Baddeleyite-Type High-Pressure Phase of TiO2

Low density phases of TiO2 by cluster self-assembly

The interest in titanium dioxide (TiO2 ) phases is growing due to the number of applications in cosmetics, food industry and photocatalysis, an increase that is driven by its exceptional properties when engineered at the nanoscale like in the form of nanoparticles. Our goal is to discover unknown low-density phases of TiO2 , with potential for applications in various fields. We then use well-known TiO2 clusters as fundamental building blocks to be self-assembled into unique structures to study their distinct characteristics. Density functional calculations are employed to relax the structures and identify the most stable TiO2 structures within an energy range of 0.1 eV per atom from the rutile and anatase phases, which are confirmed, validating our methodology. Going beyond conventional phases, we found two-dimensional TiO2 structures, previously explored in separate studies, and showing typical structures of transition metal dichalcogenide layers, that forge a bridge between different TiO2 structures. It is noteworthy that our investigation uncovered an entirely novel class of TiO2 structures featuring hexagonal cages like beehive channels, opening novel phases with huge potential. These discovered low-density phases are interesting, particularly the hexagonal cage structures with remarkable large gaps, because they introduce other dimensions for uncharted applications in the ever-growing TiO2 landscape.

Nonaqueous Chemistry of Group 4 Oxo Clusters and Colloidal Metal Oxide Nanocrystals

We review the nonaqueous precursor chemistry of the group 4 metals to gain insight into the formation of their oxo clusters and colloidal oxide nanocrystals. We first describe the properties and structures of titanium, zirconium, and hafnium oxides. Second, we introduce the different precursors that are used in the synthesis of oxo clusters and oxide nanocrystals. We review the structures of group 4 metal halides and alkoxides and their reactivity toward alcohols, carboxylic acids, etc. Third, we discuss fully condensed and atomically precise metal oxo clusters that could serve as nanocrystal models. By comparing the reaction conditions and reagents, we provide insight into the relationship between the cluster structure and the nature of the carboxylate capping ligands. We also briefly discuss the use of oxo clusters. Finally, we review the nonaqueous synthesis of group 4 oxide nanocrystals, including both surfactant-free and surfactant-assisted syntheses. We focus on their precursor chemistry and surface chemistry. By putting these results together, we connect the dots and obtain more insight into the fascinating chemistry of the group 4 metals. At the same time, we also identify gaps in our knowledge and thus areas for future research.

High-Pressure Phase Diagram of the Ti-O System

Titanium oxides are technologically important compounds. The chemistry of the Ti-O system is quite rich, largely because of the multiple oxidation states that titanium atoms can take. In this work, using a combination of variable-composition evolutionary crystal structure prediction (USPEX code) and data mining (Materials Project), we predicted all of the stable titanium oxides in the pressure range 0-200 GPa and found that 27 compounds can be stable at different pressures. We resolved contradictions between previous works and predicted four hitherto-unknown stable phases: P2₁/c-TiO₃, I4/mmm-Ti₃O₂, Imm2-Ti₅O₂, and R3̅-Ti₁₂O₅. We also showed that the high-pressure P6̅m2-TiO phase is an electride.

Characterisation of oxygen defects and nitrogen impurities in TiO2 photocatalysts using variable-temperature X-ray powder diffraction

TiO₂-based powder materials have been widely studied as efficient photocatalysts for water splitting due to their low cost, photo-responsivity, earthly abundance, chemical and thermal stability, etc. In particular, the recent breakthrough of nitrogen-doped TiO₂, which enhances the presence of structural defects and dopant impurities at elevated temperatures, exhibits an impressive visible-light absorption for photocatalytic activity. Although their electronic and optical properties have been extensively studied, the structure-activity relationship and photocatalytic mechanism remain ambiguous. Herein, we report an in-depth structural study of rutile, anatase and mixed phases (commercial P25) with and without nitrogen-doping by variable-temperature synchrotron X-ray powder diffraction. We report that an unusual anisotropic thermal expansion of the anatase phase can reveal the intimate relationship between sub-surface oxygen vacancies, nitrogen-doping level and photocatalytic activity. For highly doped anatase, a new cubic titanium oxynitride phase is also identified which provides important information on the fundamental shift in absorption wavelength, leading to excellent photocatalysis using visible light.

Riesite, a New High Pressure Polymorph of TiO2 from the Ries Impact Structure

This paper describes riesite, a new high-pressure polymorph of TiO2 from the Ries impact structure, Germany. Riesite occurs in shock-induced melt veins within xenoliths of bedrock in suevite. It is structurally closely related to srilankite from which it differs by having two distinct cation sites rather than one and through its monoclinic symmetry. It is indicative that riesite forms only upon release from the shock state upon back transformation from akaogiite.

Low-pressure-responsive heat-storage ceramics for automobiles

The accumulated heat energy of a heat-storage material is typically released over time. If a heat-storage material could preserve its accumulated heat energy for a prolonged period, the applicability of such materials would be expanded greatly. Herein we report a newly fabricated heat-storage material that can store latent heat energy for a long period and release the heat energy upon demand by applying an extremely low pressure. This material is a block-type lambda trititanium pentoxide (block-type λ-Ti₃O₅). The block-type λ-phase accumulates a large heat energy of 237 kJ L⁻¹ and exhibits a pressure-induced phase transition to beta trititanium pentoxide. The pressure-induced phase transition occurs by applying only several tens of bars, and half of the fraction transforms by 7 MPa (70 bar). Such a low-pressure-responsive heat-storage ceramic is effective to reuse excessive heat in automobiles or waste heat at industrial factories.

Structural Phase Transition and Metallization of Nanocrystalline Rutile Investigated by High-Pressure Raman Spectroscopy and Electrical Conductivity

We investigate the structural, vibrational, and electrical transport properties of nanocrystalline rutile and its high-pressure polymorphs by Raman spectroscopy, and AC complex impedance spectroscopy in conjunction with the high-resolution transmission electron microscopy (HRTEM) up to ~25.0 GPa using the diamond anvil cell (DAC). Experimental results indicate that the structural phase transition and metallization for nanocrystalline rutile occurred with increasing pressure up to ~12.3 and ~14.5 GPa, respectively. The structural phase transition of sample at ~12.3 GPa is confirmed as a baddeleyite phase, which is verified by six new Raman characteristic peaks. The metallization of the baddeleyite phase is manifested by the temperature-dependent electrical conductivity measurements at ~14.5 GPa. However, upon decompression, the structural phase transition from the metallic baddeleyite to columbite phases at ~7.2 GPa is characterized by the inflexion point of the pressure coefficient and the pressure-dependent electrical conductivity. The recovered columbite phase is always retained to the atmospheric condition, which belongs to an irreversible phase transformation.

Similarity Between Amorphous and Crystalline Phases: The Case of TiO2

Amorphous and crystalline materials differ in their long-range structural order. On the other hand, short-range order in amorphous and crystalline materials often appears similar. Here, we use a recently introduced method for obtaining quantitative measures for structural similarity to compare crystalline and amorphous materials. We compare seven common crystalline polymorphs of TiO₂, all assembled out of TiO₆ or TiO₇ polyhedral building blocks, to liquid and amorphous TiO₂ in a quantitative two-dimensional similarity plot. We find high structural similarity between a model of amorphous TiO₂, obtained by ab initio molecular-dynamics, and the B-TiO₂ crystalline polymorph. The general approach presented here sheds new light on a long-standing controversy in the structural theory of amorphous solids.

The influence of temperature, pressure and Ag doping on the physical properties of TiO2 nanoceramics

Undoped and Ag-doped TiO₂ ceramics have been prepared at temperatures between 500-1000 °C and under pressures up to 8 GPa. Their crystal structures and physical properties were investigated by means of EDX, SEM, TEM, X-ray powder diffraction, and magnetization M, specific heat C_p and electrical resistance ρ measurements. It is found that the anatase-structured As-cast powder transforms into rutile and columbite-type at 500 °C and 5.5 GPa. The stabilization of the latter phase is fulfilled under a pressure of 8 GPa and at temperatures above 800 °C. On the basis of experimental results, we conclude that the physical properties of TiO₂ can be tailored along with its crystal structure. In particular, magnetic properties change from paramagnetic in anatase and rutile to magnetic correlations and in all likelihood magnetic-field-induced antiferromagnetic short-range order in columbite-structured TiO₂. Contrasting behaviour in the temperature dependences of specific heat between anatase/rutile and columbite-type TiO₂ is obvious. Differently from anatase/rutile, the C_p of columbite-type TiO₂ exhibits a low-temperature excess, being interpreted as due to magnetic correlations, or else the prevalence of soft modes. An analysis of ρ(T) for columbite-type TiO₂ in the temperature range of 280-400 K reveals the presence of a new trapping state at an energy level of ∼28 meV within the originally forbidden gap. Furthermore, thermal fluctuation-induced tunnelling and hopping conductivities are suggested to govern in a lower temperature range. We recognize that the Ag-doped contents do not alter the crystal structure but considerably enhance magnetic correlations, compared to undoped samples.

Nanocrystals in compression: unexpected structural phase transition and amorphization due to surface impurities

We report an unprecedented surface doping-driven anomaly in the compression behaviors of nanocrystals demonstrating that the change of surface chemistry can lead to an interior bulk structure change in nanoparticles. In the synchrotron-based X-ray diffraction experiments, titania nanocrystals with low concentration yttrium dopants at the surface are found to be less compressible than undoped titania nanocrystals. More surprisingly, an unexpected TiO2(ii) phase (α-PbO2 type) is induced and obvious anisotropy is observed in the compression of yttrium-doped TiO2, in sharp contrast to the compression behavior of undoped TiO2. In addition, the undoped brookite nanocrystals remain with the same structure up to 30 GPa, whereas the yttrium-doped brookite amorphizes above 20 GPa. The abnormal structural evolution observed in yttrium-doped TiO2 does not agree with the reported phase stability of nano titania polymorphs, thus suggesting that the physical properties of the interior of nanocrystals can be controlled by the surface, providing an unconventional and new degree of freedom in search for nanocrystals with novel tunable properties that can trigger applications in multiple areas of industry and provoke more related basic science research.

Pressure induced structural transformations of anatase TiO2 nanotubes probed by Raman spectroscopy and synchrotron X-ray diffraction

Pressure-induced transformations of anatase TiO₂ nanotubes probed by in situ Raman spectroscopy and synchrotron X-ray diffraction reveal novel compression behaviors.

Prediction of 10-fold coordinated TiO2 and SiO2 structures at multimegabar pressures

We predict by first-principles methods a phase transition in TiO2 at 6.5 Mbar from the Fe2P-type polymorph to a ten-coordinated structure with space group I4/mmm. This is the first report, to our knowledge, of the pressure-induced phase transition to the I4/mmm structure among all dioxide compounds. The I4/mmm structure was found to be up to 3.3% denser across all pressures investigated. Significant differences were found in the electronic properties of the two structures, and the metallization of TiO2 was calculated to occur concomitantly with the phase transition to I4/mmm. The implications of our findings were extended to SiO2, and an analogous Fe2P-type to I4/mmm transition was found to occur at 10 TPa. This is consistent with the lower-pressure phase transitions of TiO2, which are well-established models for the phase transitions in other AX2 compounds, including SiO2. As in TiO2, the transition to I4/mmm corresponds to the metallization of SiO2. This transformation is in the pressure range reached in the interiors of recently discovered extrasolar planets and calls for a reformulation of the equations of state used to model them.

External stimulation-controllable heat-storage ceramics

Commonly available heat-storage materials cannot usually store the energy for a prolonged period. If a solid material could conserve the accumulated thermal energy, then its heat-storage application potential is considerably widened. Here we report a phase transition material that can conserve the latent heat energy in a wide temperature range, T<530 K and release the heat energy on the application of pressure. This material is stripe-type lambda-trititanium pentoxide, λ-Ti3O5, which exhibits a solid-solid phase transition to beta-trititanium pentoxide, β-Ti3O5. The pressure for conversion is extremely small, only 600 bar (60 MPa) at ambient temperature, and the accumulated heat energy is surprisingly large (230 kJ L(-1)). Conversely, the pressure-produced beta-trititanium pentoxide transforms to lambda-trititanium pentoxide by heat, light or electric current. That is, the present system exhibits pressure-and-heat, pressure-and-light and pressure-and-current reversible phase transitions. The material may be useful for heat storage, as well as in sensor and switching memory device applications.

Ten-fold coordinated polymorph and metallization of TiO2under high pressure

A stable and metallic CaC₂-type structure of TiO₂is identified with the highest coordination number among the known phases.

Polymorphic phase transition among the titania crystal structures using a solution-based approach: from precursor chemistry to nucleation process

Nanocrystalline titania are a robust candidate for various functional applications owing to its non-toxicity, cheap availability, ease of preparation and exceptional photochemical as well as thermal stability. The uniqueness in each lattice structure of titania leads to multifaceted physico-chemical and opto-electronic properties, which yield different functionalities and thus influence their performances in various green energy applications. The high temperature treatment for crystallizing titania triggers inevitable particle growth and the destruction of delicate nanostructural features. Thus, the preparation of crystalline titania with tunable phase/particle size/morphology at low to moderate temperatures using a solution-based approach has paved the way for further exciting areas of research. In this focused review, titania synthesis from hydrothermal/solvothermal method, conventional sol-gel method and sol-gel-assisted method via ultrasonication, photoillumination and ILs, thermolysis and microemulsion routes are discussed. These wet chemical methods have broader visibility, since multiple reaction parameters, such as precursor chemistry, surfactants, chelating agents, solvents, mineralizer, pH of the solution, aging time, reaction temperature/time, inorganic electrolytes, can be easily manipulated to tune the final physical structure. This review sheds light on the stabilization/phase transformation pathways of titania polymorphs like anatase, rutile, brookite and TiO2(B) under a variety of reaction conditions. The driving force for crystallization arising from complex species in solution coupled with pH of the solution and ion species facilitating the orientation of octahedral resulting in a crystalline phase are reviewed in detail. In addition to titanium halide/alkoxide, the nucleation of titania from other precursors like peroxo and layered titanates are also discussed. The non-aqueous route and ball milling-induced titania transformation is briefly outlined; moreover, the lacunae in understanding the concepts and future prospects in this exciting field are suggested.

Quenching ilmenite with a high-temperature and high-pressure phase using super-high-energy ball milling

The mass production of highly dense oxides with high-temperature and high-pressure phases allows us to discover functional properties that have never been developed. To date, the quenching of highly dense materials at the gramme-level at ambient atmosphere has never been achieved. Here, we provide evidence of the formation of orthorhombic Fe2TiO4 from trigonal FeTiO3 as a result of the high-temperature (>1250 K) and high-pressure (>23 GPa) condition induced by the high collision energy of 150 gravity generated between steel balls. Ilmenite was steeply quenched by the surrounding atmosphere, when iron-rich ilmenite (Fe2TiO4) with a high-temperature and high-pressure phase was formed by planetary collisions and was released from the collision points between the balls. Our finding allows us to infer that such intense planetary collisions induced by high-energy ball milling contribute to the mass production of a high-temperature and high-pressure phase.

Non-aqueous sol–gel synthesis through a low-temperature solvothermal process of anatase showing visible-light photocatalytic activity

A novel, facile method based on a non-aqueous sol–gel solvothermal process has been developed to synthesise spherical TiO₂ nanoparticles (NPs) in one pot.

High-pressure synthesis, crystal structure, and phase stability relations of a LiNbO3-type polar titanate ZnTiO3 and its reinforced polarity by the second-order Jahn-Teller effect

A polar LiNbO3-type (LN-type) titanate ZnTiO3 has been successfully synthesized using ilmenite-type (IL-type) ZnTiO3 under high pressure and high temperature. The first principles calculation indicates that LN-type ZnTiO3 is a metastable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement using synchrotron powder X-ray diffraction data reveals that LN-type ZnTiO3 crystallizes into a hexagonal structure with a polar space group R3c and exhibits greater intradistortion of the TiO6 octahedron in LN-type ZnTiO3 than that of the SnO6 octahedron in LN-type ZnSnO3. The estimated spontaneous polarization (75 μC/cm(2), 88 μC/cm(2)) using the nominal charge and the Born effective charge (BEC) derived from density functional perturbation theory, respectively, are greater than those of ZnSnO3 (59 μC/cm(2), 65 μC/cm(2)), which is strongly attributed to the great displacement of Ti from the centrosymmetric position along the c-axis and the fact that the BEC of Ti (+6.1) is greater than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-type ZnTiO3 is greater than that of LiNbO3 (62 μC/cm(2), 76 μC/cm(2)), indicating that LN-type ZnTiO3, like LiNbO3, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-type ZnTiO3 is 24 times greater than that of LN-type ZnSnO3. The findings indicate that the intraoctahedral distortion, spontaneous polarization, and the accompanying SHG response are caused by the stabilization of the polar LiNbO3-type structure and reinforced by the second-order Jahn-Teller effect attributable to the orbital interaction between oxygen ions and d(0) ions such as Ti(4+).

Structural stability of TiO2 at high pressure in density-functional theory based calculations

A new study on the pressure-induced phase transitions of TiO(2) has been performed using all-electron density-functional theory based computations with the projector augmented wave and the linearized augmented plane wave methods considering five experimentally observed structures. The static results yield a picture that is consistent with experiments, i.e., phase transitions with pressure are predicted as rutile --> monoclinic baddeleyite (MI) --> orthorhombic I (OI) --> cotunnite (OII) on compression, and OII --> OI --> MI --> columbite (TiO(2)II) on decompression. The elasticities of these five polymorphs are compared. Except for the baddeleyite structure, which is considerably softer than the other polymorphs, all phases show a zero pressure bulk modulus in the range of 200-240 GPa, consistent with compression results and the single crystal elastic constant; on the basis of these results we can say that the cotunnite phase is not a superhard TiO(2) polymorph as has been suggested previously. We further find that the rutile and columbite structures are energetically very similar, with the columbite structure favored slightly. All polymorphs are predicted as insulating with comparable band gaps (∼1.7-2.3 eV). Crystal field splitting for the Ti 3d electronic states leads to two distinct conduction bands in rutile and TiO(2)II for energies smaller than 8 eV, while there is a single conduction band for the other high pressure structures.

The glass transition and diffusion in simulated liquid TiO2

The glass transition and diffusion in liquid TiO₂ have been studied in a model containing 3000 atoms via molecular dynamics (MD) simulation. The density dependence of the glass transition temperature, T_g, of liquid TiO₂ has been found and is discussed. Diffusion of atomic species in 3.80 g cm^-3 TiO₂ models has been investigated over a wide temperature range from 2100 to 7000 K. We found that the temperature dependence of the diffusion constant of atomic species follows an Arrhenius law at relatively low temperatures above the melting point, and at higher temperatures it deviates from an Arrhenius law. Differences between the structures of amorphous TiO₂ models at three different densities in the range from 3.80 to 4.20 g cm^-3 have been found and are discussed. In addition, a transition from a low-density liquid (ldl) form to a high-density liquid (hdl) form was found and is discussed.

Size-dependent pressure-induced amorphization in nanoscale TiO2

We investigated the size-dependent high-pressure phase transition behavior of nanocrystalline anatase TiO2 with synchrotron x-ray diffraction and Raman spectroscopy to 45 GPa at ambient temperature. Pressure-induced amorphization results in a high-density amorphous (HDA) form when the starting crystallite size is < 10 mm. The HDA-TiO2 transforms to a low-density amorphous form at lower pressures. Harnessing the nanometer length scale thus provides a new window for experimental investigation of amorphization in poor glass formers and a synthesis route for new amorphous materials.

Density Functional Theory Calculations of Dense TiO2 Polymorphs: Implication for Visible-Light-Responsive Photocatalysts

Structural parameters and electronic band gaps of dense TiO(2) polymorphs, i.e., alpha-PbO(2), baddeleyite, fluorite, and cotunnite types of structures, were calculated using a first-principles density functional method with local-density approximation. The ambient phases, i.e., rutile and anatase, with known theoretical and experimental data were used to ensure the validity of the calculations. The fluorite-type TiO(2) turned out to have the narrowest band gap, 1.08 or 2.18 eV after applying a very approximate band gap correction, due to highly symmetrical TiO(8) polyhedra with Ti(3d) and O(2p) orbitals in the most mixed state. Ti with eight coordinated oxygens, as feasible under high pressure or residual stress, may have potential applications as a visible-light-responsive photocatalyst.

Laser ablation condensation of alpha-PbO(2)-type TiO(2)

A high-pressure phase of TiO(2) with an alpha-PbO(2)-type structure has been synthesized via very energetic Nd-YAG laser pulse irradiation of oxygen-purged Ti target. The nanometer-size alpha-PbO(2)-type particles were (11;0), (010), and (001) faceted but the larger ones were spherical. The combined effects of rapid heating and cooling, the nanophase effect, and dense surfaces account for the formation of coherently strained alpha-PbO(2) particles. The refined cell volume indicated a considerable residual stress to stabilize the dense structure to ambient condition.