Coordination Number Of Ti Research Articles

Clues from Ab Initio Calculations on Titanium Isotopic Fractionation in Tholeiitic and Calc-Alkaline Magma Series

Magmatic differentiation produces positive correlations between δ49Ti and SiO2. The equilibrium Ti isotope fractionation factors of Ti-bearing minerals are essential for understanding the mechanisms driving this isotopic fractionation. We present ab initio-derived mean force constants of Ti-bearing minerals (barium orthotitanate, potassium titanium oxide, fresnoite, diopside, geikielite, karrooite, titanite, pseudobrookite, anatase, and titanium oxide) based on density functional theory (DFT) to calculate equilibrium isotopic fractionation factors. We find that the main driver for Ti isotopic fractionation is its coordination, with four-, five-, and sixfold-coordinated Ti characterized by mean force constants of 547, 462, and 310 N/m, respectively. The coordination number of Ti in silicate melts is thought to be lower than in minerals, driving magmas toward higher δ49Ti values by fractional crystallization. The mineral-melt fractionation factors allow modeling of the observed Ti isotope trends in tholeiitic and calc-alkaline rocks. Our model results indicate that to first order, the steeper δ49Ti trend observed in tholeiitic versus calc-alkaline magmas is most likely due to enhanced removal of Ti into sequestered minerals at low SiO2 concentration in tholeiitic series compared to calc-alkaline series. The δ49Ti–SiO2 differentiation trends, however, depend on Ti coordination in the melt and the strengths of Ti bonds in diverse Fe- and Ti-oxides, which are still uncertain. Our results show that Ti isotopes can be used to reconstruct the crystallization history and identify the magmatic series parentage of magmas that otherwise lack context, but further work is needed to identify the drivers behind Ti isotopic fractionation in igneous rocks.

Amorphous Redox-Rich Polysulfides for Mg Cathodes.

The lack of appropriate cathodes is restraining the advances of Mg batteries. Crystalline cathode materials suffer from sluggish reaction kinetics and low-capacity delivery. The finite type of crystalline structure further confines the rational design of cathode materials. Herein, we proposed amorphization and anion enrichment as a brand-new strategy to not only enhance the solid-state ion diffusion and provide more ion-storage sites in amorphous structure but also contribute to the local transfer of multiple electrons through the additional anionic redox centers. Accordingly, a series of amorphous titanium polysulfides (a-TiSx, x = 2, 3, and 4) were designed, which significantly outperformed their crystalline counterparts and achieved a highly competitive energy density of ∼260 Wh/kg. The unique Mg2+ storage mechanism involves the dissociation/formation of S–S bonds and changes in the coordination number of Ti, namely, a mixture of conversion and intercalation reaction, accompanied by the joint cationic (Ti) and anionic (S) redox-rich chemistry. Our proposed amorphous and redox-rich design philosophy might provide an innovative direction for developing high-performance cathode materials for multivalent-ion batteries.

The role of the compositions of HZSM-5 zeolites modified with nanosized anatase in propane and ethanol conversion

For the first time catalytic systems based on HZSM-5 zeolites (Si/Al=12, 25, 40) with nanosized anatase (NA) were obtained in situ by a modified hydrothermal method and a sol-gel method. Initial HZSM-5 and NA/HZSM-5 were characterized (XRPD, XAS, FT-IR, DSC, BET, XPS, SEM, TPD) and analyzed in the reactions of propane (liquefied petroleum gas) conversion (РС; temperature range 200–870 °С) with the formation of the main products propylene (C3H6, PP) and ethylene (C2H4, ETH) and liquid ethanol conversion (EC; temperature range 150–360 °C) with the formation of diethyl ether ((C2H5)2O, DEE) and ETH. In the PC reaction the maximum selectivity (SETH = 75%) and conversion (αP = 65%) were obtained on HZSM-5(40) (650 °С). With an increase in the NA content in NA/HZSM-5 SETH value increases up to 85%. In the EC reaction SDEE/ETH~98–99% was reached on HZSM-5(25) (200 °C) and HZSM-5(40) (300 °C); the αE to ETH is comparable (94%) for all HZSM-5 (300 °C). Modification of HZSM-5 with NA leads to a shift in the EC reaction temperature to lower temperature region (αE to ETH ~98% at 280 °C) with SDEE/ETH~94–100% for NA/HZSM-5(25) and NA/HZSM-5(12) obtained by a modified hydrothermal method. It was established the relationship between HZSM-5 and NA/HZSM-5 catalytic activity and their characteristics: Si/Al, NA content and Ti coordination number (NA/HZSM-5), the content of "zeolite water" with OH groups, specific surface area, meso- and micropores volume and spherical particles size (HZSM-5), surface energy of adsorption centers (HZSM-5 and NA/HZSM-5), and hence the content of acid sites, the strength of which is also determined by the temperature of the catalytic reaction. The reaction mechanisms have been presented. The obtained NA/HZSM-5 catalysts have a higher or comparable catalytic activity to commercial ones, but low cost, ease of preparation, and versatility.

Observation of 9-Fold Coordinated Amorphous TiO2 at High Pressure.

Knowledge of the structure in amorphous dioxides is important in many fields of science and engineering. Here we report new experimental results of high-pressure polyamorphism in amorphous TiO2 (a-TiO2). Our data show that the Ti coordination number (CN) increases from 7.2 ± 0.3 at ∼16 GPa to 8.8 ± 0.3 at ∼70 GPa and finally reaches a plateau at 8.9 ± 0.3 at ≲86 GPa. The evolution of the structural changes under pressure is rationalized by the ratio (γ) of the ionic radius of Ti to that of O. It appears that the CN ≈ 9 plateau correlates with the two 9-fold coordinated polymorphs (cotunnite, Fe2P) with different γ values. This CN-γ relationship is compared with those of a-SiO2 and a-GeO2, displaying remarkably consistent behavior between CN and γ. The unified CN-γ relationship may be generally used to predict the compression behavior of amorphous AO2 compounds under extreme conditions.

Estimation of TiO2-FeO-Na2O slag viscosity through molecular dynamics simulations for an energy efficient ilmenite smelting process

Along with the increasing demand for the TiO2 pigment, the ilmenite smelting process has also become significant because it can utilize both rock- and sand-type ilmenite. However, due to the high liquidus temperature of the TiO2 slag system, the smelting process is highly energy consuming. In the present study, the viscosity of molten ilmenite slag was estimated using molecular dynamic simulations at a high temperature to achieve an appropriate and efficient slag design. To verify the validity of the simulation technique, experimental measurements were performed in parallel and their results were compared. The effects of FeO and Na2O addition on viscosity of TiO2 slag were also investigated. The addition of Na2O as a flux enhanced the ilmenite smelting process by not only lowering the liquidus temperature but also slowing the drastic viscosity increase. Statistical information obtained from the molecular dynamic simulations revealed a three-dimensional TiO6 octahedral network structure. The relationship between viscosity and structural change with varying FeO and Na2O concentrations was explored considering the coordination number of Ti and various bonding types.

Mixed valence cerium substitution in Gd2-xCexTi2O7+δ pyrochlores

Pyrochlore-type oxides having the formula Gd1.6Ce0.4Ti2O7+δ have been synthesized by ceramic and co-precipitation methods and annealed at 1300 °C under different environments. Rietveld refinement of powder X-ray diffraction (XRD) patterns indicated that the products adopt the pyrochlore-type oxide structure with Ce ions substituting for Gd ions. Analysis of Ce 3d X-ray photoelectron spectroscopy (XPS) spectra confirmed the presence of both Ce3+ and Ce4+, which was corroborated by analysis of Ce L3-edge X-ray absorption near edge spectroscopy (XANES) spectra. Incorporation of Ce4+ imparts disorder on the Ti octahedra as observed by analysis of Ti K-edge XANES spectra. Due to the presence of Ce4+ in the A-site of the pyrochlore-type structure, oxygen is likely intercalated into the vacant anion 8b site to charge balance the system. This intercalation results in an increase of the average Ti coordination number from 6 to between 6 and 7, which leads to the formula unit being better represented as Gd1.6Ce0.4Ti2O7+δ.

Ti K-edge XANES study on the coordination number and oxidation state of Titanium in pyroxene, olivine, armalcolite, ilmenite, and silicate glass during mare basalt petrogenesis

Lunar mare basalts are a product of partial melting of the lunar mantle under more reducing conditions when compared to those expected for the Earth’s upper mantle. Alongside Fe, Ti can be a major redox sensitive element in lunar magmas, and it can be enriched by up to a factor of ten in lunar basaltic glasses when compared to their terrestrial counterparts. Therefore, to better constrain the oxidation state of Ti and its coordination chemistry during lunar magmatic processes, we report new X-ray absorption near edge structure (XANES) spectroscopy measurements for a wide range of minerals (pyroxene, olivine, Fe–Ti oxides) and basaltic melt compositions involved in partial melting of the lunar mantle. Experiments were conducted in 1 bar gas-mixing furnaces at temperatures between 1100 and 1300 °C and oxygen fugacities (fO2) that ranged from air to two orders of magnitude below the Fe–FeO redox equilibrium. Run products were analysed via electron microprobe and XANES Ti K-edge. Typical run products had large (> 100 µm) crystals in equilibrium with quenched silicate glass. Ti K-edge XANES spectra show a clear shift in energy of the absorption edge features from oxidizing to reducing conditions and yield an average valence for Fe–Ti oxides (armalcolite and ilmenite) of 3.6, i.e., a 40% of the overall Ti is Ti3+ under fO2 conditions relevant to lunar magmatism (IW − 1.5 to − 1.8). Pyroxenes and olivine have average Ti valence of 3.75 (i.e., 25% of the overall Ti is trivalent), while in silicate glasses Ti is exclusively tetravalent. Pre-edge peak intensities also indicate that the coordination number of Ti varies from an average V-fold in silicate glass to VI-fold in the Fe–Ti oxides and a mixture between IV and VI-fold coordination in the pyroxenes and olivine, with up to 82% [IV]Ti4+ in the pyroxene. In addition, our results can help to better constrain the Ti3+/∑Ti of the lunar mantle phases during magmatic processes and are applied to provide first insights into the mechanisms that may control Ti mass-dependent equilibrium isotope fractionation in lunar mare basalts.

Anatase phase evolution and its stabilization in ion beam sputtered TiO2 thin films

Thin films of titanium oxide (TiO2) were prepared by ion beam sputtering at room temperature under various oxygen partial pressure and annealed at 350 °C and higher. Complete target oxidation is observed at O2/Ar pressures much lower than the conventional sputtering. The films are analyzed using X-ray diffraction, Raman spectroscopy, optical transmittance, and soft X-ray absorption spectroscopy. As-deposited thin films are all amorphous. Their Ti coordination number is less than that of crystalline phases, anatase and rutile. Upon post-heating above 350 °C in vacuum, films prepared with a ratio O2/Ar between 0.25 and 0.66 remain amorphous. Films prepared below this range, develop Ti6O11 and Ti3O5 grains, those prepared above this range become anatase. The band gap of the films varied between 3.12 and 3.36 eV, their refractive index between 2.07 and 2.81. Dielectric modeling of the transmittance spectra shows a broad Gaussian distribution of resonance frequency oscillators suggesting the as-deposited films to be disordered.

Pressure-induced structural changes in titanophosphate glasses studied by neutron and X-ray total scattering analyses

In this paper, the densification mechanism of two titanophosphate glasses (binary 73TiO2-27P2O5 and ternary 68TiO2-5Al2O3-27P2O5) is investigated. The glasses are densified by isostatic N2-mediated pressure treatment at elevated temperature and the pressure-induced structural changes are studied by neutron and X-ray total scattering analyses. The medium range structure change is revealed from the change in the first sharp diffraction peak (FSDP), where the density increase by pressurization is correlated with the decreased medium range repeating spacing calculated from the position of FSDP. The hot compression treatment also increases the medium range order, as shown by the increased correlation length calculated from the width of FSDP. The short range structure changes are examined by separating PO and TiO pair distribution functions (PDF) using differential neutron and X-ray PDFs. Pressurization is also found to increase the coordination number of Ti, as previously observed for other network formers in densified glasses.

Consequences of Nitrogen Doping and Oxygen Enrichment on Titanium Local Order and Photocatalytic Performance of TiO2 Anatase

Extended X-ray absorption fine structure (EXAFS) investigation of the oxygen-rich titania formed via the thermal treatment of N-doped TiO2 has revealed that the removal of N-dopants is responsible for the creation of defect sites in the titanium environment, thus triggering at high temperatures (500–800 °C) the capture of atmospheric oxygen followed by its diffusion toward the vacant sites and formation of interstitial oxygen species. The effect of the dopants on Ti coordination number and Ti–Oint and Ti–Nint bond distances has been estimated. The photocatalytic p-cresol degradation tests have demonstrated that the interband states formed by the N-dopants contribute to a greater extent to the visible-light activity than the oxygen interstitials do. However, under the UV irradiation the oxygen-rich titania shows higher efficiency in the pollutant degradation, while the N-dopants in N–TiO2 play the role of recombination sites. The presence of the surface nitrogen species in TiO2 is highly beneficial for the...

Nitrogen Location and Ti–O Bond Distances in Pristine and N-Doped TiO2 Anatase Thin Films by X-ray Absorption Studies

This Article reports detailed extended X-ray absorption fine structure (EXAFS) analysis of spray pyrolysis synthesized TiO2 thin films to locate the position of nitrogen in N-doped phase pure (anatase) TiO2 and evaluate Ti–O bond distances and the coordination number of Ti. The thin films were synthesized at three different substrate temperatures viz. 400, 450, and 500 °C leading to concomitant occurrence of varying nitrogen concentration (N/Ti) of 0.282, 0.237, and 0.24, respectively. The coordination number extracted from the EXAFS fitting revealed that films synthesized at 450 °C are stoichiometric, whereas those synthesized at 400 and 500 °C are substoichiometric. The intensity of near edge peaks in XAS was employed to delineate the contribution from both symmetry of TiO6 polyhedra and particle size. The axial and equatorial Ti–O bond distances, corresponding to the different nitrogen concentrations, extracted from the EXAFS fitting, showed good agreement with those calculated from density functional ...

An investigation of the electronic structure and structural stability of RE2Ti2O7 by glancing angle and total electron yield XANES

Pyrochlore-type oxides (A2B2O7) have been investigated for nuclear waste sequestration applications due to their overall resistance to radiation-induced structural damage. This damage can be simulated by implanting the materials with high-energy ions to aid in understanding how the structure of a crystalline material responds to radioactive decay of incorporated nuclear waste elements. RE2Ti2O7 (RE=La–Lu), Yb1.85Ca0.15Ti2O7−δ, and Yb2Ti1.85Fe0.15O7−δ were synthesized by the ceramic method and investigated by Ti K-edge X-ray absorption near edge spectroscopy (XANES) and powder X-ray diffraction before and after being implanted by 2MeV Au− ions. Surface sensitive total electron yield (TEY) XANES spectra and glancing-angle (GA) XANES spectra were collected to study the ion-implanted materials. It was necessary to use these surface sensitive detection methods because the 2MeV Au− ions only implant to ∼450nm into the surface of the materials. An effective sample mounting procedure has been developed that uses conductive silver paint to collect high quality TEY-XANES spectra from the damaged surface layer of the implanted pellets so as to minimize charging effects. This investigation has shown that the local coordination number of Ti decreased after implantation, and that the degree of damage observed decreases with decreasing radius of the RE cation. The study has also shown that the oxygen vacancies generated by a slight substitution of Yb3+ for Ca2+ or Ti4+ for Fe3+ does not considerably affect the resistance of Yb2Ti2O7 to radiation-induced damage.

Experimental approach and atomistic simulations to investigate the radiation tolerance of complex oxides: Application to the amorphization of pyrochlores

Both experimental approach and atomistic simulations are performed in order to investigate the influence of the composition of pyrochlores on their radiation tolerance. Therefore, Gd2Ti2O7 and Gd2Zr2O7 were irradiated with 4MeV Au and 92MeV Xe ions in order to study the structural changes induced by low and high-energy irradiations. XRD results show that, for both irradiations, the structural modifications are strongly dependent on the sample composition: Gd2Ti2O7 is readily amorphized, whereas Gd2Zr2O7 is transformed into a radiation-resistant anion-deficient fluorite structure. Using atomistic simulations with new interatomic potentials derived from the SMTB-Q model, the lattice properties and the defect formation energies were calculated in Gd2Ti2O7 and Gd2Zr2O7. Calculations show that titanates have a more covalent character than zirconates. Moreover, in Gd2Ti2O7 the formation of cation antisite defects leads to strong local distortions around Ti-defects and to a decrease of the Ti coordination number, which are not observed in Gd2Zr2O7. Thus, the radiation resistance is related to the defect stability: the accumulation of structural distortions around Ti-defects could drive the Gd2Ti2O7 amorphization induced by irradiation.

Atomic scale simulations of pyrochlore oxides with a tight-binding variable-charge model: implications for radiation tolerance

Atomistic simulations with new interatomic potentials derived from a tight-binding variable-charge model were performed in order to investigate the lattice properties and the defect formation energies in Gd2Ti2O7 and Gd2Zr2O7 pyrochlores. The main objective was to determine the role played by the defect stability on the radiation tolerance of these compounds. Calculations show that the titanate has a more covalent character than the zirconate. Moreover, the properties of oxygen Frenkel pairs, cation antisite defects and cation Frenkel pairs were studied. In Gd2Ti2O7 the cation antisite defect and the Ti-Frenkel pair are not stable: they evolve towards more stable defect configurations during the atomic relaxation process. This phenomenon is driven by a decrease of the Ti coordination number down to five which leads to a local atomic reorganization and strong structural distortions around the defects. These kinds of atomic rearrangements are not observed around defects in Gd2Zr2O7. Therefore, the defect stability in A2B2O7 depends on the ability of B atoms to accommodate high coordination number (higher than six seems impossible for Ti). The accumulation of structural distortions around Ti-defects due to this phenomenon could drive the Gd2Ti2O7 amorphization induced by irradiation.

Shallow n-type doping by transition-metal impurities (Hf, Zr, or Ti) in amorphous In2O3

Through first-principles electronic-structure calculations, we suggest that because the transitionmetal (TM)-d levels are located deep within the conduction band, TM (M = Hf, Zr, or Ti)-doped amorphous In2O3 (a-In2O3) can provide shallow donors, even though the effective mass of the a-In2O3 is slightly increased. We examined the microscopic structure of the doped oxides. The effective coordination number of In is little changed by the doping. The atomic strtuctures of Sn, Hf, and Zr are similar, but the coordination number of Ti is smaller than those of the other dopants. The effective masses of the electron of the TM-doped a-In2O3 system are larger than those of the undoped a-In2O3 and Sn-doped a-In2O3.

Titanium local structure in tektite probed by X-ray absorption fine structure spectroscopy

The local structure of titanium in tektites from six strewn fields was studied by Ti K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) in order to provide quantitative data on Ti-O distance and Ti coordination number. The titanium in tektites possessed different coordination environment types. XANES spectra patterns revealed resemblance to high-temperature TiO(2)-SiO(2) glass and TiO(2) anatase. All samples showed that the valence of Ti is 4+. Based on the Ti-O distances, coordination numbers and radial distribution function determined by EXAFS analyses, the tektites were classified into three types: type I, Ti occupies a four-coordinated tetrahedral site with Ti-O distances of 1.84-1.79 Å; type II, Ti occupies a five-coordinated trigonal bipyramidal or tetragonal pyramidal site with Ti-O distances of 1.92-1.89 Å; type III, Ti occupies a six-coordinated octahedral site with Ti-O distances of 2.00-1.96 Å. Although Ti occupies the TiO(6) octahedral site in most titanium minerals under ambient conditions, some tektites have four- and five-coordinated Ti. This study indicated that the local structure of Ti might change in impact events and the following stages.

Composition-dependent changes in the NEXAFS spectra of TiN–Cu films

The nanostructure of Ti in TiN–Cu films, grown by pulsed laser deposition, is studied using Ti– K–NEXAFS spectroscopy. The characteristics of the NEXAFS spectra are strongly affected by both the different growth conditions and the Ti concentration in the films. More specifically, the area under the pre-edge peak is strongly correlated with the coordination number of Ti. It is found that in the TiN–Cu films grown from the intermetallic TiCu target, the area under the pre-edge peak decreases as the Ti concentration increases from 50 to 95 at.%. In the films grown from Ti and Cu elemental targets, the area under the pre-edge peak decreases as a function of the nitrogen partial pressure. The detected changes in the pre-edge peak can be attributed to the formation of Ti–Cu bonds.

Reactions during Electrochemically Induced Nucleation of Mullite from a MgO/Al2O3/TiO2/SiO2/B2O3/CaO Melt

Glasses with the composition 52.6 SiO 2 ·18.7 Al 2 O 3 ·14.3 MgO·7.7 TiO 2 ·4.7 B 2 O 3 ·2.0 CaO were crystallized using electrochemically induced nucleation at temperatures from 1200 to 1400 °C. The crystals grew with their fastest growing axis pointing away from the surface of the electrode. At the cathode, Ti 4+ is reduced to Ti 3+ , which was proved by EPR and UV—vis—NIR spectroscopy. The silicate network is also reduced to elemental silicon to a small extent and forms an alloy with the platinum cathode. The higher coordination number of Ti 3+ should lead to a drop in viscosity due to its role as a glass modifier and, hence, fully explains the nucleation. The orientations of three neighboring crystals were analyzed by electron backscatter diffraction (EBSD).

First-principles study of thinTiOxand bulklike rutile nanowires

We have systematically investigated structural, electronic and magnetic properties of very thin ${\text{TiO}}_{x}$ $(x=1,2)$ nanowires as well as bulklike (110) rutile nanowires by using the first-principles plane-wave pseudopotential calculations based on density functional theory. A large number of different possible structures have been searched via total-energy calculations in order to find the ground-state structures of these nanowires. Three-dimensional structures are more energetically stable than planar ones for both of the stoichiometries (i.e., $x=1,2$). The stability of ${\text{TiO}}_{x}$ nanowires is enhanced with its increasing radius as a result of reaching sufficient coordination number of Ti and O atoms. All stoichiometric ${\text{TiO}}_{2}$ nanowires studied exhibit semiconducting behavior and have nonmagnetic ground state. There is a correlation between binding energy $({E}_{b})$ and energy band gap $({E}_{g})$ of ${\text{TiO}}_{2}$ nanowires. In general, ${E}_{b}$ increases with increasing ${E}_{g}$. In TiO nanowires, both metallic and semiconductor nanowires result. In this case, in addition to paramagnetic TiO nanowires, there are also ferromagnetic ones. We have also studied the structural and electronic properties of bulklike rutile (110) nanowires. There is a crossover in terms of energetics, and bulklike nanowires are more stable than the thin nanowires for larger radius wires after a critical diameter. These (110) rutile nanowires are all semiconductors.

Atomic structure of nanometer-sized amorphousTiO2

Amorphous titania $({\text{TiO}}_{2})$ is an important precursor for synthesis of single-phase nanocrystalline anatase. We synthesized amorphous titania by hydrolysis of titanium ethoxide at the ice point. Transmission electron microscopy examination and nitrogen gas adsorption indicated that the particle size of the synthesized titania is $\ensuremath{\sim}2\text{ }\text{nm}$. Synchrotron wide-angle x-ray scattering (WAXS) was used to probe the atomic correlations in this amorphous sample. Atomic pair-distribution function (PDF) derived from Fourier transform of the WAXS data was used for reverse Monte Carlo (RMC) simulations of the atomic structure of the amorphous ${\text{TiO}}_{2}$ nanoparticles. Molecular-dynamics simulations were used to generate input structures for the RMC. X-ray-absorption spectroscopy (XAS) simulations were used to screen candidate structures obtained from the RMC by comparing with experimental XAS data. The structure model that best describes both the WAXS and XAS data shows that amorphous ${\text{TiO}}_{2}$ particles consist of a highly distorted shell and a small strained anataselike crystalline core. The average coordination number of Ti is 5.3 and most Ti-O bonds are populated around $1.940\text{ }\text{\AA{}}$. Relative to bulk ${\text{TiO}}_{2}$, the reduction in the coordination number is primarily due to the truncation of the Ti-O octahedra at the amorphous nanoparticle surface and the shortening of the Ti-O bond length to the bond contraction in the distorted shell. The pre-existence of the anataselike core may be critical to the formation of single-phase nanocrystalline anatase in crystallization of amorphous ${\text{TiO}}_{2}$ upon heating.