4-fold Coordination Research Articles

Enhancing the oxide-ionic conductivity of Ba3Mo1+xNb1-2xGexO8.5 at intermediate temperatures: the effect of site-selective Ge4+-substitution.

Significant oxide-ionic conductivity has been recently reported for a family of cation-deficient hexagonal perovskite derivatives Ba3M2O8.5 (M = Mo/W6+ and Nb5+/V5+). Herein, strong 4-fold coordination geometry preferring Ge4+ ions are doped into Ba3Mo1+xNb1-2xGexO8.5 to manipulate the oxygen distribution within palmierite-like layers for the enhancement of oxide-ionic conductivity. Rietveld refinement of the neutron diffraction data of Ba3Mo1.2Nb0.6Ge0.2O8.5 reveals that Ge4+-ions are selectively incorporated into the palmierite-like layers, owing to their strong 4-fold coordination environment preference. Such a site-selective doping behavior leads to an increase in the occupation proportion of the O3 site and a concomitant decrease in the occupancy factor for O2. Ionic conduction measurements show that the bulk conductivity of Ba3Mo1.2Nb0.6Ge0.2O8.5 is about twice higher than that of the parent compound at intermediate temperatures (300-500 °C). Furthermore, bond-valence site energy (BVSE) landscape analysis reveals that the oxygen ionic conduction of Ba3Mo1+xNb1-2xGexO8.5 is dominated by the two-dimensional pathways along the palmierite-like layers, despite the three-dimensional (3D) oxygen diffusion pathways being present in the hybrid structure, which strongly confirms that the enhancement in ionic conductivity at intermediate temperatures is attributed to the site-selective Ge4+-substitution-induced redistribution of oxygen ions within the palmierite-like layers.

Combined effect of strain and screw dislocation on the ferromagnetic behavior of ZnO nanowires

Owing to its excellent structural flexibility, a noteworthy strain adaptability was discovered from the screw dislocation located in zinc oxide (ZnO) nanowires (NWs). Based on density functional theory (DFT), the electronic structure and magnetic coupling mechanism of Zn vacancies (VZn) in this dislocation while sustaining inhomogeneous strain patterns were explored. The electronic structure analysis revealed that a double screw characteristic could be observed from the center of the dislocation, where O atoms were under 3-fold coordination. This was beneficial to the capture of numerous VZn. Meanwhile, the most stable configuration was obtained under a −2% strain, which enhanced the stability and interaction of VZn, allowing the couplings of VZn to form steady VZn pairs. A stable ferromagnetic (FM) state then originated from the p-p spin couplings of surrounding O-2p orbits. It was deduced that the long-range FM ordering could be regulated by the combined effect of strain and screw dislocation.

Iron Speciation in Different Saharan Dust Advections and Effect of the Procedural Blank on the Results From X-ray Absorption Spectroscopy and Selective Leaching Experiments

In this work, we applied X-ray Absorption Spectroscopy (XAS) and selective leaching experiments for investigating iron speciation in different dust advections collected on different unwashed quartz fiber filters. XAS analysis evidenced a predominance of Fe(III) in 6-fold coordination for Saharan dust and a trend towards Fe(II) and 4-fold coordination in the order: Saharan dust, mixed Saharan, and non-Saharan aerosol samples. The role of the sampling substrate was evaluated explicitly, including in the analysis a set of blank filters. We were able to pinpoint the possible contribution to the overall XAS spectrum of the residual Fe on quartz as the concentration decrease towards the blank value. In particular, the filter substrate showed a negligible effect on the structural trend mentioned above. Furthermore, selective leaching experiments evidenced a predominance of the residual fraction on Fe speciation and indicated the lowest Fe concentrations for which the blank contribution is <20% are 1 μ g for the first three steps of the procedure (releasing the acid-labile, reducible and oxidizable phases, respectively) and 10 μ g for the last step (dissolving the insoluble residuals).

Localized High Concentration Electrolytes for High Voltage Lithium–Metal Batteries: Correlation between the Electrolyte Composition and Its Reductive/Oxidative Stability

We demonstrate a first-principles screening methodology as an effective tool to explore electrolyte formulations for the new generation of high energy density rechargeable batteries. We study the liquid structure and electronic properties in dilute electrolytes, high concentration electrolytes (HCE), and localized high concentration electrolytes (LHCE), with focus on electrolyte formulations based on lithium bis­(fluorosulfonyl)­imide (LiFSI), dimethyl carbonate (DMC), and bis­(2,2,2-trifluoroethyl) ether (BTFE) as a diluent. We describe the solvation complexes in the dilute electrolyte and explore structural changes triggered by the increase in lithium salt concentration for HCEs and the diluent effects in LHCEs. In HCE formulations, there is a 4-fold coordination environment of lithium-ions as in the dilute electrolyte, but the number of lithium-ion interactions with O atoms from FSI– anions dominates. In these solutions, the ability of the FSI– anions to interact with multiple lithium-ions allows complex 3D network formation and influences the reductive/oxidative behavior of the electrolyte. Interestingly, in LHCEs, the BTFE diluent molecules do not change the 3D solution structure when diluting the HCE formulation from 5.49 to 3.83 M. However, there is a composition threshold where the structural and electronic behavior may change. We show that diluting the HCE electrolyte with BTFE down to 1.77 M breaks the three-dimensional solution structure into an island-like solvation complex. We relate these structural changes to the electronic properties of the electrolytes finding a causal relationship between the reductive/oxidative behavior and the lithium–oxygen interaction mechanisms in the solvated complexes. The coordination with lithium-ions lowers the electrolyte LUMO and HOMO levels: the higher is the number of interactions with lithium-ions, the more likely the solvent molecule, FSI– anion, or diluent molecule is to be reduced and the less likely it is to become oxidized. The evolution of the solvated ion structure in HCE and LHCE suggests a close connection to a corresponding change in the lithium-ion transport mechanisms for these electrolytes.

Calcium orthocarbonate, Ca2CO4-Pnma: A potential host for subducting carbon in the transition zone and lower mantle

A novel structure of calcium orthocarbonate Ca2CO4-Pnma has been discovered using ab-initio crystal structure prediction methods (AIRSS and USPEX) based on the density functional theory. This phase appears above 13 GPa and remains stable to at least 50 GPa and 2000 K according to the calculations within quasi-harmonic approximation. Thus, the discovered phase can be stable at Earth's transition zone and lower mantle P-T conditions. The carbon atoms in the discovered phase are presented in 4-fold coordination, and its structure is similar to the high-pressure and high-temperature α'H-Ca2SiO4 phase.

Methane dehydrogenation on 3d 13-atom transition-metal clusters: A density functional theory investigation combined with Spearman rank correlation analysis

The rational use of non-renewable energy sources such as methane (CH4) has been considered a promising alternative to several high-value chemicals. Therefore, there is a considerable effort to identify the main physical-chemical features for modulating CH4 dehydrogenation, in particular, at the nanoscale regime, where new effects could promote the breaking of the C–H bond. Thus, at this work, we report a theoretical investigation of the dehydrogenation of CH4 based on a step-by-step process on 3d 13-atom transition-metal (TM) clusters (TM = Fe, Co, Ni, Cu) through ab initio density functional theory calculations combined with the Spearman rank correlation analysis and the unity bond index-quadratic exponential potential (UBI-QEP) model. Our results revealed that the adsorption/interaction of CH4 and its dehydrogenated species (CH3, CH2, CH, and C) is guided by three factors, namely, (i) charge transfer from the TM13 clusters to the CHn species, (ii) enhancement of the sp-d coupling between the electronic states of the CHn species (sp-) and TM13 (d-states) systems, (iii) number of unpaired electrons in the CHn species, in which (ii) and (iii) increases by decreasing n, which is consistent with the tendency of the C atom to restore its 4-fold coordination. The H co-adsorption promotes an increasing in the adsorption/interaction energy as it promotes an enhancement of sp-d hybridization, which has a maximum for, Fe13 and Co13 and minimum for Cu13. From the Spearman correlation analysis, we identified that the effective coordination number, the Hirshfeld charge on the C atom, and the distance between the CHn and TM13 systems drive the magnitude of the adsorption energy, in particular, at the H co-adsorption regime. The thermodynamic and kinetic properties obtained via the UBI-QEP model indicates that the CH4 dehyrogenation is slightly favorable on Ni13 cluster, while kinetically CH4 dehydrogenation process would take place much easier on Ni13, Fe13, and Co13 rather than on the Cu13 cluster.

Structural Role of Phosphate in Metaluminous Sodium Aluminosilicate Glasses As Studied by Solid State NMR Spectroscopy

In this contribution we present a detailed study of the effect of the addition of small to intermediate amounts of P2O5 (up to 7.5 mol %) on the network organization of metaluminous sodium aluminosilicate glasses employing a range of advanced solid state NMR methodologies. The combined results from MAS, MQMAS (multiple quantum MAS), or MAT (magic angle turning) NMR spectroscopy and a variety of dipolar based NMR experiments-27Al{31P}-, 27Al{29Si}-, 29Si{31P}-, and 31P{29Si}-REDOR (rotational echo double resonance) NMR spectroscopy as well as 31P{27Al}- and 29Si{27Al}-REAPDOR (rotational echo adiabatic passage double resonance) NMR-allow for a detailed analysis of the network organization adopted by these glasses. Phosphate is found as QP2, QP3, and QP4 (with the superscript denoting the number of bridging oxygens), the QP4 units can be safely identified with the help of 31P MAT NMR experiments. Al exclusively adopts a 4-fold coordination. The withdrawal of a fraction of the sodium cations from AlO4 units that is needed for charge compensation of the QP2 units necessitates an alternative charge compensation scheme for these AlO4 units via formation of QP4 units or oxygen triclusters. The dipolar NMR experiments suggest a strong preference of P for Al with an average value of ca. 2.4 P-O-Al connections per phosphate tetrahedron. P is thus mainly integrated into the network via P-O-Al bonding, the formation of Si-O-P bonding plays only a minor role.

Carbon phosphide nanosheets and nanoribbons: insights on modulating their electronic properties by first principles calculations.

A carbon phosphide (CP) monolayer, a 2D structure derived from the same 3-fold coordination found both in graphene and phosphorene, has been successfully synthesized in an experiment recently. In this paper, we investigated the modulation of electronic structures and transport characteristics of 2D nanosheets and quasi-1D nanoribbons of CP nanomaterials in the α-phase by using first-principles density functional theory simulation. The calculated band structures show that the band gap of 2D CP nanosheets progressively increases as the uniform biaxial strain changes from compression to stretching. However, the biaxial strain cannot change the indirect band gap behavior of the original 2D CP nanosheet. In addition, the band structures of quasi-1D nanoribbons with different styles of H-passivated zigzag edges have also been studied. The results show that the H-passivated zigzag PC ribbons with two P edges are semiconductors with indirect band gaps, and the gaps decrease with increasing width of ribbons. However, the H-passivated CP nanoribbons with one P-atom terminated edge in combination with one P-atom edge, and H-passivated CC nanoribbons with two C-atom terminated edges display metallic behaviors. The semi-conductive or metallic behaviors of zigzag CP nanoribbons can be explained by presenting the wave function of their energy band around the Fermi level. Finally, the electronic transport properties of different CP nanoribbon based nanojunctions are studied in which arise the interesting negative differential resistance or rectification effects in their current-voltage characteristic curves.

Synthesis, Crystal Structure, and Luminescence Properties of a White-Light-Emitting Nitride Phosphor, Ca0.99Eu0.01AlSi4N7.

Colorless transparent single crystals of a new europium-doped calcium aluminum silicon nitride, Ca0.99Eu0.01AlSi4N7, were synthesized by heating a mixture of binary nitrides (Ca3N2, EuN, AlN, Si3N4, and Mg3N2) at 2150 °C under a nitrogen gas pressure of 0.85 MPa. The X-ray diffraction reflections from a single crystal were indexed with orthorhombic cell parameters a = 11.6819(3) Å, b = 21.0193(6) Å, and c = 4.91770(10) Å. The crystal structure was isomorphic to that of SrAlSi4N7:Eu (space group Pna21). One of the Ca/Eu sites was split into two sites of 4-fold and 8-fold nitrogen coordination. A white-light emission spectrum with two peaks at wavelengths of 498 and 614 nm was observed from the single crystals under 400 nm near-ultraviolet-light irradiation. The CIE1931 chromaticity of the white light was plotted at x = 0.378 and y = 0.429; the mean color-rendering index Ra was 81.

Anomalous structure of MgCO3 liquid and the buoyancy of carbonatite melts

MgCO3 is one of the most important components of mantle-derived carbonatite melts, and yet also one of the most difficult to study experimentally. Attempts to constrain its thermodynamic properties are hampered by decarbonation, which occurs at only ∼500°C, far below its metastable 1 bar melting temperature. Molecular dynamic simulations, however, can predict the thermodynamic properties of the MgCO3 liquid component in spite of experimental challenges. Using the recently developed empirical potential model for high-pressure alkaline-earth carbonate liquids (Hurt and Wolf, 2018), we simulate melts in the MgCO3-CaCO3-SrCO3-BaCO3 system from 773 to 2373 K up to 20 GPa. At 1 bar, MgCO3 liquid assumes a novel topology characterized by a 4-fold coordination of the metal cation (Mg) with both the carbonate molecule and oxygen ion; this is distinct from the other alkaline-earth carbonate liquids in which the metal cation is in ∼6- and ∼8-fold coordination with carbonate and oxygen. With increasing pressure, MgCO3 liquid structure becomes progressively more like that of (Ca, Sr, Ba)CO3 liquids with Mg2+ approaching 6-fold coordination with carbonate groups. The novel network topology of MgCO3 liquid results in a melt that is significantly more buoyant and compressible than other alkaline-earth carbonate liquids. Simulations of mixed MgCO3-bearing melts show that metal cation coordination with O and C is independent of bulk composition. Mixed simulation also reveal that molar volume, compressibility, enthalpy and heat capacity do not mix ideally with (Ca, Sr, Ba)CO3 liquids at 1 bar, a consequence of preferential metal-cation ordering in MgCO3-bearing mixtures. As pressure increases, however, mixing progressively approaches ideality with respect to molar volume, becoming nearly ideal by 12 GPa. The model is further applied to mantle-derived primary carbonatite melts with compositions, temperatures and pressures determined by published phase equilibrium experiments. The voluminous structure of liquid MgCO3 results in a buoyant melt that inhibits a density crossover with the surrounding mantle. Assuming FeCO3 liquid also adopts the same anomalous high-volume structure as MgCO3, we predict that even the most Fe-rich ferrocarbonatites would remain buoyant and be barred from sinking or stagnating in the mantle.

Magnetic coupling properties of Zn vacancies in ZnO nanowire with screw dislocation

The electronic structure and magnetic coupling property of Zn vacancy (VZn) in ZnO nanowires (ZnONWs) with screw dislocations were investigated using first-principle calculation based on density functional theory (DFT). The optimized configuration of the screw dislocation shows that all of the atoms in dislocation cores are 3-fold coordination, favoring the formation of VZn. These VZn are ferromagnetic, which is achieved by spin polarization of the surrounding 3-fold coordination O atoms. Furthermore, the periodicity of the screw dislocations along [0001] direction can facilitate the aggregation and interaction of VZn. As a result, the p-p coupling between O atoms around VZn is continuous along [0001], which leads to long range ferromagnetism (FM) ordering of the system. Also, the dislocations can capture carriers, which realize the control of magnetic moment.

Fe(II) Hydride Complexes for the Homogeneous Dehydrocoupling of Hydrazine Borane: Catalytic Mechanism via DFT Calculations and Detailed Spectroscopic Characterization

The catalytic dehydrocoupling of hydrazine borane (N2H4·BH3, HB) using two molecularly defined PNP Fe(II) hydride complexes [(PNHP)Fe(HBH3)(H)(CO)] (PNHP = HN[CH2CH2P(i-Pr)2], 1-BH3) and [(PNP)Fe(H)(CO)] (PNP = N[CH2CH2P(i-Pr)2], 2) is reported. Both catalysts are highly active and recyclable, and they can potentially release up to 2.56 equiv of hydrogen from HB, giving a thermally unstable solid dehydrocoupling product. For the stable dehydrogenation of HB of up to 1.0 equiv of hydrogen, we have analyzed the resulting insoluble BxNyHz polymeric residue using 11B solid-state NMR spectroscopy techniques combined with density functional theory (DFT) calculations of 11B NMR parameters for a comprehensive library of molecular BxNyHz fragments compatible with the polymeric BxNyHz structure. These studies were further supported by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis. On this basis, we propose a detailed mechanism for the stable dehydrogenation of HB of up to 1.0 equiv of hydrogen and the subsequent polymerization reaction, leading to the insoluble polymeric residue. This residue with the general chemical formula of BNH3 is shown to be consistent with a polymer repeat unit of five- or six-ring amino-borane structures, supported by the fact that the 11B magic-angle-spinning (MAS) NMR experiments only reveal boron in 4-fold coordination.

Binding and Competition of Be2+ and Ca2+ with Model Phospholipid Membranes

Membrane surfaces, carrying multiple electronegative groups, attract metal ions and provide a rich environment for multiple ion-mediated signaling events. Be2+, a small divalent ion known for its toxicity and ability to activate abnormal immune responses is recognized as a strong ligand for anionic lipids. Be2+'s tight binding to the surface of phosphatidylserine-containing (PS) membranes was recently shown to diminish the recognition of phosphatidylserine by macrophages. The major hypothesis for the mechanism of this process is that Be2+ is able to outcompete and partially displace Ca2+ from its binding sites. This displacement may be due to a very different 4-fold coordination of Be2+ by oxygens, and the inability of small Be2+ to substitute for six-coordinated Ca2+ in physiological reactions. FTIR is a highly informative technique in studies of ligand binding and ion coordination at membrane interfaces. Despite ITC studies showing that Be2+ has the capacity to compete with Ca2+ on the surface of PS liposomes, there have been no spectroscopic studies of Be2+ interactions with phospholipids. In this work, we apply the ATR-FTIR technique and present the spectral ‘signature’ of Be2+ binding to PS manifested as a dominant 1095 cm-1 line apparently arising from ion binding to phosphate oxygens. Be2+ produces no visible alteration of carbonyl bands and appears to partially suppress carboxyl vibrational bands. Dialysis experiments show that FTIR can be used to watch the ionic equilibria and determine binding constants for specific atomic groups in the phospholipids. We show that Be2+ tightly binds not only to anionic PS, but also to zwitterionic PC. We also observe Be2+ readily displacing more abundant Ca2+ from phosphate head groups.

The density of Li2CO3-Na2CO3-K2CO3-Rb2CO3-Cs2CO3-CaCO3-SrCO3-BaCO3 liquids: New measurements, ideal mixing, and systematic trends with composition

Abstract The densities of 15 liquids in the Li2CO3-Na2CO3-K2CO3-Rb2CO3-Cs2CO3-CaCO3-SrCO3-BaCO3 system were measured at 1 bar with the Pt double-bob Archimedean method between 758 and 1455 K. Melt compositions contain ≤50 mol% CaCO3, SrCO3 and BaCO3, and ≤100% Rb2CO3 and Cs2CO3. These results were combined with double-bob density measurements from the literature on nine liquids in the Li2CO3-Na2CO3-K2CO3-CaCO3 system to calibrate a new linear volume equation for multicomponent carbonate liquids. The regression led to the following fitted partial molar volumes (±1σ cm3/mol) at 1100 K for Li2CO3 (41.22 ± 0.09), Na2CO3 (53.27 ± 0.11), K2CO3 (71.59 ± 0.13), Rb2CO3 (80.78 ± 0.11), Cs2CO3 (94.00 ± 0.09), CaCO3 (40.18 ± 0.16), SrCO3 (44.33 ± 0.22) and BaCO3 (50.99 ± 0.19). At 1100 K, the thermal expansion coefficients of all alkali carbonate liquid components are indistinguishable within 1-sigma error (22.07 ± 1.66 × 10−5 K−1), but differ from the thermal expansion coefficients of all alkaline-earth carbonate liquid components, which are also indistinguishable within 1-sigma error (16.40 ± 2.85 × 10−5 K−1). The linear volume equation recovers the measurements within analytical error (±0.3%). The partial molar volumes of all eight carbonate components increase linearly along two different trends, one for the alkali carbonates (R2 = 0.999) and another for the alkaline earth carbonates (R2 = 0.999) as a function of cation volumes, where metal cation-oxygen coordination numbers are obtained from molecular dynamic simulations in the literature (ranging from 4- to 6-fold among the alkali metals and 7- to 8-fold among the alkaline-earth metals). The linear fits lead to two different partial molar volumes (∼38 and ∼31 cm3/mole) for the carbonate ion (CO32−) at 1100 K, which reflects a more open topological arrangement of the carbonate ions when in 4-fold coordination with the alkali metals vs. 6-fold coordination with alkaline earth metals. The results permit the partial molar volume of MgCO3 and FeCO3 in multicomponent carbonate liquids to be calculated if the oxygen and carbonate ion coordination with Mg2+ and Fe2+ are known. For example, in the case where Mg2+ and Fe2+ are in 6-fold coordination with both oxygen and carbonate, the estimated partial molar volumes at 1100 K are 34.4 (±0.6), and 35.1 (±0.6) cm3/mol, respectively, with a thermal expansion coefficient of 16.40 (±2.85) 10−5 K−1. In contrast, if Mg2+ and Fe2+ are in 4-fold coordination with both oxygen and carbonate, the estimated partial molar volumes at 1100 K are 40.0 (±0.6), and 40.4 (±0.6) cm3/mol, respectively, with a thermal expansion coefficient of 22.07 (±1.66) 10−5 K−1. To resolve these different estimates of molar volume, molecular dynamic simulations are needed to determine the respective coordination environments for Mg2+ and Fe2+ in carbonate liquids.

In-situ observation of the structural change in MgO-B2O3-SiO2 glass at high pressure and the permanent structural change

A 41.6MgO-27.1B2O3-31.2SiO2 glass was synthesized by container-less levitation method. B K-edge X-ray absorption near edge structure (XANES) spectroscopy was used to quantify the coordination of boron in the as-made glass, resulting in ~72% of boron in 3-fold coordination. Raman spectroscopy indicated that the glass structure includes depolymerized borate units, which are pyroborate (B2O54−) and ring and/or chain-type metaborate (B3O63−, (BO2O−)∞) groups. The structural changes in the glass under hydrostatic condition were investigated by high-pressure Raman spectroscopy within a diamond anvil cell at pressures up to 8.5 GPa. On the borate structure, metaborate groups decreased with pressure. In contrast, triborate groups, which include tetrahedrally coordinated boron and have a fully polymerized borate network, increased; suggesting that the coordination transformation of B and polymerization were induced via compression. However, the modification of the borate structure units seems to be reversible at least up to 8.5 GPa. As for the silicate network, Q0 and Q1 structures were responsible to compression. Specifically, Q0 could be replaced by Q1 with elevating pressure, which means that the silicate network becomes polymerized with pressure. Whereas, structure of glass samples recovered from the pressure higher than 5 GPa displayed depolymerized silicate network, as dissociation of Q2 to Q1. The observed structural changes in the silicate network implied a partial change of the coordination number of Si4+ ions, which can be enhanced by the presence of a modifier cation with high field strength such as Mg2+.

Molten and glassy tellurium(IV) oxosulfato complexes in the TeO2 –K2S2O7 system studied by Raman spectroscopy: Stoichiometry, vibrational properties and molecular structure

Tellurium(IV) oxide dissolves in molten potassium pyrosulfate following a reaction that leads to TeIV oxosulfato complexes. High temperature Raman spectroscopy is used under static equilibrium conditions at temperatures of 550 – 740oC for inferring the structural properties of the complex(es) formed in TeO2-K2S2O7 molten mixtures (XTeO20 = 0 – 0.70) under O2 atmosphere. The composition effects on the Raman spectra together with the outcome of a quantitative exploitation of the relative Raman band intensities point to a 3:1 TeO2:K2S2O7 stoichiometry, which combined with the observed vibrational properties is suggestive of a (Te=O)3O2(SO4)22- structural formula for the simplest building unit formed. A 3-fold coordination around Te becomes evident comprising an oxide ligand and two oxygen atoms bridging to adjacent Te or S atoms. A gradual association of building units takes place with increasing XTeO20. Cooling results in formation of stable glasses with a concomitant change in the coordination number of TeIV from 3 to 4 by means of a structural mechanism suggestively involving a TeO2/2(=O)→ TeO4/2 transformation. Totally ten bands are attributed to the TeIV oxosulfato complex; the most characteristic ones pertain to the 1040 cm-1 band assigned to symmetric stretching unidentate sulphate and to the 850 cm-1 band assigned to Te=O terminal stretching, the latter band being attenuated on going from the molten to the glassy state due to the TeO2/2(=O)→ TeO4/2 interconversion.

Fractionation of the isotopes of boron between granitic melt and aqueous solution at 700 °C and 800 °C (200 MPa)

The fractionation of boron's isotopes, 10B and 11B, between granitic melt (M) and aqueous solution (V), represented as Δ11BM-V, was investigated between 700 °C and 800 °C at 200 MPaH2O. One series of experiments used the Macusani obsidian (MAC) to reassess the Δ11BM-V values reported by Hervig et al. (2002). Another series of experiments employed a synthetic glass (HG) made to the composition of the hydrous haplogranite minimum (Ab36Or29Qtz35) at 200 MPaH2O. Boron was added as boric acid (NIST SRM 951a) to two aliquots of HG glass to bring the concentration of B2O3 in each to ~2.5 (‘Low B’) and ~5 wt% B2O3 (‘High B’). Thus, values of Δ11BM-V were examined as a function of B2O3 and temperature.Results from the current study show significant fractionation of boron's isotopes between granitic melt and aqueous solution at these high temperatures. Values of Δ11BM-V for ‘Low B’ and ‘High B’ glasses range from −10.9 (±1.3‰) to −6.1 (±1.7‰) at 800 °C and −7.8 (±1.5‰) to −5.3 (±1.3‰) at 700 °C, and the Δ11BM-V values do not correlate with the concentration of B in melt (glass). One experiment (CGB88) conducted at 750 °C and 200 MPaH2O using MAC yielded a Δ11BM-V value (−8.4‰) similar to that reported by Hervig et al. (2002). The Δ11BM-V values reported in this study form a linear trend with temperature through the Δ11B values for boromuscovite-aqueous solution, illite-aqueous solution, and melt-aqueous solution. Therefore, based on the results of this study, we concur with Hervig et al. (2002) that the fraction of B in 4-fold coordination in hydrous granitic melt is significant, if this is the fundamental cause of the fractionation effect. Spectroscopic studies on hydrous glass compositions similar to those reported in this study might elucidate the coordination environment of B in melts to the extent that its structural state is quenchable.

Spectroscopic features of ultrahigh-pressure impact glasses of the Kara astrobleme

The state of substances under ultrahigh pressures and temperatures (UHPHT) now raises a special interest as a matter existing under extreme conditions and as potential new material. Under laboratory conditions only small amounts of micrometer-sized matter are produced at a pressure up to 100 GPa and at room temperature. Simultaneous combination of ultrahigh pressures and temperatures in a lab still requires serious technological effort. Here we describe the composition and structure of the UHPHT vein-like impact glass discovered by us in 2015 on the territory of the Kara astrobleme (Russia) and compare its properties with impact glass from the Ries crater (Germany). A complex of structural and spectroscopic methods presents unusual high pressure marks of structural elements in 8-fold co-ordination that had been described earlier neither in synthetic nor natural glasses. The Kara natural UHPHT glasses being about 70 Ma old have well preserved initial structure, presenting some heterogeneity as a result of partial liquation and crystallization differentiation where an amorphous component is proposed to originate from low level polymerization. Homogeneous parts of the UHPHT glasses can be used to deepened fundamental investigation of a substance under extreme PT conditions and to technological studies for novel material creations.

Monoclinic to cubic phase transformation and photoluminescence properties in Hf1−xSmxO2 (x = 0–0.12) nanoparticles

Bulk hafnium oxide (HfO2) exhibits the monoclinic phase at room temperature which transforms to tetragonal and cubic phases at 1700 and 2600 °C, respectively, under ambient conditions. For the first time, we observe a monoclinic to stable cubic phase transformation at room temperature in nanoparticles of HfO2 by incorporating Sm3+ ions up to 12 at. %. Although the monoclinic structure is retained at 1 at. % of Sm, a mixed phase of monoclinic and cubic is observed at intermediate Sm concentration (5–11 at. %). Le-Bail profile refinement of X-ray diffraction patterns confirms the monoclinic to cubic phase transformation with increasing Sm3+ ion concentration. While the significant difference in ionic radii of Sm3+ and Hf4+ ion induces strain in the lattice above 9 at. %, a lower valency of Sm produces oxygen vacancy leading to 8-fold coordination with Sm3+ ion and stabilizes the cubic phase at room temperature. Not only the particle size obtained from transmission electron micrograph (TEM) matches well with the size calculated from the Williamson-Hall plot, the lattice spacing estimated from high resolution TEM also confirms the monoclinic and cubic phases in HfO2 and Hf0.88Sm0.12O2, respectively. Apart from phase transformation induced by Sm3+ ions, photoluminescence studies demonstrate an excellent emission in near green and red regions in Hf1−xSmxO2 nanoparticles. A schematic energy band diagram has been proposed based on the excitation and emission processes involved in HfO2 and Hf0.99Sm0.01O2 nanoparticles.

Metalation and coordination reactions of 2H-meso-trans-di(p-cyanophenyl)porphyrin on Ag(111) with coadsorbed cobalt atoms.

We investigated the metalation and coordination reactions of Co with 2H-5,15-bis(para-cyanophenyl)-10,20-bisphenylporphyrin (2HtransDCNPP) on a Ag(111) surface by scanning tunneling microscopy. At room temperature (RT), 2HtransDCNPPs self-assemble into a supramolecular structure stabilized by intermolecular hydrogen bonding. The metalation of 2HtransDCNPP is achieved either by depositing Co atoms onto the supramolecular structure at RT, or, alternatively, by depositing the molecules onto a submonolayer Co-precovered Ag(111) surface with a subsequent heating to 500 K. In addition, the molecules coordinate to Co atoms through the N atoms in the peripheral cyano groups with a preference of isolated 4-fold coordination motifs at RT.