Hexavalent Cations Research Articles

Improved quantum-mechanical model for evaluating the difficult synthesis of nitride and oxygen perovskites

Most nitride perovskites have been reported using computational prediction; very few have been synthesized experimentally. Using an improved quantum-mechanical model, we predicted that four hexavalent cations, Mo6+, Os6+, Po6+ and Ru6+, can dope the nitride perovskites LaWN3 and LaReN3, and we explain why most of the numerically predicted nitride perovskites are more difficult to synthesize than oxygen perovskites. Based on first principles, the calculated results for the structural and stable properties of the doped perovskites LaW1-xBxN3 (BMo6+, Os6+, Po6+ and Ru6+) also confirm the accuracy of the improved model. The proposed model suggests that there are two potential barriers in nitride perovskites, whereas oxygen perovskites have one potential barrier and a potential well between the cations and anions. The numerical results show that the width of the potential well or second potential barrier plays a crucial role in the successful synthesis of perovskites. This model is of great value for the future synthesis of perovskites.

WinPclclas, a Windows Program for Pyrochlore Supergroup Minerals

Abstract A Microsoft® Visual Basic program, WinPclclas, has been developed to calculate the chemical formulae of pyrochlore supergroup minerals (PSM) based on data obtained from wet-chemical and electron-microprobe analyses. These are also classified following the nomenclature scheme adopted by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association for PSM (IMA-10) and including both subsequent group and species updates. The program considers classification among the 33 IMA-approved PSM, as well as ten species that have not been approved, adhering to the general formula A2B2X6Y and considering the dominant ions at the A, B, and Y sites. It considers PSM represented by the pyrochlore, microlite, roméite, elsmoreite, and betafite groups, along with unassigned members of the ralstonite and coulsellite groups. Mineral formulae of the PSM are calculated based on the ideal number of B cations being two apfu, owing to the potential for vacancies at the A, X, and Y sites. WinPclclas operates in four stages: (1) it estimates cation and anion contents provided by input chemical data; (2) it determines the dominant cation and anion at the A, B, and Y sites; (3) it assigns the PSM to one of the seven groups, based on the dominant cation at the B site (i.e., W6+, Nb5+, Ta5+, Sb5+, Ti4+, Al3+, and Mg2+); and (4) it classifies the pyrochlore species into an appropriate group, based on the dominant cation at the A site and the dominant anion at the Y site. The H2O (wt.%) content for the PSM is estimated by stoichiometric constraints. WinPclclas allows users to: (1) enter up to 51 input variables for mineral-chemical analyses; (2) type and load multiple PSM compositions in the data entry section; (3) edit and load the Microsoft® Excel files used in calculating, classifying, and naming the PSM, together with the total monovalent to hexavalent cation, and (4) store all the calculated parameters in the output of a Microsoft® Excel file for further data evaluations. The program is distributed as a self-extracting setup file, including the necessary support files used by the program, a help file, and representative sample data files.

Structural complexity of Y6BO12 fluorite-related ternary oxides

Oxides with generic stoichiometry M7O12 occupy, in a generic phase diagram, an intermediate place between fluorite and bixbyite structure types. Their structure is derived from the ideal fluorite structure, and it is characterized by a more or less pronounced ordering of the O vacancies within the average fluorite sublattice. We believe that ternary oxides with this kind of formula provide interesting degrees of flexibility for understanding the structural characteristic of these structures in the context of fission product stabilization in nuclear fuels and in the field of actinide waste forms as well. We would like to discuss the structural characteristics of the chemical bonds in ternary systems consisting of trivalent and hexavalent cations. Eventually, in compounds with generic formula Y6B+6O12, the B cation can be either a transition metal (W, Mo) or hexavalent uranium. We believe that studying the polyhedron of the sevenfold coordinated Y ion is particularly interesting to understand the flexibility of the chemical bonds: it is often described in the literature as a mono-capped trigonal prism, with bond lengths exhibiting an extremely large dispersion, ranging for instance from 2.19 to 2.70 Å in Y6WO12. We would discuss the implications of this large dispersion of distances on the chemical bond characteristics and compare DFT models with established experimental knowledge on pristine and irradiated specimens.

Adsorptive removal of Cd(II) from contaminated water via hexavalent molybdenum-containing layered double hydroxide: Ni/Mo-LDH

ABSTRACTThis article states the first application of nickel/molybdenum-layered double hydroxide (LDH) as a new adsorbent for the removal of cadmium ions [Cd(II)] from aqueous solutions. The essential structure of Ni/Mo-LDH of di and hexavalent cations was elucidated by X-ray photoelectron spectroscopy, X-ray diffraction, and Fourier transmission infrared spectroscopy. The maximum adsorption capacity of adsorbent is 53.6 mg g−1 at a temperature of 298 K, pH of 5.5, mass of 0.5 g L−1, time of 105 min, and initial concentration of 175 mg L−1. This adsorbent was distinguished by self and fast separation from adsorption media due to its layered, mesoporous structure and the presence of hexavalent molybdenum cation.

Diffusion of Zr, Hf, Nb and Ta in rutile: effects of temperature, oxygen fugacity, and doping level, and relation to rutile point defect chemistry

We performed experiments with thin film diffusion couples to simultaneously measure diffusion coefficients of Zr, Hf, Nb and Ta parallel to the a- and c-axes of synthetic rutile in a gas mixing furnace at controlled oxygen fugacity at temperatures between 800 and $$1100\,^{\circ }\hbox {C}$$ . Depth profiles of the diffusion couples were measured using secondary-ion mass spectrometry. Some of the diffusion profiles show a concentration dependence, which indicates different diffusion mechanisms above and below a particular trace-element concentration level ( $$\sim \,1000\,\upmu \hbox {g}/\hbox {g}$$ ). The diffusion coefficients for the mechanism dominant at high-concentration levels are approximately two orders of magnitude smaller than for the low-concentration mechanism. Below the critical concentration the diffusion coefficient is constant, as consistently shown in all of the experiments. For this diffusion coefficient we have found that $$D_{\text{Zr}} \sim D_{\text{Nb}}> D_{\text{Hf}}>> D_{\text{Ta}}$$ , and diffusion is isotropic for the four elements at all investigated T and $$f\hbox {O}_2$$ conditions. At $$1000\,^{\circ }\hbox {C}$$ for log $$f\hbox {O}_2 < $$ FMQ+1, the diffusion coefficients decrease with increasing oxygen fugacity where D is proportional to $$f\hbox {O}_2^n$$ with exponents $$n \approx -0.25$$ for Zr and Hf and $$n \approx -0.30$$ for Nb and Ta. Diffusivites of Nb and Ta strongly differ from each other at all investigated conditions, thus providing the potential to fractionate these geochemical twins, as suggested earlier. The present data and literature data for Zr and Ti self diffusion are interpreted and predicted based on published quantitative point defect models. Two end-member diffusion mechanisms were identified for impurity diffusion of Zr: (i) an interstitialcy mechanism involving $$\hbox {Ti}^{3+}$$ on interstitial sites, which is dominant at approximately log $$f\hbox {O}_2 < $$ FMQ+2; (ii) a vacancy mechanism involving Ti vacancies, which is dominant at approximately log $$f\hbox {O}_2> $$ FMQ+2. The point defect calculations also explain the observed effects of heterovalent substitutions, such as $$\hbox {Nb}^{5+}$$ for $$\hbox {Ti}^{4+}$$ at high concentration levels for changes in the diffusion mechanism and hence diffusion rates. In the case of rutile, this concentration effect becomes much more sensitive to the substitution level at lower temperature. In natural rutile penta- and hexavalent cations may largely be charge balanced by mono-, di- and trivalent cations, such that the doping effect on diffusion may be reduced or may even be reversed. The Arrhenius relationships established here may therefore not be directly applicable to natural rutile. We obtained the following Arrhenius relationships (with diffusion coefficients D in $$\hbox {m}^2/\hbox {s}$$ , $$f\hbox {O}_2$$ in Pascal and T in Kelvin), which are only applicable for log $$f\hbox {O}_2 < $$ FMQ+2: $$\begin{aligned} \log D_{\text{Zr}}= & {} (-0.40 \pm 0.47) + (-0.253 \pm {0.019}) \log \frac{f\text{O}_2}{10^{-7}} - \frac{414\pm 11\,\hbox {kJ/mol}}{\text{R}T \ln 10}\\ \log D_{\text{Hf}}= & {} (-0.08 \pm 0.63) + (-0.266 \pm 0.023) \log \frac{f\text{O}_2}{10^{-7}} - \frac{428\pm 15\,\hbox {kJ/mol}}{\text{R}T \ln 10}\\ \log D_{\text{Nb}}= & {} (-0.19 \pm 0.36) + (-0.294 \pm 0.014) \log \frac{f\text{O}_2}{10^{-7}} - \frac{421\pm 9 \,\hbox {kJ/mol}}{\text{R}T \ln 10}\\ \log D_{\text{Ta}}= & {} (0.45 \pm 0.73) + (-0.304 \pm 0.015) \log \frac{f\text{O}_2}{10^{-7}} - \frac{463\pm 18\,\hbox {kJ/mol}}{\text{R}T \ln 10} \end{aligned}$$

Recent advances in perovskites: Processing and properties

The perovskite structure is one of the most wonderful to exist in nature. It obeys to a quite simple chemical formula, ABX3, in which A and B are metallic cations and X, an anion, usually oxygen. The anion packing is rather compact and leaves interstices for large A and small B cations. The A cation can be mono, di or trivalent, whereas B can be a di, tri, tetra, penta or hexavalent cation. This gives an extraordinary possibility of different combinations and partial or total substitutions, resulting in an incredible large number of compounds. Their physical and chemical properties strongly depend on the nature and oxidation states of cations, on the anionic and cationic stoichiometry, on the crystalline structure and elaboration techniques, etc. In this work, we review the different and most usual crystalline representations of perovskites, from high (cubic) to low (triclinic) symmetries, some well-known preparation methods, insisting for instance, in quite novel and original techniques such as the mechanosynthesis processing. Physical properties are reviewed, emphasizing the electrical (proton, ionic or mixed conductors) and catalytic properties of Mn- and Co-based perovskites; a thorough view on the ferroelectric properties is presented, including piezoelectricity, thermistors or pyroelectric characteristics, just to mention some of them; relaxors, microwave and optical features are also discussed, to end up with magnetism, superconductivity and multiferroisme. Some materials discussed herein have already accomplished their way but others have promising horizons in both fundamental and applied research. To our knowledge, no much work exists to relate the crystalline nature of the different perovskite-type compounds with their properties and synthesis procedures, in particular with the most recent and newest processes such as the mechanosynthesis approach. Although this is not intended to be a full review of all existing perovskite materials, this report offers a good compilation of the main compounds, their structure and microstructure, processing and relationships between these features

Water decontamination via the removal of Pb (II) using a new generation of highly energetic surface nano-material: Co+2Mo+6 LDH

CoMo(CO32−) layered double hydroxide of a highly energetic surface, as a new LDH consisting of divalent and hexavalent cations (M+2/M+6-LDH), was prepared by a homogeneous co-precipitation method. The structure and morphology of the prepared material was confirmed by several analytical techniques namely; X-ray diffraction analysis (XRD), X-ray fluorescence (XRF), Fourier transform infra-red (FT-IR) spectroscopy, differential scanning calorimetry and thermal gravimetric analysis (DSC-TGA), N2 adsorption–desorption isotherm and scanning electron microscope (SEM). The highly energetic surface of the prepared LDH was demonstrated via the X-ray photoelectron spectroscopy (XPS). The surface energy is due to the formation of +4 surface charges in the brucite layer between Co+2 and Mo+6. The prepared LDH was applied as a novel adsorbent for the removal of Pb (II) from its aqueous solution at different experimental conditions of time, temperature and initial Pb (II) concentrations. The change of the Pb (II) concentrations; due to adsorption, was monitored by atomic absorption spectrophotometer (AAS). The maximum uptake of Pb (II) by the Co Mo LDH was (73.4mg/g) at 298K. The Pb (II) adsorption was found to follow Langmuir isotherm and pseudo second order model. The adsorption process was spontaneous and endothermic. The interference of other cations on the removal of the Pb (II) was studied. Na+ and K+ were found to increase the adsorption capacity of the Co Mo LDH toward Pb (II) while it was slightly decreased by the presence of Mn+2 and Cu+2. The synthesized LDH showed a great degree of recoverability (7 times) while completely conserving its parental morphology and adsorption capacity. The mechanism of the lead ions removal had exhibited more reliability through a surface adsorption by the coordination between the Mo+6 of the brucite layers and the oxygen atoms of the nitrates counter ions.

Extraction and Metalation of Porphyrins in Fluorous Liquids with Carboxylic Acids and Metal Salts

Porphyrins have found application in a remarkable variety of areas such as sensors, ion selective electrodes, photodynamic therapy, and energy-transfer systems. Here, we demonstrate the extraction of 5,10,15,20-tetraphenylporphyrin (TPhP) and 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) into a mixture of perfluorohexanes (FC-72) through noncovalent interactions with Krytox (1), a carboxylic acid terminated perfluoropolyether. We found that 1 transfers two protons to the TPhP tetrapyrrole ring to create the porphyrin dication (H2TPhP2+) in FC-72 while up to six protons are transferred to the TPyP pyridyl and tetrapyrrole nitrogens to create a hexavalent cation macrocycle in the fluorous phase. The total charge on TPyP is controlled by adjusting the concentration of 1 in the fluorous phase. In addition, we observed extraction of ZnTPyP from CDCl3 with 1/FC-72, while ZnTPhP is not extracted by 1/FC-72. We prepared the Zn salt of 1 and found that it extracts (from CDCl3) and metalates TPyP but not TPhP. Competitive binding between the porphyrins and an ethanol cosolvent hinders the extraction of both TPhP and TPyP and inhibits the formation of the TPyP hexacation in FC-72. By controlling the concentration of porphyrin, 1, and ethanol, it is possible to reversibly solubilize TPyP in the fluorous phase through noncovalent interactions between the pyridyl moieties and 1 while leaving the tetrapyrrole ring available to interact with metals or other substrates. In addition, both porphyrins and ZnTPyP are easily recovered from the fluorous phase using commercially available fluorous solid-phase extraction cartridges. Understanding noncovalent interactions in fluorous matrices should lead to development of more robust devices for sensing and energy transfer.

Sc2(WO4)3 and Sc2(MoO4)3 and Their Solid Solutions: 45Sc, 17O, and 27Al MAS NMR Results at Ambient and High Temperature

The mobilities of trivalent ions are generally considered to be low in solid oxides and thus trivalent ionic conductors are rare. Sc2(WO4)3 and related compounds have been described as Sc3+ ionic conductors. However, there has not been conclusive experimental evidence whether Sc3+ is the major or the only mobile species. Here, we report 45Sc, 17O, and 27Al MAS NMR studies at ambient and high temperature of Sc2(WO4)3-type compounds including Sc2(WO4)3, Sc2(MoO4)3 (scandium tungstate and scandium molybdate), and their solid solutions, as well as aluminum-doped Sc2(WO4)3. 45Sc MAS NMR spectra at ambient temperature of the Sc2(WO4)3−Sc2(MoO4)3 binary are consistent with random mixing of W6+ and Mo6+ on tetrahedral sites. 17O spectra show that the details of the end-member short-range structure are preserved throughout this solid solution, indicating that lattice distortion is minimized by the close similarity of the radii of the hexavalent cations. Al3+ substitutes only into the octahedral sites. At high temperatures (600−700 °C), only the 17O high spectra show noticeable line shape changes, whereas 45Sc high-temperature spectra maintain a constant peak width and shift in position only, suggesting that the oxide anion may be a more mobile species but not excluding the possibility of a minor number of mobile scandium ions. 17O high-temperature NMR data also suggest that oxide ions bonded to molybdenum ions may be more mobile than those in tungstate groups.

Tungsten-Doped TiO2 vs Pure TiO2 Photocatalysts: Effects on Photobleaching Kinetics and Mechanism

Sol gel prepared tungsten (VI) doped TiO2 powders were evaluated as photocatalysts for crystal violet (CV) bleaching under UV irradiation, in the presence of oxygen. The incorporation of the hexavalent cation affects both surface and bulk properties. The optimum activity observed results from the competition between higher surface areas and lower specific activities per adsorption site. At pH 4.0, doped particles share a common photodegradation mechanism, where the relative contribution of the aromatic structure rupture over the loss of terminal methyl groups (N-demethylation) is more relevant than in the case of non-doped titania. This behavior can be minimized shifting the pH into basic condition.

Preparation of new layered double hydroxide, Zn-Mo LDH

Layered double hydroxide (LDH) is prepared conventionally with bivalent and trivalent cations. We recently reported that the preparation of Zn-Ti LDH consisting of bi and tetravalent cations is possible through the decomposition of urea. In this study, Zn-Mo LDH consisting of bi and hexavalent cations were prepared and reacted with organic monocarboxylic and dicarboxylic acids. Interlayer spacing of the prepared LDH (Zn-Mo-CO3) contained carbonate anions between the layers was 0.72 nm. The spacing was small compared to usual LDH (Zn-Al-CO3) of 0.76 nm in the case of carbonate anion as the guest. Also, TG analysis indicated that the electrostatic force between the Zn-Mo layers and carbonate anions was larger than those of Zn-Al LDH. Certainly, the carbonate anions in Zn-Mo LDH decomposed at 260°C while they in usual LDH decomposed at 230–240°C. ESCA showed that Mo5+ had changed to Mo6+ through the preparation procedure. These results showed the preparation of layered double hydroxides consisting of bivalent and hexavalent cations. By the intercalation of Zn-Mo LDH with suberic acid at 60°C, a sharp peak was observed at 1.06 nm and the peak of LDH itself (0.72 nm) disappeared. This result has suggested that the intercalation of organic acid into new LDH was performed completely.

ATP from Subplasmalemmal Mitochondria Controls Ca2+-dependent Inactivation of CRAC Channels

A sustained Ca2+ entry is the primary signal for T lymphocyte activation after antigen recognition. This Ca2+ entry mainly occurs through store-operated Ca2+ channels responsible for a highly selective Ca2+ current known as I(CRAC). Ca2+ ions act as negative feedback regulators of I(CRAC), promoting its inactivation. Mitochondria, which act as intracellular Ca2+ buffers, have been proposed to control all stages of CRAC current and, hence, intracellular Ca2+ signaling in several types of non-excitable cells. Using the whole-cell configuration of the patch clamp technique, which allows control of the intracellular environment, we report here that respiring mitochondria located close to CRAC channels can regulate slow Ca2+-dependent inactivation of I(CRAC) by increasing the Ca2+-buffering capacity beneath the plasma membrane, mainly through the release of ATP.

Crystal structure and magnetism of the double perovskites Sr3Fe2TeO9 and Ba3Fe2TeO9: a neutron diffraction study

Sr3Fe2TeO9 and Ba3Fe2TeO9 double perovskites have been prepared in polycrystalline form by solid state reaction, in air. These materials have been studied by X-ray diffraction (XRD), neutron powder diffraction (NPD) and magnetic measurements. At room temperature, the crystal structure of Sr3Fe2TeO9 is tetragonal, space group I4/m, with a = b = 5.55902(4) A, c = 7.885(1) A, whereas Ba3Fe2TeO9 presents a hexagonal lattice, space group P63/mmc, with a = 5.7670(1) A, c = 14.1998(4) A. The structure of Sr3Fe2TeO9, which can ideally be rewritten as Sr2Fe(Fe1/3Te2/3)O6, can be described as the result of a single anti-phase tilting of the FeO6 and (Fe,Te)O6 octahedra along the c axis, exhibiting a tilting angle of 4.5° at RT. Ba3Fe2TeO9 shows a different stacking of the Fe/Te octahedra: the structure is constituted by dimer units of (Fe,Te)O6 octahedra sharing a face along the c axis; the dimers are connected, sharing corners, by a single layer of FeO6 octahedra. In both Sr and Ba compounds a certain degree of antisite disordering has been detected, implying the presence of about 15% Te at Fe positions. Magnetic measurements show the onset of ferrimagnetic ordering at relatively high temperatures of 717 and 711 K for the Sr and Ba compounds, respectively; however the magnetization isotherms at 2 K exhibit, for H = 50 kOe, maximum magnetization values close to 0.8 μB f.u.−1 and 0.35 μB f.u.−1 for Sr and Ba compounds respectively, although full saturation is not reached. The extremely weak magnetic scattering contribution observed on the low-temperature NPD patterns for Sr3Fe2TeO9 is in contrast with the well-established ferrimagnetic structures observed for other members of the Sr3Fe2B″O9 double perovskite series (B″ = U, Mo, W). This distinct behavior is discussed as a function of the chemical nature of the different B″ hexavalent cations.

Comparative study of the singularity in the optical properties of congruent doped LiNbO3 crystals

A comparison of the optical properties such as optical absorption band edge, ordinary and extraordinary refractive indices, powder second harmonic generation and the non-linear coefficient, d33, of congruent LiNbO3 crystals doped with di-, tri- and hexa-valent cations is reported. It has been found that the referred parameters show a clear concentration singularity depending on the valency of the cation. A charge compensation mechanism is proposed to explain the role of the cation. The experimental data of no and ne are described by a generalised Sellmeier equation considering the different ions sites in the crystal.

Natural and synthetic compounds with kröhnkite-type chains: review and classification

Abstract The crystal structure of kröhnkite [Na2Cu(SO4)2·2H2O], which contains infinite chains composed of CuO6 octahedra corner-linked with SO4 tetrahedra, was originally determined 1952 by Dahlman. Since then, a large number of both minerals (e.g., collinsite, fairfieldite, etc.) and synthetic compounds with closely related struc-tures have been investigated, but the structural relation was not always recognised. In the present review we compare the structures of AnM(XO4)2·2H2O (A = mono- or divalent cation, n = 1, 2; M = di- or trivalent cation; X = penta- or hexavalent cation) compounds with kröhnkite-type infinite chains, and propose a structural classification into six types. The rod group symmetries of the chains are compared. Furthermore, the linkage of these chains to double chains (in krausite-type compounds) and sheets (in yavapaiite-, merwinite-, bafertisite-type and similar structures) is discussed, and the occurrence of octahedral-tetrahedral chains topologically identical to those in kröhnkite (referred to as kröhnkite-type) and similar chains (referred to as kröhnkite-like) in several related compounds is pointed out.

A Novel Uranyl Sulfate Cluster in the Structure of Na6(UO2)(SO4)4(H2O)2

The sodium uranyl sulfate hydrate, Na6(UO2)(SO4)4(H2O)2, has been synthesized and the crystal structure has been determined by single-crystal X-ray diffraction. The structure crystallizes in space group P1, a=5.5503(5), b=11.2456(9), and c=14.256(1) Å, α=91.483(2), β=92.583(2), and γ=97.588(2)°, V=880.6(2) Å3, Z=2, and has been refined on the basis of F2 for all unique data collected with monochromatic MoKα X-radiation and a CCD-based detector to an agreement factor (R1) of 3.38%, calculated using 5617 unique observed reflections (∣Fo∣≥4σF). The structure contains a novel uranyl sulfate cluster with composition [(UO2)(SO4)4]6− that is composed of a uranyl pentagonal bipyramid and four sulfate tetrahedra. Three sulfate tetrahedra are linked to the uranyl pentagonal bipyramid by the sharing of vertices, and the other shares an equatorial edge of the uranyl pentagonal bipyramid. This is the second structure known to involve the sharing of an edge of a uranyl polyhedron with a tetrahedron containing a hexavalent cation. The uranyl sulfate clusters are linked through bonds to Na cations, and by H bonding.

EXSOLUTION AND BREAKDOWN OF SCANDIAN AND TUNGSTENIAN Nb Ta Ti Fe Mn PHASES IN NIOBIAN RUTILE

Niobian rutile commonly exsolves titanian ferrocolumbite to manganocolumbite or ixiolite with elevated contents of Nb, Mn and Sc, W or Sn, but further breakdown of the exsolved phase has not been reported to date. Niobian rutile from Eptevann, southern Norway, exsolved titanian-scandian phase E1 of the A 3+ B O 4 type, which subsequently broke down to a (Sc,Mn)-enriched and U-depleted phase E2 (11 to 12.8 wt.% Sc 2 O 3 ), a Ti-rich but (Sc,Mn,U)-poor phase E3, and a (U,Y)-rich but (Ti,Sc,Mn)-depleted, metamict and hydrous phase X (2.5 to 3 wt.% UO 2 ). Niobian rutile from Ilmen Mountains, Russia, exsolved a (Ti,U)-poor, Mn-rich tungstenian ixiolite (10 to 14 wt.% WO 3 ), which subsequently broke down into a Ti-poor but W-rich phase (~30 wt.% WO 3 ) and a U-enriched but W-depleted mineral (~6 wt.% WO 3 , 0.7 to 1.5 wt.% UO 2 ). The second-stage breakdown of the exsolved phases reflects the metastable nature of these minerals, which feature disordered populations of divalent to hexavalent cations in an exclusively octahedral array of oxygen anions, with consequent long-range disorder and local imbalances of electrostatic charges. Under low-temperature conditions, insufficient for further exsolution and ordering, the exsolved phases readily respond to the action of aqueous fluids by breakdown to more cation-selective phases. However, incomplete separation of different categories of cations and non-integral stoichiometries of the products suggest that they did not attain equilibrium.

The effect of Ca-Tschermaks component on trace element partitioning between clinopyroxene and silicate melt

We have studied the influence of Ca-Tschermaks (Calcium Tschermaks or CaTs) content of clinopyroxene on the partitioning of trace elements between this phase and silicate melt at fixed temperature and pressure. Ion probe analyses of experiments carried out in the system Na 2O–CaO–MgO–Al 2O 3–SiO 2, at 0.1 MPa and 1218°C, produced crystal-melt partition coefficients ( D) of 36 trace elements (Li, Cl, Sc, Ti, V, Cr, Fe, Co, Ge, Sr, Y, Zr, Nb, Mo, Ru, Rh, In, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta and W), for clinopyroxene compositions between 10 and 32 mol% CaTs. Partition coefficients for 2+ to 5+ cations show, for each charge, a near parabolic dependence of log D on ionic radius of the substituting cation, for partitioning into both the M1 and M2 sites of clinopyroxene. Fitting the results to the elastic strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal-melt partition coefficients from elastic moduli. Nature 372, 452–454] we obtain results for the strain-free partition coefficients of theoretical cations ( D 0), with site radius r 0, and for the site's Young's Modulus ( E). In agreement with earlier data our results show that increasing ivAl concentration in cpx is matched by increasing D, E M1, E M2 and D 0 for tri-, tetra- and pentavalent cations. The degree of fractionation between chemically similar elements (i.e. Ta/Nb, Zr/Hf) also increases. In contrast, D values for mono-, di- and hexavalent cations decrease with increasing ivAl in the cpx. The large suite of trace elements used has allowed us to study the effects of cation charge on D 0, r 0 and E. We have found that D 0 and r 0 decrease with increasing cation charge, e.g. r 0=0.66 Å for 4+ cations and 0.59 Å for 5+ cations substituting into M1. Values of E M1 and E M2 increase with cation charge as well as with increasing ivAl content. The increase in E M2 is linear and close to the trend set by Hazen and Finger [Hazen, R.M., Finger, L.W., 1979. Bulk modulus-volume relationship for cation–anion polyhedra. J. Geophys. Res. 84 (10) 6723–6728] for oxides. E M1 values are much higher and do not fit the trend predicted by the Hazen and Finger relationship.

Trivalent Ion Conduction in Molybdates Having Sc2(WO4)3-Type Structure

A molybdate series with Sc2(WO4)3-type structure, which has been demonstrated to be suitable for trivalent ion conduction, was prepared, and the trivalent ion conducting characteristics of this series were compared with those of the tungstate series. The ionic conductivities of the molybdate series become considerably higher in comparison to those of the tungstate series. Since the ionic radius of Mo6+ is smaller than that of W6+, the oxide anions that form a tetrahedron unit with Mo6+ ion form stronger bonds than those that form a tetrahedron unit with W6+. Stronger hexavalent cation and oxide anion bonding develops more suitable surroundings for trivalent ion migration in the Sc2(WO4)3-type structure. The trivalent ion conducting properties in the molybdate solids were directly characterized by dc electrolysis and EPMA measurements.

Point defect analysis and microstructural effects in pure and donor-doped ceria

Ceria (CeO 2− x ) can be made electronically conductive by doping with pentavalent or hexavalent cations. A technique has been developed, within the framework of standard Kröger-Vink defect analysis, by which the concentrations of three coexisting major defect species (pentavalent impurity, free electrons, and one of several possible point defects) can be modeled in ceria or other MO 2-type oxides. This analysis provides diagnostic information regarding the dominant defect behavior that cannot be obtained from the standard Brouwer approximation. Comparisons of microstructures and electrical conductivity were made between Nb-doped ceria synthesized by a conventional mixed-oxide route and by hydroxide coprecipitation. The mixed oxide materials contained silicate intergranular phases that acted as sinks for the dopant. Coprecipitated materials were virtually free of intergranular contamination and exhibited electrical conductivities approximately 25% higher than the mixed oxide materials. Finally, experimental results on the electrical conductivity of Nb-, Ta-, and W-doped ceria indicate that all are electronic conductors, with the W-doped materials showing the highest overall conductivity in air in the temperature range 600–900°C.