2O3 ternary system are not detected at the studied temperatures.

Phase relations and lattice parameter trends in Nd3+−substituted UO2 system under reducing and oxidizing conditions

While Am-substituted UO2 solid solutions have been considered as advanced fuels for heterogeneous transmutation of americium, U-substituted Am2O3 with cubic structure has been envisaged as a promising candidate for radioisotope heat unit and radioisotope thermoelectric generator. Considering Nd2O3 as a surrogate for Am2O3, a series of Nd3+-substituted UO2 samples with the general formula U1-xNdxO2±δ (0 ≤ x ≤ 1, δ ≥ 0) have been investigated for phase relations under reducing and oxidizing conditions. In this direction, samples have been synthesized by a solid-state route and thoroughly characterized by X-ray diffraction and Raman spectroscopy. XRD studies revealed the formation of fluorite-type solid solution at x ≤ 0.7 and a biphasic phase field consisting of C-type (U,Nd)O2−δ phase and A-type Nd2O3 at x ≥ 0.8 in reducing conditions. In the fluorite-type regime, due to the aliovalent Nd3+-substitution in UO2, the charge neutrality was achieved primarily through oxygen vacancy creation as evident from lattice parameter trends as well as Raman spectroscopic investigation that exhibit Raman band due to oxygen vacancies. The reduced samples have a high tendency to get oxidized even during storage under ambient conditions leading to a decrease in lattice parameters, which can be attributed to the formation of the higher oxidation state of uranium (U5+/U6+) following a charge compensation mechanism. Synthesis in the oxidizing conditions yielded a biphasic phase field at x ≤ 0.2, a single-phase fluorite-type phase field at 0.3 ≤ x ≤ 0.7, a biphasic mixture of fluorite-type (U,Nd)O2−δ and trigonal Nd6UO12 phases at x = 0.8 and another biphasic mixture of Nd6UO12 and Nd2O3 phases at x = 0.9. The transition of reduced single-phase fluorite-type solid solution at x ≤ 0.2 to a biphasic mixture upon oxidation has been probed by a high-temperature XRD.

Mechanism of electrotransport in superionic conductor Sr-=SUB=-0.7-=/SUB=-La-=SUB=-0.15-=/SUB=-Lu-=SUB=-0.15-=/SUB=-F-=SUB=-2.3-=/SUB=- with the fluorite-type structure

The ionic conductivity of the superionic conductor Sr0.7La0.15Lu0.15F2.3 with the fluorite-type structure (type CaF2, sp. gr. Fm3m) had measured by the impedance spectroscopy in the temperature range 385-794 K. Bulk crystals of a three-component solid solution (lattice parameter a=5.7726(1) Angstrem) had obtained from the melt by the directed crystallization technique. Temperature dependence ionic conductivity sigmadc(T) satisfies the Arrhenius--Frenkel equation with the activation enthalpy of the electrical conductivity Delta Hsigma=0.706± 0.05 eV. The value of sigmadc is 1.5·10-5 S/cm at 500 K. Based on the analysis of electrophysical and structural studies of fluorite-type solid solutions in SrF2-LaF3 and SrF2-LuF3 systems, the microscopic model of ion transport in the Sr0.7La0.15Lu0.15F2.3 crystal was proposed. The concentration and mobility of charge carriers were calculated: nmob=7.8·1020 cm-3 and μmob=1.2·10-7 cm2/Vs at 500 K. Keywords: superionic conductors, impedance spectroscopy, single crystals, fluorides, rare earth ions, fluorite structure.

Механизм электропереноса в суперионном проводнике Sr-=SUB=-0.7-=/SUB=-La-=SUB=-0.15-=/SUB=-Lu-=SUB=-0.15-=/SUB=-F-=SUB=-2.3-=/SUB=- со структурой флюорита

The ionic conductivity of the superionic conductor Sr0.7La0.15Lu0.15F2.3 with the fluorite-type structure (type CaF2, sp. gr. Fm-3m) had measured by the impedance spectroscopy in the temperature range 385–794 K. Bulk crystals of a three-component solid solution (lattice parameter a = 5.7726(1) A) had obtained from the melt by the directed crystallization technique. Temperature dependence ionic conductivity σdc(T) satisfies the Arrhenius–Frenkel equation with the activation enthalpy of the electrical conductivity Hσ = 0.706 ± 0.05 eV. The value of σdc is 1.5 10–5 S/cm at 500 K. Based on the analysis of electrophysical and structural studies of fluorite-type solid solutions in SrF2–LaF3 and SrF2–LuF3 systems, the microscopic model of ion transport in the Sr0.7La0.15Lu0.15F2.3 crystal was proposed. The concentration and mobility of charge carriers were calculated: nmob = 7.8 × 1020 cm–3 and μmob = 1.2 × 10–7 cm2/Vs at 500K.

Enhanced Electrocatalytic Activity and Surface Exsolution in PrOx-Substituted Cerium Gadolinium Oxide

The electrocatalytic activity of Pr-substituted cerium gadolinium oxide (CGO) was examined to assess its suitability as a composite air electrode in solid oxide cells. The solid-state reaction of CGO and Pr6O11 yielded fluorite-type solid solution, (Ce0.9Gd0.1)1–xPrxO2−δ (x = 0, 0.17 and 0.33), with an increased lattice parameter. Electrochemical tests of electrolyte-supported cells with CGO-PrOx and LSCF-CGO-PrOx electrodes exhibited that the PrOx addition significantly improves the electrical conductivity and electrocatalytic activity for oxygen reduction reaction (ORR). In-depth characterization showed that the improved ionic and electrical conductivities of CGO-PrOx are associated with high concentration of oxygen vacancies created by replacement of Ce4+ ions with Pr3+ ions. TEM analysis revealed that annealing of CGO-PrOx results in the exsolution of PrOx nanoparticles, indicating local lattice strain around Pr3+ and eventually their outward migration. TPD, TPR, and TPO measurements further demonstrated that CGO-PrOx has greater oxygen reduction reaction (ORR) activity at low temperatures (400–600 °C) when compared to CGO. These findings suggest the high potential of Pr-substituted CGO for low-temperature solid oxide cell applications.

Ionic Conductivity of Crystals of Fluorite-Type Solid Solution of the PbF2–CdF2–MnF2 System

Ionic Conductivity of Crystals of Fluorite-Type Solid Solution of the PbF2–CdF2–MnF2 System

Deciphering the Nature of Ru Sites in Reductively Exsolved Oxides with Electronic and Geometric Metal–Support Interactions

The reductive exsolution of metallic Ru from fluorite-type solid solutions Ln2Ru0.2Ce1.8O7 (Ln = Sm, Nd, La) leads to materials with metal–support interactions that influence the electronic state a...

Ion distribution models for defect fluorite ZrO2 - AO1.5 (A = Ln, Y) solid solutions: I. Relationship between lattice parameter and composition

The composition dependence of the lattice constant, a, in AxB1-xO2-0.5xV0.5x, fluorite-type solid solutions (B = Zr, A = {Nd-Yb, Y}, V = oxygen vacancy) is characterized by changes in slope at x ~ 1/3, which cannot be described by existing models. Moreover, over the range of 0.15 ≤ x ≤ 1/3 the a vs. x data on all systems can be fitted with a linear equation, a = q + rx, with the same intercept, q, however, over the range of 1/3 ≤ x ≤ 1/2 the data bifurcate requiring different intercepts for A={Nd-Gd] and for A={Dy-Yb,Y} systems, implying steeper slopes for the first group. An adequate description is achieved via close-packing models involving two anion species, O2- and V, and three combinations of cation species, namely 8A, 7B, 8B (0 ≤ x ≤ 1/3), 7A, 8A, 7B (1/3 ≤ x ≤ 1/2) and 8A, 6B, 7B (1/3 ≤ x ≤ 1/2), that take into an account the constraint on the average coordination number, K(x), of K(x) = 8 – 2x and two additional short-range order (SRO) constrains referring to vacancy-vacancy avoidance and vacancy-to-Zr association. The consistent fit to a vs. x data with these models requires the effective size of the oxygen vacancy to be larger than the radius of the oxygen anion by ~11%. The latter observation seems to play an important role in the understanding of structure-property relationships in AxB1-xO2-0.5xV0.5x systems.

Ion distribution models for defect fluorite ZrO2 - AO1.5 (A = Ln, Y) solid solutions: II. Thermodynamics of mixing and ordering

Thermodynamic mixing properties of AxB1-xO2-0.5xV0.5x, fluorite-type solid solutions (B = Zr, A = {Nd-Yb, Y}, V = oxygen vacancy) are modelled as functions of four parameters, ΔH1, ΔH2, ΔH3 and ΔH4, which correspond to the enthalpy effects of the reactions 6A + 8B = 7A + 7B (1), 6A + 8B = 8A + 6B (2), 6B + 8B = 7B + 7B (3) and 6A + 8A = 7A + 7A (4), involving six cation species, 6A, 7A, 8A, 6B, 7B and 8B. The model predicts that the disordered configuration containing all cation species evolves with the decreasing temperature such that 6-fold coordinated cations tend to vanish within 0 ≤ x ≤ 1/2 domain, while 8-fold coordinated cations become extinct within 1/2 ≤ x ≤ 1 domain. The further evolution within the intervals of 0 ≤ x ≤ 1/3, 1/3 ≤ x ≤ 1/2, 1/2 ≤ x ≤ 2/3 and 2/3 ≤ x ≤ 1 favours the extinction of 7A, 8B, 7B and 6A cation species, respectively. With the further decrease in the temperature 6-fold B and 8-fold A cations reappear within the domains of 1/3 ≤ x ≤ 1/2 and 1/2 ≤ x ≤ 2/3 via the reaction 7A + 7B = 8A + 6B. The configurational entropy reduces along with these transformations. The model fits structural and calorimetric data on Zr-based AxB1-xO2-0.5xV0.5x systems and provides hints to understanding of ionic conductivity and radiation susceptibility data.

Probing structural evolution in thorium–cerium mixed oxides thermally annealed in oxidizing and reducing conditions

ThO2 is a promising matrix either for incineration or direct disposal of plutonium and (Th, Pu)O2 is a potential advanced fuel. In this view, a ThO2–CeO2 mixed oxide system has been investigated under oxidizing as well as reducing conditions wherein CeO2 has been employed as a non-radioactive surrogate for PuO2. Selected compositions in Th1−xCexO2 (0 ≤ x ≤ 0.5) series were prepared by gelcombustion synthesis and thoroughly characterized by XRD and Raman spectroscopy. Both the studies confirm the formation of a fluorite-type (F-type) solid solution throughout the series after calcination of gelcombusted powders at 873 K in air. The F-type phase is retained even on annealing these solid solutions at an elevated temperature 1773 K in air. Lattice parameter of the F-type phase decreases as a function of ceria content in accordance with the Vegard’s law. On the contrary, the position of the F2g Raman band in these solid solutions exhibits non-ideal behavior. Annealing of calcined samples at 1873 K in a reducing atmosphere (Ar−8% H2) yields a single phase of F-type structure at x ≤ 0.2, while a biphasic phase field at x ≥ 0.3. Characteristic Raman vibration bands have been utilized to identify the presence of possible defects (due to Ce4+/Ce3+ substitution in ThO2) that are otherwise difficult to detect by the XRD studies. Detailed structure and defect analysis in ThO2–CeO2 mixed oxides will not only give valuable insight to the fuel designer but also bring out fundamental chemical aspects of such mixed oxides.

Ion Distribution Models for Defect Fluorite Zro &lt;sub&gt;2&lt;/sub&gt; - AO &lt;sub&gt;1.5&lt;/sub&gt; (A = &lt;i&gt;Ln&lt;/i&gt;, Y) Solid Solutions: I. Relationship between Lattice Parameter and Composition

The composition dependence of the lattice constant, a, in AxB1-xO2-0.5xV0.5x, fluorite-type solid solutions (B = Zr, A = {Nd-Yb, Y}, V = oxygen vacancy) is characterized by changes in slope at x ~ 1/3, which cannot be described by existing models. Moreover, over the range of 0.15 ≤ x ≤ 1/3 the a vs. x data on all systems can be fitted with a linear equation, a = q + rx, with the same intercept, q, however, over the range of 1/3 ≤ x ≤ 1/2 the data bifurcate requiring different intercepts for A={Nd-Gd] and for A={Dy-Yb,Y} systems, implying steeper slopes for the first group. An adequate description is achieved via close-packing models involving two anion species, O2- and V, and three combinations of cation species, namely 8A, 7B, 8B (0 ≤ x ≤ 1/3), 7A, 8A, 7B (1/3 ≤ x ≤ 1/2) and 8A, 6B, 7B (1/3 ≤ x ≤ 1/2), that take into an account the constraint on the average coordination number, K(x), of K(x) = 8 - 2x and two additional short-range order (SRO) constrains referring to vacancy-vacancy avoidance and vacancy-to-Zr association. The consistent fit to a vs. x data with these models requires the effective size of the oxygen vacancy to be larger than the radius of the oxygen anion by ~11%. The latter observation seems to play an important role in the understanding of structure-property relationships in AxB1-xO2-0.5xV0.5x systems.

Oxygen Nonstoichiometry and Transport Properties of Mixed-Conducting Ce0.6–xLa0.4Pr x O2–δ

The oxygen nonstoichiometry and electrical conductivity of fluorite-type solid solutions Ce0.6‒xLa0.4Pr x O2–δ (x = 0.1–0.2) were studied in the oxygen partial pressure range 10–19–0.35 atm at 1023–1223 K. It was confirmed that the Pr4+/3+ and Ce4+/3+ redox pairs, which determine the concentration of p- and n-type electron charge carriers, play the dominant roles under oxidizing and reducing conditions, respectively. The conductivity vs. charge carrier concentration dependencies in these conditions are almost linear. Increasing praseodymium content leads to a substantially higher hole conductivity and an expanded range of the oxygen nonstoichiometry variations at high oxygen partial pressures. Under reducing conditions when praseodymium cations become trivalent opposite trends are observed on doping.

Synthesis and luminescence studies of CaF2:Yb:Pr solid solutions powders for photonics

Praseodymium- and ytterbium-doped calcium fluoride powder samples were prepared by co-precipitation from aqueous solutions followed by annealing at 600 °C. Phase and microstructure analyses of these fluorite-type solid solution samples were carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM), which have demonstrated the average particle size to be about 30 nm, ∼100 nm after drying at 45 °C and annealing at 600 °C, respectively. The fluorescence studies under 443 nm excitation in 3H4 – 3P2 band of Pr3+ ions allowed us to determine the optimal chemical compositions of CaF2:Yb:Pr samples with the high Pr3+->Yb3+ energy transfer efficiency exceeding 90% and down-conversion luminescence efficiency up to 50%. The optimal compositions are: Yb:Pr = 10.0:0.05; 10.0:0.1; and 5.0:0.05 mol%. This confirms that the energy transfer from praseodymium to ytterbium occurred via quantum cutting mechanism, and the optimal composition for highly efficient phosphor application appears to be CaF2:Yb(10.0 mol%):Pr(0.1 mol%).

A direct in situ observation of water-enhanced proton conductivity of Eu-doped ZrO2: Effect on WGS reaction

Eu-doped ZrO2 solid solutions have been synthesized in order to prepare proton conductors as water-enhancer additives for the WGS reaction. Elemental characterization has been carried out revealing homogeneous dopant distribution resulting in fluorite-type solid solutions for Eu2O3 contents up to ∼9 mol.%. Representative samples of the Eu-doped ZrO2 series have been analysed by Impedance Spectroscopy (IS) in inert, oxygen and wet conditions. The solid solution with 5 mol.% of Eu2O3 has presented the highest conductivity values for all tested conditions indicating an optimal amount of dopant. Moreover, the presence of vapour pressure results in an increment of the conductivity at temperatures lower than 300 °C, meanwhile at higher temperatures the conductivity is the same than that in inert conditions. To elucidate these results, in situ DRIFTS studies were carried out. These experiments evidenced the existence of water dissociation at oxygen vacancies (band at 3724 cm−1) as well as the presence of physisorbed water at temperatures up to ∼300 °C where the band at 5248 cm−1 characteristic of these species disappeared. These results points to a layer model where the physisorbed water interacts with surface hydroxyls generated by dissociated water that improves the proton conductivity through Grotthuss’ mechanism in the RT-300 °C temperature range. These samples were successfully tested in WGS reaction as additive to a typical Pt-based catalyst. The presence of the mixed oxide reveals an increase of the catalyst’ activity assisted by the proton conductor, since improves the water activation step.

Synthesis of gadolinium-doped thorium dioxide via a wet chemical route: Limitations of the co-precipitation method

A wet chemical route (oxalate precipitation) was chosen to prepare gadolinium-doped thoria powders as a feed for pellet fabrication. The oxalate and derived oxide powders were characterized by thermogravimetric analysis coupled to evolved gas analysis, BET, XRD and SEM. Dilatometry was used to study the sintering behavior. From homogeneous solutions containing selected mixtures of gadolinium (III) and thorium (IV) nitrates solid precipitates were obtained, that were composed of two phases. Calcination at 700 °C or 1000 °C was insufficient to obtain single phases for any of the compositions. Rietveld analysis of calcined powders confirmed the presence of both a Th-rich phase (ThxGd1−xO2−x/2) and a Gd-rich phase ((GdyTh1−y)2O3+(1−y)) in which limited substitution of the foreign element (Gd or Th, respectively) occurred. A fluorite-type solid solution formed during sintering at 1750 °C. The unit cell parameters of sintered pellets were determined for ThxGd1−xO2−(1−x)/2 mixed oxides in the composition range: 0 ≤ x ≤ 0.30 and agree with reported values.

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.

Dielectric properties of bismuth niobate films using LaNiO3 bottom electrode

Bi3NbO7 (BNO) thin films were deposited on Pt/TiO2/SiO2/Si (100) and LaNiO3 bottom electrode substrates at room temperature from the polymeric precursor method. X-ray powder diffraction was used to investigate the formation characteristics and stability range of the tetragonal modification of a fluorite-type solid solution. The results showed that this tetragonal, commensurately modulated phase forms through the intermediate formation of the incommensurately modulated cubic fluorite phase followed by the incommensurate-commensurate transformation. LaNiO3 (LNO) bottom electrode strongly promotes the formation of high intensity (111) texture of BNO films. The dielectric constants of the films increased from 192 to 357 at 1 MHz with the bottom electrode while the leakage current behavior at room temperature of the films decreased from 10−7 to 10−8 A/cm2 at a voltage of 5 V. The reduction of dc leakage current is explained on the basis of relative phase stability and improved microstructure of the material. The capacitance density of 75 fC/µm2, dielectric loss of 0.04 % at 1 MHz, and breakdown strength of about 0.30 MV/cm is compatible with embedded decoupling capacitors applications.

Ion mobility and conductivity in fluorite-type solid solutions in the KF–MF2–BiF3 systems (M = Ba, Cd) according to 19F NMR and conductivity data

Ion mobility and transport properties of the fluorite-type K0.41Ba0.09Bi0.5F2.09, K0.41Cd0.09Bi0.5F2.09, K0.45Ba0.05Bi0.5F2.05, and K0.45Cd0.05Bi0.5F2.05 solid solutions were studied by the 19F NMR spectroscopy and conductivity measurements. Analysis of 19F NMR spectra made it possible to assess the temperature effect on the nature of ion motions in the fluoride sublattice, identify types of ion mobility, and determine the respective temperature ranges (150–470K). The results showed that the dominant type of ion motion is diffusion of fluoride ions in these solid solutions at temperatures above 350K. The samples have the ionic conductivity above 10−2S/cm at 463K.

Fabrication and structural characterization of bismuth niobate thin films grown by chemical solution deposition

Bi3NbO7 (BNO) thin films were deposited on Pt/TiO2/SiO2/Si (100) substrates at room temperature from the polymeric precursor method. X-ray powder diffraction and transmission electron microscopy were used to investigate the formation characteristics and stability range of the tetragonal modification of a fluorite-type solid solution. The results showed that this tetragonal, commensurately modulated phase forms through the intermediate formation of the incommensurately modulated cubic fluorite phase followed by the incommensurate-commensurate transformation. The 200 nm thick BNO films exhibit crystalline structure, a dielectric constant of 170, capacitance density of 200 nF/cm2, dielectric loss of 0.4 % at 1 MHz, and a leakage current density of approximately 1 × 10−7 A/cm2 at 5 V. They show breakdown strength of about 0.25 MV/cm. The leakage mechanism of BNO film in high field conduction is well explained by the Schottky and Poole–Frenkel emission models. The 200 nm thick BNO film is suitable for embedded decoupling capacitor applications directly on a printed circuit board.