Ti Valence Research Articles

Porous-suspended graphite carbon doped TiO2 improves the band structure of photocatalysts to enhance photocatalytic degradation of tetracycline in water

ABSTRACT In order to improve visible light utilization efficiency and enrich low-concentration antibiotics in water systems, a new type of floating TiO2 graphite carbon (FTDGC) photocatalyst was prepared by the one-pot method. These floating materials have good adsorption properties and sufficient mechanical strength, which makes them easy to recycle and reuse in processing. Its photocatalytic performance was evaluated by photocatalytic degradation of tetracycline (TC) in water. The results showed that FTDGC had a high adsorption degradation capacity for TC and the photocatalytic degradation process was accorded with a first-order kinetic equation. The Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy indicated the generation of Ti–C–Ti valence bonds and Ti–C–O bonds on the TiO2 surface with the modification of graphite carbon materials. Optimization of preparation conditions revealed that as the amount of activator phosphoric acid increased and the porosity of the solid surface increased, the external exposure of TiO2 made the effect of the composite catalyst more pronounced. The recycling experiment showed that FTDGC still had good degradation ability for TC after five times reuse. The catalysts developed in this research can provide a new idea and reference for the pretreatment of refractory organic wastewater, as well as for the reuse of medium water.

Cation Vacancies Enable Anion Redox in Li Cathodes.

Conventional Li-ion battery intercalation cathodes leverage charge compensation that is formally associated with redox on the transition metal. Employing the anions in the charge compensation mechanism, so-called anion redox, can yield higher capacities beyond the traditional limitations of intercalation chemistry. Here, we aim to understand the structural considerations that enable anion oxidation and focus on processes that result in structural changes, such as the formation of persulfide bonds. Using a Li-rich metal sulfide as a model system, we present both first-principles simulations and experimental data that show that cation vacancies are required for anion oxidation. First-principles simulations show that the oxidation of sulfide to persulfide only occurs when a neighboring vacancy is present. To experimentally probe the role of vacancies in anion redox processes, we introduce vacancies into the Li2TiS3 phase while maintaining a high valency of Ti. When the cation sublattice is fully occupied and no vacancies can be formed through transition metal oxidation, the material is electrochemically inert. Upon introduction of vacancies, the material can support high degrees of anion redox, even in the absence of transition metal oxidation. The model system offers fundamental insights to deepen our understanding of structure-property relationships that govern reversible anion redox in sulfides and demonstrates that cation vacancies are required for anion oxidation, in which persulfides are formed.

Electrochemical reduction processes and extraction of titanium with different valence states in molten CaO-CaF2 slags

Titanium is an excellent structural and functional material. At present, the production of titanium has long process cycle, high energy consumption, serious pollution and other characteristics, so the development of low-cost titanium production methods has extremely far-reaching significance. In this work, the electrochemical reduction process of Ti(IV) and Ti(II) in the CaF2-CaO melt were investigated by cyclic voltammetry and square wave voltammetry measurements at 1740 K, respectively. In the CaF2-CaO-5 wt% TiO2 melt, Ti(IV) ions were reduced to Ti by one step. One measure was taken to lower the valence state of Ti ions: Ti(IV) ions in the melt was changed to Ti(II) by adding Ti0. Ti(IV) ions were reduced in two steps: in the CaF2-CaO-5 wt% TiO2-Ti0 melt, Ti(IV) → Ti(II) and Ti(II) → Ti, respectively, and in the CaF2-CaO-5 wt% TiO, the Ti(II) ions were reduced in a single step: Ti(II) → Ti. It was found that both migration and mass transfer affected the reduction processes. In contrast to the Ti(IV) reduction to Ti process, the Ti(II) reduction to Ti process showed a higher diffusion rate. Electrolytes and electrodes were analyzed after titanium electrodeposition by XRD. It was found that there were no metal titanium was detected in the electrolyte placed close to the cathode after electrodeposition. It believed that the formed metallic Ti reacted with dissolved O2- in the melt to form TiO. The reduction process of Ti(IV) on the iron electrode was investigated, the metal Ti was reduced by alloying process, the oxidation of Ti was hindered, there was a dense structure layer of CaTiO3 outside the reduced metal Ti that prevented the ions transfer and affected the Ti(IV) ions migration process, which might be the main factor that the Ti(IV) reduction process was controlled by migration. The lower valence state of Ti in the melt promoted the formation of Ca2TiO4, which increased the ion migration rate and decreased the mass transfer rate. It was suggested that titanium or titanium alloys may be prepared by reducing the valence of Ti(IV) in minerals and altering the melt composition. It is feasible to prepare titanium and titanium alloys by MOE short process using ore as raw material.

(Invited) Revealing Charge Storage Processes at the Interfaces of Supercapacitor Electrodes

The fast development of electrochemical energy storage devices has revolutionized almost every aspect of modern life by enabling portable electronic devices, electric vehicles, and grid storage of renewable energy. To satisfy the growing industrial and consumer needs for energy storage systems with high output power and short recharge time, the electrode materials should have excellent high-rate performance.1 Electrical double-layer (EDL) capacitors exhibit high-rate capability and long-cycling life as they store charges electrostatically. Using ionic liquid (IL) as the electrolyte leads to a large voltage window but low capacitance. In this first session of the presentation, I will introduce a mathematical model to simulate the IL ion packing structure in nanopores. This model is capable of estimating the EDLC capacitance based on pore size distribution. Then, we further revealed that mixture IL ions can be selectively driven into the pores of different confinement levels, which was confirmed by solid-state NMR, and further explained by DFT simulation2.In the second part of the presentation, I will shift to pseudocapacitors, which are expected to have a much higher charge storage capacity than EDL capacitors and a much higher rate than batteries as they store energy through fast surface redox reactions.3 The emerging 2D material family, 2D transition metal carbides/nitrides MXenes, shows outstanding pseudocapacitive performance due to their ionophilicity, metallic conductivity, and highly reactive surfaces. The proper coupling between electrolyte and electrode is critical to increase energy and power density. In the organic electrolyte, we found that the solvent has a strong impact on the ion/solvent arrangement in 2D MXene material and hence the charge storage capability.4 In the aqueous electrolytes, we successfully introduced surface redox reactions to MXene electrodes by using the water-in-salt electrolytes and by adjusting the initial valence of Ti in Ti3C2 MXenes, which dramatically increases the capacitance of MXene in the neutral aqueous electrolytes5.References Simon, P.; Gogotsi, Y., Nat. Mater. 2020, 19 (11), 1151-1163. Wang, X.; Mehandzhiyski, A. Y.; et al., J. Am. Chem. Soc. 2017, 139 (51), 18681-18687. Fleischmann, S.; Mitchell, J. B.; et al., Chem. Rev. 2020, 120 (14), 6738-6782. Wang, X.; Mathis, T. S.; et al., Nat. Energy 2019, 4 (3), 241-248. Wang, X.; Mathis, T. S.; et al., ACS Nano 2021, 15 (9), 15274-15284.

Modulation of Bipolar Ultraviolet Current in TiO2 Nanofilms for Switching Logic Devices via Ti Valence State Control

Modulation of Bipolar Ultraviolet Current in TiO₂ Nanofilms for Switching Logic Devices via Ti Valence State Control

Dielectric relaxations without colossal permittivity in Mg and Nb co-doped BaTiO3 ceramics

Magnesium (Mg) and Niobium (Nb) co-doped BaTiO3 ceramics are prepared using a solid-state reaction method under an air atmosphere, and their crystal structures, microstructures, valence states and dielectric properties are studied. The ceramics adopt a cubic perovskite structure, and the lattice parameter increases with doping content owing to the larger ionic radii of dopants. The complex permittivity and electric modulus as a function of frequency and temperature are analyzed. Dielectric relaxations without colossal permittivity are observed. The relaxation processes are thermally activated and their variations with temperature are analyzed. The activation energy values for the relaxation processes are determined by the Arrhenius relationship and increase with doping content. By analyzing the electric modulus, the activation values for high-frequency relaxation are found to be associated with the electrical properties of grain, and the contribution of grain boundary could be neglected based on internal barrier layer capacitor (IBLC) model. The electron hopping between the different Ti valences, which is confirmed by X-ray photoelectron spectroscopy, is supposed to play an important role in high-frequency relaxation. The permittivity plateaus are related to the inhomogeneous structures of grain and grain boundary, and the low-frequency permittivity plateau is attributed to the space-charge polarization at grain boundaries. By comparing with Cu/Nb co-doped BaTiO3, the absence of colossal permittivity is ascribed to high resistivity of the samples. Charge density and migration as well as the insulating grain boundaries are critical for the formation of space charge and a high permittivity.

Novel (1-x)MgCu2Nb2O8 − xTiO2 composite ceramics with high Q×f and near-zero τ f

For the MCu2Nb2O8 (M=Ca, Mg, Zn or Ni) composite ceramics, it is still a difficult problem to achieve both high Q×f valule and near-zero temperature coefficient of resonant frequency (τ f ). In this paper, the (1-x)MgCu2Nb2O8 − xTiO2 (0 ≤ x ≤ 0.28) ceramics were fabricated by the two-step sintering (TSS) technique. Results of x-ray diffraction (XRD) and Raman spectroscopy proved that the ceramic samples were composite phases of MgNb2O6, CuO and TiO2. The chemical compositions and valences of Mg, Cu, Nb, O and Ti were investigated by x-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM) showed that proper TiO2 addition could promote growth of grains and increase the average grain size. Appropriate TiO2 content and enhanced densification contributed to the 0.84MgCu2Nb2O8 − 0.16TiO2 ceramics sintered at 935 °C with ε r = 18.75, Q×f = 34,398 GHz (f 0 = 7.8 GHz), and τ f = −0.56 ppm/°C, exhibiting potential applications in Low Temperature Co-fired Ceramic (LTCC) devices.

Intensifying Electrochemical Activity of Ti3C2Tx MXene via Customized Interlayer Structure and Surface Chemistry.

MXene, a new intercalation pseudocapacitive electrode material, possesses a high theoretical capacitance for supercapacitor application. However, limited accessible interlayer space and active sites are major challenges to achieve this high capacitance in practical application. In order to stimulate the electrochemical activity of MXene to a greater extent, herein, a method of hydrothermal treatment in NaOH solution with reducing reagent-citric acid is first proposed. After this treatment, the gravimetric capacitance of MXene exhibits a significant enhancement, about 250% of the original value, reaching 543 F g-1 at 2 mV s-1. This improved electrochemical performance is attributed to the tailoring of an interlayer structure and surface chemistry state. An expanded and homogenized interlayer space is created, which provides enough space for electrolyte ions storage. The -F terminations are replaced with O-containing groups, which enhances the hydrophilicity, facilitating the electrolyte's accessibility to MXene's surface, and makes MXene show stronger adsorption for electrolyte ion-H+, providing sufficient electrochemical active sites. The change in terminations further leads to the increase in Ti valence, which becomes more prone to reduction. This work establishes full knowledge of the rational MXene design for electrochemical energy storage applications.

Nitrogen doping mediated oxygen vacancy and Ti valence regulation to enhance photocatalytic H2 generation

The band gap width has always been the main reason for the low photocatalytic performance of TiO2 semiconductors. “Band gap engineering” is often used to change the band gap width of TiO2. On this basis, the impurity band gap was induced below the TiO2 conduction band by regulating N element. What's interesting was that N element adjusted the valence state of adjacent Ti element and caused the generation of oxygen vacancy. This is due to the extraction of neutral oxygen atoms, resulting in electron transfer to the Ti cation band structure. The 300-N-T catalyst exhibited the best photocatalytic hydrogen production performance under visible light, which was 307.65 μmol/(g·h) and was 61.53 times higher than that of anatase. And the concentration of Ti3+ defects and oxygen vacancy reduced with decreasing of N content in TiO2. According to Kubelke Munk formula, the band gap of 300-N-T photocatalyst was narrowed to 2.73 eV, which indicated that there are impurity energy levels near the CB and VB of TiO2 material. The Ti3+ ions as the active site of the photocatalytic reaction was confirmed by instantaneous photocurrent response and EIS characteristic. This study revealed the structure-activity relationship between the content of N element, Ti3+ ions and oxygen vacancy in TiO2 materials, as well as the decisive process of defect induced visible light catalysis.

Influences of substituting of (Ni1/3Nb2/3)4+for Ti4+on the phase compositions, microstructures, and dielectric properties of Li2Zn[Ti1−x(Ni1/3Nb2/3)x]3O8(0 ≤x≤ 0.3) microwave ceramics

Complex ions substitution is gaining more attention as an appealing method of modifying the structure and performance of microwave ceramics. In this work, Li₂Zn[Ti_1-<em>x</em>(Ni_1/3Nb_2/3)<em>_x</em>]₃O₈ (LZTNN<em>x</em>, 0 ≤ <em>x</em> ≤ 0.3) ceramics were designed based on complex ions substitution strategy, following the substitution rule of radius and valence to investigate the relationships between phase composition (containing oxygen vacancies and Ti³⁺ ions), microstructure, and microwave dielectric characteristics of LZTNN<em>x</em> ceramics. The samples maintained a single Li₂ZnTi₃O₈ solid solution phase as <em>x</em> ≤ 0.2 whereas the <em>x</em> = 0.3 sample produced a secondary phase with LiNbO₃ structure. The appropriate amount of (Ni_1/3Nb_2/3)⁴⁺ substitution could slightly improve the densification of LZTNN<em>x</em> ceramics due to the formation of Li₂ZnTi₃O₈ solid solution accompanied by a decrease in average grain size. The presence of a new <em>A</em>_1g Raman active band at about 848 cm^-1 indicated that the local symmetry changed, affecting the atomic interactions of LZTNN<em>x</em> ceramics. The variation in dielectric constant (ε<em>_r</em>) was closely related to molar volume ionic polarizability, and the temperature coefficient of resonant frequency (τ<em>_f</em>) was related to the bond valence of Ti. The increase in density, the absences of Ti³⁺ ions and oxygen vacancies, and the reduction in damping behavior were responsible for decreased dielectric loss. The LZTNN0.2 ceramic sintered at 1120 ^oC exhibited favorable microwave dielectric properties: ε<em>_r</em> = 22.13, quality factor (<em>Q</em>×<em>f)</em> = 97350 GHz, and τ<em>_f</em> = −18.6 ppm/^oC, which might be a promising candidate for wireless communication applications in highly selective electronics.

Interlayer Structure and Chemistry Engineering of MXene-Based Anode for Effective Capture of Chloride Anions in Asymmetric Capacitive Deionization.

Negatively charged surfaces and readily oxidizabile characteristics fundamentally restrict the use of MXene building blocks as anodes for anion intercalation. Herein, by embedding bacterial cellulose nanofibers with conformal polypyrrole coating (BC@PPy) and populating them between MXene (Ti3C2Tx) interlayers, we enable the fabricated MXene/BC@PPy (MBP) composite films to be highly efficient anodes for Cl--capturing in asymmetric capacitive deionization (CDI) systems. Performance gains are realized due to the surface electronegativity of MXene nanosheets becoming compensated by positively charged BC@PPy nanofibers, alleviating electrostatic repulsion, thus realizing reversible Cl- intercalation. More crucially, the anodization voltage of MBP is effectively enhanced as a result of the increase of the Ti valence state in MXene nanosheets with the addition of the BC@PPy spacer. Furthermore, BC@PPy nanopillars effectively enlarge the interlayer space for facile Cl- de-/intercalation, improve the vertical electron transfer between loosely deposited MXene nanosheets, and perform as additional active materials for Cl--capturing. Consequently, the MBP anode exhibits a promising desalination capacity of up to 17.56 mg g-1 at 1.2 V with a high capacity retention of 94.6% after 30 cycles in an asymmetric CDI system. This work offers a simple and effective strategy to unlock the application potential of MXene building blocks as anodes for Cl--capturing in electrochemical desalination.

XAFS Analysis of Sulfuric Acid Solution Related to the Effect of Bi in Suppressing the Disproportionation Reaction of Mn&lt;sup&gt;3+&lt;/sup&gt; in Mn/Ti Redox Flow Cells

In sulfuric acid (H2SO4) aqueous solutions containing manganese (Mn) and titanium (Ti) ions, which are used for the positive electrolyte in redox flow (RF) batteries, the addition of a small quantity of bismuth (Bi) has been found to be very effective in suppressing the MnO2 precipitation in the charging process from Mn2+ to Mn3+. The effect of the addition of Bi on the structural change in these solutions has been analyzed using x-ray absorption fine structure (XAFS) techniques. X-ray absorption near edge structure (XANES) spectra indicated that some Mn4+ ions were generated only when Bi was not added. It was also confirmed that the Mn-O octahedral coordination was maintained at a high state of charge (SOC). On the other hand, no change was observed in the valence of Ti and Bi ions even at high SOCs. Extended x-ray absorption fine structure (EXAFS) measurements combined with fitting analysis clarified that the addition of Bi resulted in the generation of Mn-O-Bi coordination at the SOCs of 70 and 90 %. Bi-O-Mn coordination may suppress the MnO2 precipitation at a high SOC.

Synthesis of Oxide Interface-Based Two-Dimensional Electron Gas on Si.

Two-dimensional electron gas (2DEG) at the interface of amorphous Al2O3/SrTiO3 (aAO/STO) heterostructures has received considerable attention owing to its convenience of fabrication and relatively high mobility. The integration of these 2DEG heterostructures on a silicon wafer is highly desired for electronic applications but remains challanging up to date. Here, conductive aAO/STO heterostructures have been synthesized on a silicon wafer via a growth-and-transfer method. A scanning transmission electron microscopy image shows flat and close contact between STO membranes and a Si wafer. Electron energy loss spectroscopic measurements reveal the interfacial Ti valence state evolution, which identifies the formation of 2D charge carriers confined at the interface of aAO/STO. This work provides a feasible strategy for the integration of 2DEG on a silicon wafer and other desired substrates for potential functional and flexible electronic devices.

Effect of HF etching on titanium carbide (Ti3C2Tx) microstructure and its capacitive properties

Two-dimensional multi-layered microstructure generated by hydrofluoric acid etching is essential for the high capacitance MXenes. However, the mechanisms dictating the formation, evolution and properties of MXenes during etching remain elusive due to the lack of direct observation. Herein, using ex-situ scanning electron microscopy combined with high resolution transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy, we have revealed the fine variation of microstructure and properties of multi-layered titanium carbide Ti3C2Tx etched for different time. The results show that the laminar cracks always formed from the first initial laminar crack at the edge site of the outermost side of the Ti3AlC2 particle, which induces continuous nucleation of other laminar cracks in the nearby area. The early laminar cracks undergo growth accompanying with increased macro-layer density of Ti3C2Tx and the Al(F,O,C)3 precipitation in subsequent etching process. The formation of laminar cracks is revealed to be correlated with the release of local internal stress in Ti3AlC2. In addition, it is found that there are significant changes of Ti valence state, CC and Ti-C bonding as well as Ti-F, CO and Al-O surface groups during etching. Maximum specific capacitance coincides with maximum lattice parameter c of Ti3C2Tx and maximum number of macro-layers, while over-etching produces in-plane microcracks, Ti dissolution, and reduced specific capacitance. The results provide important insight to facilitate optimal synthesis of this material.

Electrophoretic deposition of Ti3C2Tx MXene nanosheet N-carbon cloth as binder-free supercapacitor electrode material

Despite the existing advances of Ti3C2Tx MXene as supercapacitor electrode material, its capacitance can be further enhanced through the composite strategy. In this work, the nitrogen-doped superhydrophilic carbon cloth (ENCC) was firstly prepared by N-doping of carbon cloth (CC), and then Ti3C2Tx MXene nanosheets were electrophoretically deposited to yield the binder-free Ti3C2Tx(EPD)/ENCC as supercapacitor electrode material. As a result, the composite electrode exhibited an area-specific capacitance of 2080.1 mF·cm−2 at 1 mA·cm−2 current density. The analysis implied that the varying Ti valence states provided certain psedocapacitance to the capacitance of the as-prepared electrode, though the main contribution was still dominantly from the electric double layer capacitance (EDLC). Further, the assembled symmetric supercapacitor yielded a wide voltage window of 1.8 V, and a good cyclic stability with 91% capacitance retention after 10,000 charge/discharge cycles at 20 mA·cm−2. Overall, the employed composite strategy enabled the good dispersion of the MXene flakes on the surface of the flexible substrate. Meanwhile, the large number of functional groups (-OH, -F, etc.) on the surface of Ti3C2Tx MXene interacted with the carbon layer through hydrogen bonding effects, which endows it as an ideal electrode for electrochemical capacitors.

Synthesis and electronic properties of epitaxial SrNiO3/SrTiO3 superlattices

The functionality of materials is critically dependent on structures and electronic properties. The valence of transition metal (TM) cations in complex oxides is key and each TM exhibits a fixed range of stable values. For Ni, the formal oxidation state is 2+, but can reach 3+ for certain compositions. By limiting SrNiO${}_{3}$ film thickness to 1 unit cell in (SrNiO${}_{3}$)${}_{1}$/(SrTiO${}_{3}$)${}_{n}$ superlattices, the authors demonstrate that the Ni valence exceeds 3+ whereas the Ti valence remains at 4+, and that neither valence changes with $n$. This work demonstrates that the structural environment can be controlled to achieve electronic properties not normally found in bulk materials by means of superlattice formation.

Tailoring the oxidation state of metallic TiO through Ti3+/Ti2+ regulation for photocatalytic conversion of CO2 to C2H6

The regulation and stabilization of the oxidation state to promote the conversion of CO2 to C2 fuel still faces many challenges. Based on the principle of charge balance, we creatively propose a co-doping strategy to adjust the surface oxidation state distribution of metallic catalysts. A TiO-based photocatalyst co-doped with Zn and N was synthesized by ammonia assisted one-step calcination method, named ZN-TC. XPS characterization shows that Zn and N adjust the valence states of adjacent Ti elements respectively, so that the surface of TiO maintains a relatively stable Ti3+/Ti2+ ratio. Under visible light irradiation, the material can catalyze CO2 into CO (324.11 μmol·g−1·h−1) and C2H6 (10.27 μmol·g−1·h−1) in the liquid phase. The selectivity of C2H6 reached 14.45%. When irradiated with near-infrared light, ZN-TC shows 100% CO selectivity because the photon energy is not enough to support the catalytic hydrogenation of CO2. Theoretical calculations and experiments proved that Zn and N elements mainly act on the B-1 band to regulate the Ti valence state. In-situ DRIFTS and in-situ Raman tests confirmed the function of oxidation state adjustment to promote the C-C coupling on the catalyst surface to produce ethoxy groups, which ultimately led to the production of C2H6.

Effect of Carbon Source on Synthesis of Carbon Coated Lithium Titanate

Carbon coated lithium titanate (Li4Ti5O12/C) was obtained by a facile solid state approach in inert Ar atmosphere. The composition, morphology, residual carbon content and Ti valence of the samples were systematically investigated. The carbon content of Li4Ti5O12/C should be optimized, since excess carbon in the composite leads to the reduction of Ti (IV) to form Ti (III), which results in large irreversible capacity of Li4Ti5O12/C. With an optimal carbon content of 0.68wt%, the Li4Ti5O12/C sample shows high rate capabilities and good cycling ability, delivering discharge capacities of 160.8 mAh/g at 5C. The superior high rate properties are ascribed to the specific nanostructures, which enables fast electronic and ionic transport by introducing carbon coating and decreasing the particle size of lithium titanate.

Spinel-type solid solution ceramic MgAl2O4-Mg2TiO4 with excellent microwave dielectric properties

In this work, a series of (1-x)MgAl2O4-xMg2TiO4 (expressed as (1-x)MA- xMT) (0.0 ≤ x ≤ 0.9) ceramics were successfully prepared via a solid-state reaction, and the effect of Mg2TiO4 content on the phase compositions, microstructures and microwave dielectric properties of MgAl2O4 ceramics have been investigated. The MgAl2O4 could form a solid solution with Mg2TiO4, and the grain size of the (1-x)MA-xMT ceramics gradually increased with the increase of sintering temperature. The dielectric constants (εr) of (1-x)MA-xMT ceramics steadily increased with the increase of Mg2TiO4 content. The εr values were mainly affected by secondary phase and dielectric polarizability. The quality factor (Qf) of (1-x)MA-xMT ceramics was closely related to the relative density, secondary phase, cation vacancies and Ti valence; whereas the temperature coefficient of resonance frequency (τf) highly depended on the Al-site bond strength. For the sample at x = 0.9, the 0.1MA-0.9MT ceramics sintered at 1350 °C for 4 h exhibited the optimized microwave dielectric properties: εr = 12.36, Qf = 236,600 GHz and τf = −61 ppm/°C.

A high-pressure Raman study of FeTiO3 ilmenite: Fermi resonance as a manifestation of Fe-Ti charge transfer

We investigated the 300 K high-pressure behavior of ilmenite using Raman spectroscopy to 54 GPa. Upon compression, we observe a Fermi resonance between the lowest frequency Ag symmetry peaks (ν4 and ν5) between ~ 10 and ~ 30 GPa: bands that involve major components of Ti–O and Fe–O-related displacements, respectively. The peaks’ relative intensities switch at ~ 18 GPa and they also reach their minimum separation at ~ 20 GPa, indicating that their maximum resonance occurs between 18 and 20 GPa. The negative shift of the Ti–O-associated ν4 vibration under compression is fully consistent with a shift in valence of Ti from 4 + to 3 + under compression. Anomalously small mode shifts of other, more localized vibrations are also consistent with a charge transfer from Fe to Ti under compression. At higher pressures, we have not found definitive evidence for a transition to the perovskite-structure at 300 K, which has been well characterized at high pressures and temperatures. At 40 GPa, we observe an apparent reversible disordering that persists up to our highest pressure. The 300 K mode shifts of the Raman active modes in FeTiO3 under pressure are notably different from those of other ABO3 compounds (where A = Mg, Mn and B = Ti, Si); in other ilmenite-structured compounds, the peaks shift at a faster rate and there has not been any observation of Fermi resonance. Thus, iron’s complex electronic structure, and its charge transfer with titanium, appears to play a primary role in the behavior of phonons in FeTiO3 ilmenite.