Substitution Of Trivalent Cations Research Articles

Theoretical Investigation of the Role of Anion and Trivalent Cation Substitution in the Physical Properties of Lead-Free Zero-Dimensional Perovskites

Theoretical Investigation of the Role of Anion and Trivalent Cation Substitution in the Physical Properties of Lead-Free Zero-Dimensional Perovskites

Synthesis, Characterization, and Crystal Structure of Dominant Uranium(V) Brannerites in the UTi2–xAlxO6 System

The synthesis, characterization, and crystal structure of a novel (dominant) uranium(V) brannerite of composition U1.09(6)Ti1.29(3)Al0.71(3)O6 is reported, as determined from Rietveld analysis of the high-resolution neutron powder diffraction data. Examination of the UTi2-xAlxO6 system demonstrated the formation of brannerite-structured compounds with varying Al3+ and U5+ contents, from U0.93(6)Ti1.64(3)Al0.36(3)O6 to U0.89(6)Ti1.00(3)Al1.00(3)O6. Substitution of Al3+ for Ti4+, with U5+ charge compensation, resulted in near-linear changes in the b and c unit cell parameters and the overall unit cell volume, as expected from ionic radii considerations. The presence of U5+ as the dominant oxidation state in near-single-phase brannerite compositions was evidenced by complementary laboratory U L3-edge and high-energy-resolution fluorescence-detected U M4-edge X-ray absorption near-edge spectroscopy. No brannerite phase was found for compositions with Al3+/Ti4+ > 1, which would require a U6+ contribution for charge compensation. These data expand the crystal chemistry of uranium brannerites to the stabilization of dominant uranium(V) brannerites by the substitution of trivalent cations, such as Al3+, on the Ti4+ site.

Mineralogy, Geochemistry and Raman Spectroscopy of Multi-Genesis Serpentine Polymorphs of Darepahn Ophiolites

The present article pays to some Alpine-Himalayan ophiolites of Late Cretaceous age from Darepahn area in the southern part of Nain-Baft Ophiolite belt. Whole rock processed data clearly shows that the metaperidotite rocks are silica-poor with low CaO, Al2O3, and TiO2 that reflect the predominance of olivine over calcic pyroxene in the protolithes. The considerable content of Co (85.6–124.6 ppm) and very low contents of Sc (4.3–15.3 ppm) and Sr (0.7-14 ppm) provided more evidence for olivine enrichment in the rock parentages. Calculating numbers of cation per formula unites based on EMPA results for distinguished Serpentine polymorphs (classified structurally via Raman spectroscopy) display that Tschermak substitution of trivalent cations for lizardite is tetrahedrally, and for chrysotile and antigorite, it is octahedrally. These chemical differentiations, lead to identifying chemical field plots in separator diagrams between SiO2, MgO, H2O, and Al2O3. Lizardite polymorphs can consider as primarily metasomatism results thru an iron-rich fluid occurrence. It has a considerable occupation of trivalent cations (especially Fe3+) and simultaneous less than 4 Si atoms in formula structure that reflected tetrahedral Tschermak substitutions. Antigorites have some deformed recrystallized textures along with high Σoct (Total Octahedral Cations) and low H2O content that could be considered as thermal originated polymorph. Chrysotiles have bimodal MgO and FeO contents that return to magnetite formations in groundmass and octahedral Tschermak substitutions on late-stage veins. The results exemplify compositional variations of Darepahn Serpentine polymorphs as a function of textural behaviors, structural position and thermodynamic condition of the formation process.

Bandgap engineering of a lead-free defect perovskite Cs3Bi2I9 through trivalent doping of Ru3.

Inorganic defect halide compounds such as Cs3Bi2I9 have been regarded as promising alternatives to overcome the instability and toxicity issues of conventional perovskite solar cells. However, their wide indirect bandgaps and deep defect states severely limit their photoelectronic conversion efficiency when implemented in devices. Trivalent cation substitution has been proposed by previous calculations allowing the engineering of their band structures, but experimental evidences are still lacking. Herein we use the trivalent cation Ru3+ to partially replace Bi3+ in Cs3Bi2I9, and reveal their structural and optoelectronic properties, as well as the environmental stability. The Ru-doped Cs3Bi2I9 shows a decreasing bandgap with the increasing doping levels and an overall up-shift of band structure, owing to the dopant-induced defect states and thus enhanced phonon–electron coupling. As a result, upon Ru3+ doping, the narrowed bandgap and the upward shift of the band structures might facilitate and broaden their applications in optoelectronic devices.

Cation Substitution Effect on a Molecular Analogue of Perovskite Manganites

A recently synthesized Mn-containing molecule Ce2IIICeIVMn8IIIO8(O2CPh)18(HO2CPh) (Ce3Mn8III) structurally resembles the repeating unit of a bulk perovskite manganite. This resemblance brings forth the intriguing possibility of studying physical properties and effects that appear in both the molecule and the far more complex bulk perovskites, for example, effects of cation substitution, which in bulk manganites may lead to phase transition from metallic-ferromagnetic to insulating-paramagnetic as well as the colossal magnetoresistance effect. We investigate divalent and trivalent cation substitution in Ce3Mn8III molecules and its effects on ground-state magnetic configuration using first-principles-based approaches. One MnIII ion changes its valence to MnIV after trivalent (including La, Gd) cation substitution, while four MnIII ions become MnIV upon divalent (including Ca, Sr, Ba, and Pb) cation substitutions, all accompanied by vanishing local Jahn–Teller distortion around MnIV. The valence state of MnIV induced by cation substitutions can hop among Mn sites in molecule, and the calculated energy barrier for such state hopping is 0.25 to 0.6 eV/molecule. In addition, the charging energies of the Ce3Mn8III molecule and its derivatives are found to be strongly dependent on the spin direction of added electron.

Ceramic Polymer Composite Electrolytes (CPCE) for Wearable Metal Battery

For a battery used in a volume constrained system such as a wearable electronic, the volumetric packaging cost is significant and most of the packaging structure is dedicated to encapsulating sensitive electrolytes, aqueous or aprotic. Solid electrolytes are a path forward that provides battery performance durability, and safety while facilitating the use of low cost – low weight – high flexibility battery packaging materials that translate to low cost, high-density, conformal, wearable batteries. The full potential of solid electrolytes can only be realized if improvements in processing cost and stability with ultra high energy systems is improved resulting in a disruptive energy density beyond the Li-ion technology. In particular, research and development of solid-state flexible electrolytes capable of coping with massive structural reconfigurations required in next-generation electrodes such as Si, SnO2, sulfur is needed [1]. Lithium superionic conductor (LISICON)-type solid electrolytes are the most promising solid electrolytes for lithium batteries. The basic structure for LISICON (LiA2 IV(PO4)3 (AIV = Ge, Ti, Si, Zr, etc.) structure can be described as a covalent skeleton [A2P3O12]- constituted of AO6 octahedra and PO4 tetrahedra, which form 3D interconnected channels and two types of interstitial positions (M’ and M’’) where conductor cations (Li+) are distributed. The structural and electrical properties of LISICON-type compounds vary with the composition of the framework. The smallest unit cell has been obtained for LiGe2(PO4)3. By substitution of trivalent cations (Al, Cr, Ga, Fe, Sc, etc) for tetravalent cations (Ge4+ or Ti4+) in the octahedral sites, a high bulk conductivity of 3 x 10-3 S/cm at 25°C with (Li1.3Al0.3Ti1.7(PO4)3 (LATP) [2] and 5 x 10-3 S/cm with (Li1.3Al0.3Ge1.7(PO4)3 (LAGP) [3] was achieved. In addition to high Li+ ion conductivity, LAGP or LATP combines many favorable properties such as solid-state nature, broad electrochemical window (>5 V), negligible porosity and single ion conduction (high transference number, no dendrite formation, no cross-over of electrode materials to opposite side of electrodes compartment, etc.) enabling high-energy battery chemistries and circumventing safety and packaging issues of conventional lithium batteries [4]. While the pursuit of high ionic conductivity through innovative chemical structures remains as the momentum for novel lithium-ion conductors, novel material processing techniques are needed. Processing techniques have the potential to address the fact that these ceramics are brittle in nature and their thin film processing is elusive. Processing methods which can create thin (~30 micron), flexible, scalable, solid electrolytes are highly desirable for both high energy battery cells and batteries that need full conformability and flexibility. We will present novel methods such as extrusion, solution processing, electrospinning, and polymer grafting on NASICON ceramics, to develop Ceramic Polymer Composite Electrolytes (CPCE)films for conformal and wearable lithium sulfur batteries. Structural, thermal, electrical, electrochemical characterization of the CPCE will be presented. Battery performance of wearable battery cells based on free-standing and flexible electrolyte (CPCE), anode (lithiated Silicon (Si) or Germanium (Ge) nanoparticles imbedded in graphene matrix) and cathode (S + Carbon/graphene composite) coated on carbon fiber mat) will be presented and discussed.

Beneficial effects of substituting trivalent ions in the B-site of La0.5Sr0.5Mn1-xAxO3 (A = Al, Ga, Sc) on the thermochemical generation of CO and H2 from CO2 and H2O.

The effect of substitution of Al(3+), Ga(3+) and Sc(3+) ions in the Mn(3+) site of La0.5Sr0.5MnO3 on the thermochemical splitting of CO2 to generate CO has been studied in detail. Both La0.5Sr0.5Mn1-xGaxO3 and La0.5Sr0.5Mn1-xScxO3 give high yields of O2 and generate CO more efficiently than La0.5Sr0.5Mn1-xAlxO3 or the parent La0.5Sr0.5MnO3. Substitution of even 5% Sc(3+) (x = 0.05) results in a remarkable improvement in performance. Thus La0.5Sr0.5Mn0.95Sc0.05O3 produces 417 μmol g(-1) of O2 and 545 μmol g(-1) of CO, respectively, i.e. 2 and 1.7 times more O2 and CO than La0.5Sr0.5MnO3. This manganite also generates H2 satisfactorily by the thermochemical splitting of H2O.

Enhancing photoluminescence performance of SrSi2O2N2:Eu(2+) phosphors by Re (Re = La, Gd, Y, Dy, Lu, Sc) substitution and its thermal quenching behavior investigation.

Eu(2+)-doped SrSi2O2N2 has recently been identified as a viable green phosphor that in conjunction with a blue-emitting diode can be used in solid-state white-lighting sources. In this study, we attempt to improve the photoluminescence and thermal quenching behavior by codoping Re(3+) (Re = La, Gd, Y, Dy, Lu, Sc) and Li(+) instead of Sr(2+). Trivalent cation substitution at the Sr(2+) site enhances the photoluminescence intensities and also achieves better thermal stability at high temperature. The lifetime decay properties in the related substituted phosphors are investigated. Furthermore, under the 460 nm blue light irradiation, this green phosphor exhibits excellent luminescence properties with absorption and internal/external efficiencies. High-color-rendition warm-white LEDs using the phosphor have the color temperature and color rendition of 4732 K and 91.2, respectively, validating its suitability for use in solid-state white lighting.

Ceramic Polymer Composite Electrolytes (CPCE) for a Solid-State, Conformal, Wearable Battery

Wearable and conformable electronics are being developed with increasing capabilities for man portable, light weight, low volume systems; however increased capability often comes with increased power demands. For a battery used in a volume and weight constrained system such as a wearable electronic, the volumetric packaging cost is significant and most of the packaging structure is dedicated to encapsulating sensitive liquid electrolytes, aqueous or aprotic. Solid electrolytes are a path forward that provides battery performance durability, and safety while facilitating the use of low cost – light weight – mechanically flexibile battery packaging materials that translate to low cost, high-density, conformal, wearable batteries. The full potential of solid electrolytes can only be realized if improvements in processing cost and stability with ultra high energy systems results in disruptive energy density beyond the Li-ion technology. In particular, research and development of solid-state flexible electrolytes capable of coping with massive structural reconfigurations required in next-generation electrodes such as Si, SnO2, sulfur is needed[1]. Lithium superionic conductor (LISICON)-type solid electrolytes are the most promising solid electrolytes for lithium batteries. The basic structure for LISICON (LiA2 IV(PO4)3 (AIV = Ge, Ti, Si, Zr, etc.) structure can be described as a covalent skeleton [A2P3O12]- constituted of AO6 octahedra and PO4 tetrahedra, which form 3D interconnected channels and two types of interstitial positions (M’ and M’’) where conductor cations (Li+) are distributed. The structural and electrical properties of LISICON-type compounds vary with the composition of the framework. The smallest unit cell has been obtained for LiGe2(PO4)3. By substitution of trivalent cations (Al, Cr, Ga, Fe, Sc, etc) for tetravalent cations (Ge4+ or Ti4+) in the octahedral sites, a high bulk conductivity of 3 x 10-3 S/cm at 25°C with (Li1.3Al0.3Ti1.7(PO4)3 (LATP) [2] and 5 x 10-3 S/cm with (Li1.3Al0.3Ge1.7(PO4)3 (LAGP) [3] was achieved. In addition to high Li+ ion conductivity, LAGP or LATP combines many favorable properties such as solid-state nature, broad electrochemical window (>5 V), negligible porosity and single ion conduction (high transference number, no dendrite formation, no cross-over of electrode materials to opposite side of electrodes compartment, etc.) enabling high-energy battery chemistries and circumventing safety and packaging issues of conventional lithium batteries [4]. While the pursuit of high ionic conductivity through innovative chemical structures remains as the momentum for novel lithium-ion conductors, novel material processing techniques are needed. Processing techniques have the potential to address the fact that these ceramics are brittle in nature and their thin film processing is elusive. Processing methods which can create thin (~30 micron), flexible, scalable, solid electrolytes are highly desirable for both high energy battery cells and batteries that need full conformability and flexibility. We will present novel methods such as extrusion, solution processing, electrospinning, and polymer grafting on NASICON ceramics, to develop Ceramic Polymer Composite Electrolytes (CPCE)films for conformal and wearable lithium batteries. Structural, thermal, electrical, electrochemical characterization of the CPCE will be presented. Battery performance of wearable battery cells based on free-standing and flexible electrolyte (CPCE), anode (Silicon (Si) or Germanium (Ge) nanoparticles imbedded in graphene matrix) and cathode (LiNi0.5Mn1.5O4 (LNMO) or lithium cobalt oxide (LCO) coated on carbon fiber mat) will be presented and discussed.

Charge Compensation in Trivalent Doped Ca3(SiO4)Cl2

ABSTRACTCalcium chlorosilicate (Ca3(SiO4)Cl2) is seen as a potential host phase for the immobilization of Cl-rich wastes arising from pyrochemical reprocessing, a waste stream often containing a mix of both di- and trivalent cations. Substitution of trivalent cations into the lattice requires some form of charge compensation to ensure the lattice remains charge neutral overall. Whilst previous work has only examined this through the formation of Ca vacancies, this study investigates the feasibility of charge-balancing via the substitution of a monovalent cation onto the Ca sites of the lattice. To that end, a series of static lattice calculations were performed to determine the site selectivity of monovalent cations of differing size when substituted onto the Ca sites of the calcium chlorosilicate lattice and the solution energies for the overall substitution processes compared with those for charge compensation via vacancy formation. In all cases the monovalent charge-balancing species shows a clear preference for substitution onto the Ca1 site in the calcium chlorosilicate lattice. The solution energy of the substitution process increases with the increasing ionic radii of both the mono- and trivalent species as the steric stresses associated with substitution of larger cations than the Ca2+ host increase. As such, only charge-balancing using Li+, Na+ or K+ is more favourable than via formation of a Ca vacancy.

Basicity–FAME yield correlations in metal cation modified MgAl mixed oxides for biodiesel synthesis

In this paper Zn2+, Co2+, Fe3+ and La3+ were respectively introduced into Mg3Al1 mixed oxides to prepare Mg1Zn2Al1, Mg1Co2Al1, Mg3Al0.6Fe0.4 and Mg3Al0.6La0.4 oxides by calcinations at 773K. The XRD, XPS, BET, ICP-AES and CO2-TPD characterizations of samples exhibited that the effects of the substitution of trivalent cations (Fe3+, La3+) for Al3+ on the basicity of samples were stronger than the substitution of divalent cations (Zn2+, Co2+) for Mg2+ because of high O2− concentration which were coincident with the FAME yield for these catalysts. La3+ modified catalyst exhibited highest activity in transesterification.

Stabilization of the perovskite structure in Co-based mixed conductors by tri-valent cation substitution

Abstract For Co-based perovskite mixed conductors, the substitution of Co by cations with a higher valence than Co has been known to be effective in stabilizing the cubic perovskite structure. Nevertheless, such a substitution may reduce the number of the oxygen vacancies and be unfavorable for increasing oxygen permeability. In order to prepare cubic perovskite mixed conductors with greater oxygen permeability, the effect of substituting the tri-valent cation, In3 +, in the Co-based system (Ba,Sr)(Co,In)O3-δ, was investigated and compared to the effect of substituting Y, Al, and Fe. For (BaxSr1-x)(Co0.9In0.1)O3 − δ and (BaxSr1 − x)(Co0.9Y0.1)O3 − δ, cubic perovskite single phase was found to be attained at high Ba concentrations of x = 0.9 and 1.0 ≥ x ≥ 0.5. This differed from the cases of (BaxSr1 − x)(Co0.9Al0.1)O3 − δ and (BaxSr1 − x)(Co0.9Fe0.1)O3 − δ in which pure perovskite appeared in the low Ba concentration ranges of 0.2 ≥ x ≥ 0 and 0.5 ≥ x ≥ 0, respectively. A disk sample of (Ba0.9Sr0.1)(Co0.9In0.1)O3 − δ with 1 mm thickness showed high oxygen permeability of 5.69 cm3/min/cm2 at 900 °C, which was 24% higher than the permeability of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3 − δ.

Stabilization of cubic perovskite structure upon trivalent cation substitution in Co-based mixed conductors (Ba Sr1-)(Co0.9Y0.1)O3-δ

Abstract The stabilization of perovskite through trivalent cation substitution is a promising strategy for the development of oxygen permeation membranes. In this study, perovskite stabilization through yttrium substitution in (Ba x Sr 1- x )(Co 0.9 Y 0.1 )O 3-δ was investigated and compared with that of Al. A perovskite single phase of (Ba x Sr 1- x )(Co 0.9 Y 0.1 )O 3-δ was attained at a Ba concentration of 1.0 ≥ x ≥ 0.5, which was higher than the range at which the hexagonal polymorph occurred. This differed from (Ba x Sr 1- x )(Co 0.9 Al 0.1 )O 3-δ in which the perovskite occurred at a lower Ba concentration than the hexagonal polymorph. In addition, (Ba 0.9 Sr 0.1 )(Co 0.9 Y 0.1 )O 3-δ showed good oxygen permeability on a level similar to that of (Ba 0.5 Sr 0.5 )(Co 0.8 Fe 0.2 )O 3-δ .

Neighboring-Cation Substitution Tuning of Photoluminescence by Remote-Controlled Activator in Phosphor Lattice

Highly efficient red phosphors with superior intrinsic properties that are excited by ultraviolet or blue light-emitting diodes are important white light sources for our daily life. Nitride-based phosphors, such as Sr2Si5N8:Eu(2+) and CaAlSiN3:Eu(2+), are commonly more red-shifted in photoluminescence and have better thermal/chemical stability than oxides. Cation substitutions are usually performed to optimize photoluminescence and thermal quenching behavior. However, the underlying mechanisms are unclear in most cases. Here we show that neighboring-cation substitution systematically controls temperature-dependent photoluminescence behavior in CaAlSiN3:Eu(2+) lattice. Trivalent cation substitution at the Ca(2+) site degrades the photoluminescence in high-temperature environments but achieves better thermal stability when the substituted cation turns monovalent. The neighboring-cation control of lifetime decay is also observed. A remote control effect that guides Eu(2+) activators in selective Ca(2+) sites is proposed for neighboring-cation substitution while the compositional Si(4+)/Al(3+) ratio adjusts to the valence of M(n+) (n = 1-3) cation. In the remote control effect, the Eu(2+) activators are surrounded with nitride anions which neighbor with M(3+)-dominant and Si(4+)/Al(3+)-equivalent coordination when M is trivalent, but shift to the site where surrounded nitride anions neighbor with M(+)-dominant and Si-rich coordination when M is monovalent. This mechanism can efficiently tune optical properties, especially thermal stability, and could be general to luminescent materials, which are sensitive to valence variation in local environments.

Hydrogen incorporation in crystalline zircon: Insight from ab initio calculations

The OH stretching vibration frequencies of crystalline zircon that contains water have been investigated by quantum mechanical calculation using CRYSTAL09 and several hybrid functionals. Incorporation mechanisms considered for H in zircon include: (1) hydrogarnet and partial hydrogarnet-type substitution; (2) H-compensated trivalent substitution at the Si site; and (3) H-compensated trivalent substitution at the Zr site. The results provide a clear picture of the H locations in zircon and their associated IR stretching vibrations. Based on the results of structural relaxation and corresponding OH stretching calculations, we can assign the 3420 cm −1 band to a hydrogarnet or partial hydrogarnet-type substitution with polarization dependent on the extent of local relaxation (i.e., closer to E || c for partial hydrogarnet and less polarized for hydrogarnet). These results also show that the 3385 cm −1 ( E ⊥ c ) IR band originates from H incorporation that accompanies a trivalent cation substitution at the Si-site. As an example of such a substitution, the [AlO 4 /H] defect was investigated. H-incorporation coupled with a trivalent cation substitution at the Zr-site appears to be responsible for generating the 3180 cm −1 ( E ⊥ c ) IR band. The ab initio calculation on [YO 8 /H] and [LaO 8 /H] structures show that the 3180 cm −1 band represents an average between two distinct bands at ~3220–3250 and ~3100 cm −1 attributed to H locations close to or further away from the Zr-substituted atom. The energetics of these substitutions demonstrate that the overlapping of the two IR bands depends on the substituting cation, and this is controlled, in part, by the thermal history of the sample and/or by the experimental conditions under which the IR spectra are obtained. Furthermore, these results predict for the first time the existence of a peroxy-type O bond associated with the partial hydrogarnet substitution in the zircon structure that now accounts for the ~800 cm −1 bands.

Li isotopes and trace elements as a petrogenetic tracer in zircon: insights from Archean TTGs and sanukitoids

We report d 7 Li, Li abundance ((Li)), and other trace elements measured by ion probe in igneous zircons from TTG (tonalite, trondhjemite, and granodiorite) and sanukitoid plutons from the Superior Province (Canada) in order to characterize Li in zircons from typical Archean continental crust. These data are compared with detrital zircons from the Jack Hills (Western Australia) with U-Pb ages greater than 3.9 Ga for which parent rock type is not known. Most of the TTG and sanukitoid zircon domains preserve typical igneous REE patterns and CL zoning. (Li) ranges from 0.5 to 79 ppm, typical of (Li) in continental zircons. Atomic ratios of (Y ? REE)/(Li ? P) average 1.0 ± 0.7 (2SD) for zircons with magmatic composition preserved, supporting the hypothesis that Li is interstitial and charge compensates substitution of trivalent cations. This substitution results in a relatively slow rate of Li diffusion. The d 7 Li and trace element data constrain the genesis of TTGs and sanukitoids. (Li) in zircons from granitoids is significantly higher than from zircons in primitive magmas in oceanic crust. TTG zircons have d 7 Li (3 ± 8%) and d 18 O in the range of primitive mantle- derived magmas. Sanukitoid zircons have average d 7 Li (7 ± 8%) and d 18 O higher than those of TTGs supporting genesis by melting of fluid-metasomatized mantle wedge. The Li systematics in sanukitoid and TTG zircons indicate that high (Li) in pre-3.9-Ga Jack Hills detrital zircons is a primary igneous composition and suggests the growth in proto-continental crust in magmas similar to Archean granitoids.

Catalli, Sundberg receive Mineral and Rock Physics Graduate Research Awards

Krystle Catalli and Marshall Sundberg have been awarded the 2010 Mineral and Rock Physics Graduate Research Award, given annually to one or more promising young scientists for outstanding contributions achieved during their Ph.D. research. Recipients of this award are engaged in experimental and/or theoretical studies of Earth and planetary materials with the purpose of unraveling the physics and chemistry that govern their origin and physical properties. Catalli's thesis is entitled “The effect of trivalent cation substitution on the major lower mantle silicates.” Sundberg's thesis is entitled “Chemical interactions amongst phases during diffusion creep: Applications to the Earth's upper mantle.”

Lithium in Jack Hills zircons: Evidence for extensive weathering of Earth's earliest crust

In situ Li analyses of 4348 to 3362 Ma detrital zircons from the Jack Hills, Western Australia by SIMS reveal that the Li abundances (typically 10 to 60 ppm) are commonly over 10,000 times higher than in zircons crystallized from mantle-derived magmas and in mantle-derived zircon megacrysts (typically < 2 ppb). High Li concentrations in zircons (10 to 250 ppm) have also been found in igneous zircons from three continental parent rocks: granites, Li-rich pegmatites, and migmatites in pelitic metasediment. The substitution of trivalent cations (REEs and Y) in zircon correlates with Li + 1 and P + 5 , suggesting that an interstitial site for Li, as well as the xenotime substitution for P, provides charge balance for REEs. Li is thus fixed in the zircon structure by coupled substitutions, and diffusive changes in [Li] composition are rate-limited by slow diffusion of REEs. The Jack Hills zircons also have fractionated lithium isotope ratios ( δ 7Li = − 19 to + 13‰) about five times more variable than those recorded in primitive ocean floor basalts (2 to 8‰), but similar to continental crust and its weathering products. Values of δ 7Li below − 10‰ are found in zircons that formed as early as 4300 Ma. The high Li compositions indicate that primitive magmas were not the source of Jack Hills zircons and the fractionated values of δ 7Li suggest that highly weathered regolith was sampled by these early Archean magmas. These new Li data provide evidence that the parent magmas of ancient zircons from Jack Hills incorporated materials from the surface of the Earth that interacted at low temperature with liquid water. These data support the hypothesis that continental-type crust and oceans existed by 4300 Ma, within 250 million years of the formation of Earth and the low values of δ 7Li suggest that weathering was extensive in the early Archean.

Three water sites in upper mantle olivine and the role of titanium in the water weakening mechanism

Infrared spectroscopy on synthetic olivines has established that there are at least four different mechanisms by which hydrogen is incorporated into the crystal structure. Two mechanisms occur in the system MgO‐SiO2‐H2O associated with silicon and magnesium vacancies, respectively. A third mechanism is associated with trivalent cation substitution, commonly Fe3+ in natural olivine, while the fourth mechanism, which is the one most prevalent in natural olivines from the spinel‐peridotite facies of the Earth's upper mantle, is associated with Ti4+ [Berry et al., 2005]. Here first principles calculations based on density functional theory are used to derive the structure and relative energies of the two defects in the pure MgO‐SiO2‐H2O system, and possible hydrogen‐bearing and hydrogen‐free point defects in Ti4+‐doped forsterite. Calculated structures are used to compare the predicted orientation of the O‐H bonds with the experimentally determined polarization. The energies are used to discuss how different regimes of chemical environment, temperature (T), pressure (P), and both water content and water fugacity impact on which of the different hydroxyl substitution mechanisms are thermodynamically stable. We find that given the presence of Ti impurities, the most stable mechanism involves the formation of silicon vacancies containing two protons charge balanced by a Ti4+ cation occupying an adjacent octahedral site. This mechanism leads to the water‐mediated formation of silicon vacancies. As silicon is known to be the most slowly diffusing species in olivine, this provides a credible explanation of the observed water weakening effect in olivine.

Preparation and thermoelectric properties of Cd 3−xA xTeO 6 (A = In, La, and Bi) ceramics

Due to its close-shelled electron configuration, perovskite structured oxide Cd 3TeO 6 was originally regarded as an electric insulator. When Cd 3TeO 6 was electron-doped by the substitution of trivalent cations (In 3+, La 3+ and Bi 3+) and the introduction of oxygen vacancies, all samples showed bright changes on thermoelectric properties. A substantial negative temperature dependence of resistivity was observed in non-substituted Cd 3TeO 6, while substituted samples showed a slight dependence on temperature. Obviously resistivity degradation was found on air-sintered samples after nitrogen treatment. The negative Seebeck coefficient and Hall coefficient obtained indicated that electrons were the charge carriers. Among the three types of substituted cations, indium exerted the largest effect on the decrease of resistivity down to 1.3 mΩ cm at room temperature. The best thermoelectric power factor of 1.35 × 10 −4 W m −1 K −2 was obtained for Cd 2.97In 0.03TeO 6 at 300 K.