TiO4 Tetrahedra Research Articles

The influence of temperature on the Ti4+ coordination in sodium titanosilicate melts: In-situ high-temperature Raman spectroscopy study

The structure of xTiO2-(100-x)[40Na2O-60SiO2] (x = 10, 15, 25 and 30 mol%) glasses and melts was studied in situ in the temperature range of 20–1150 °C using high-temperature Raman spectroscopy. The fraction of five-fold coordinated Ti4+ ions gradually decreases as temperature increases above ∼ 600 °C in the samples with x ≤ 25 mol%. No similar changes were found for the sample with x = 30 mol%. The change in titanium coordination is related to the transformation of square-based TiO5 pyramids into TiO4 tetrahedra with the formation of additional non-bridging oxygen atoms (NBO) in the melt structure. This transformation can be described by [TiØ4O]2− + SiØ3O− ↔ TiØ3O− + SiØ2O22− and [TiØ4O]2− + SiØ4 ↔ TiØ3O− + SiØ3O− (Ø is bridging oxygen atom) disproportional reactions, which are shifted toward the right with increasing temperature. All studied melts exhibit a more uniform titanium distribution in comparison with glasses of corresponding composition.

Structures and Dissolution Behaviors of Quaternary CaO-SrO-P2O5-TiO2 Glasses.

Calcium phosphate glasses have a high potential for use as biomaterials because their composition is similar to that of the mineral phase of bone. Phosphate glasses can dissolve completely in aqueous solution and can contain various elements owing to their acidity. Thus, the glass can be a candidate for therapeutic ion carriers. Recently, we focused on the effect of strontium ions for bone formation, which exhibited dual effects of stimulating bone formation and inhibiting bone resorption. However, large amounts of strontium ions may induce a cytotoxic effect, and there is a need to control their releasing amount. This work reports fundamental data for designing quaternary CaO-SrO-P2O5-TiO2 glasses with pyro- and meta-phosphate compositions to control strontium ion-releasing behavior. The glasses were prepared by substituting CaO by SrO using the melt-quenching method. The SrO/CaO mixed composition exhibited a mixed cation effect on the glassification degree and ion-releasing behavior, which showed non-linear properties with mixed cation compositions of the glasses. Sr2+ ions have smaller field strength than Ca2+ ions, and the glass network structure may be weakened by the substitution of CaO by SrO. However, glassification degree and chemical durability of pyro- and meta-phosphate glasses increased with substituted all CaO by SrO. This is because titanium groups in the glasses are closely related to their glass network structure by SrO substitution. The P-O-Ti bonds in pyrophosphate glass series and TiO4 tetrahedra in metaphosphate glass series increased with substitution by SrO. The titanium groups in the glasses were crosslink and/or coordinate phosphate groups to improve glassification degree and chemical durability. Sr2+ ion releasing amount of pyrophosphate glasses with >83% SrO substitution was larger than 0.1 mM at day seven, an amount that reported enhanced bone formation by stimulation of osteogenic markers.

Flux Growth of Uranyl Titanates: Rare Examples of TiO4 Tetrahedra and TiO5 Square Bipyramids.

Single crystals of four new uranyl titanates have been grown via the flux growth method using mixed alkali halide fluxes. Na2(UO2)(TiO)O3 and KNa(UO2)(TiO)O3 have analogous layered structures containing titanyl (TiO2+) units coordinated into TiO5 square pyramids. Cs2(UO2)TiO4 crystallizes in the Cs2USiO6 structure type and is a rare example of a structure containing TiO4 tetrahedra. Cs2(UO2)Ti2O6 crystallizes in a new tunnel structure and contains the also rare TiO5 trigonal bipyramids. DFT studies were performed to understand the bonding in the observed titanate polyhedra. Furthermore, the luminescence properties of the compounds are reported, and leaching studies are reported for Cs2(UO2)Ti2O6.

The effect of TiO2 on the structure of Na2O-CaO-SiO2 glasses and its implications for thermal and mechanical properties

Titania represents an important compound for property modifications in the widespread family of soda lime silicate glasses. In particular, such titania-containing glasses offer interesting optical and mechanical properties, for example, for substituting lead-bearing consumer glasses. Here, we provide a systematic study of the effect of TiO2 on the structural, thermal, and mechanical properties for three series of quaternary Na2O–CaO–TiO2–SiO2 glasses with TiO2 concentrations up to 12mol% and variable Na2O, CaO, and SiO2 contents. Structural analyses by Raman and magic-angle spinning 29Si NMR spectroscopy reveal the presence of predominantly four-fold coordinated Ti[4] atoms in glasses of low and moderate TiO2 concentrations, where Si–O–Si bonds are replaced by Si–O–Ti[4] bonds that form a network of interconnected TiO4 and SiO4 tetrahedra, with a majority of the non-bridging oxygen ions likely being located at the SiO4 tetrahedra. At higher TiO2 contents, TiO5 polyhedra are also formed. Incorporation of TiO2 strongly affects the titanosilicate network connectivity, especially when its addition is accompanied by a decrease of the CaO content. However, except for the thermal expansion coefficient, these silicate-network modifications seem to have no impact on the thermal and mechanical stability. Instead, the compositional dependence of the thermal and mechanical properties on the TiO2 content stems from its effect on the network energy and packing efficiency.

Lewis Acid Catalysis of TiO4 Tetrahedra on Mesoporous Silica in Water

TiO4-deposited mesoporous silica (TDMS) was studied as a water-tolerant solid Lewis acid catalyst. TDMS prepared by a simple postgrafting technique using Ti(i-pro)4 has a large BET surface area (ca. 450 m2 g–1) and ordered mesoporous structure. Ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS), X-ray absorption near edge structure (XANES), and CO adsorption measurements with Fourier transform infrared (FTIR) spectroscopy revealed that isolated TiO4 tetrahedra on TDMS have Lewis acidity, even in the presence of water. Lewis acid centers on TDMS exhibit higher catalytic performance for the hydride transfer of pyruvaldehyde to lactic acid and the Mukaiyama-aldol condensation of benzaldehyde with 1-trimethylsilyloxy-cyclohexene in water than that of conventional heterogeneous and homogeneous Lewis acids, including scandium(III) triflate (Sc(OTf)3), which also works even in water. The high catalytic performance of TDMS can be attributed to Lewis acid catalysis of isolated TiO4 tetrahedra in water.

Optical properties of TiO2–SiO2glass over a wide spectral range

AbstractOptical properties of vitreous SiO2with 7.4 wt.% TiO2are found by dispersion analysis of reflectivity measured in the infrared, visible, and ultraviolet augmented with literature values of vacuum‐ultraviolet reflectivity and absorption. The principal infrared absorption associated with the titanium dopant occurs at 950 cm−1in a deep minimum of the host silica absorption. We attribute this to a perturbation of the silica's absorption at 1076 cm−1involving oxygen atoms bridging SiO4and TiO4tetrahedra. Strong ultraviolet absorptions of Ti4+occur just below the silica exciton peak between 5.5 and 7.8 eV. We attribute these to charge‐transfer transitions at TiO4tetrahedra; i.e., bound excitons consisting of a Ti3+ion and a hole shared by four oxygen neighbours. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Propylene Oxide Synthesis and Selective Oxidation over Supported Metal Oxide Photocatalysts with Molecular Oxygen

Abstract Recent progress of photocatalytic epoxidation of propylene with molecular oxygen is reviewed. Highly dispersed metal oxide photocatalysts, such as silica-supported V2O5, Nb2O5, and TiO2 with low loading, promote steady-state propylene oxide formation with high selectivity (>40%). The isolated tetrahedrally-coordinated species in the ligand-to-metal charge-transferred state is the photocatalytic active site. The photo-induced hole center localized on the lattice oxygen plays a significant role for the photocatalytic epoxidation. In the case of isolated TiO4 tetrahedra on silica, an electrophilic oxygen species is generated by the reaction between the hole center and molecular oxygen. The addition of alkali ions to surface VO4 tetrahedra improves the photocatalytic epoxidation ability of V2O5/SiO2 with low loading. The key step in the selective photocatalytic oxidation over isolated VO4 tetrahedra is the electron transfer from a hydrocarbon to the photo-induced hole center on the lattice oxygen, which is inserted in the photogenerated products.

Coordination, Structure, and Vibrational Spectra of Titanium in Silicates and Zeolites in Comparison with Related Molecules. An ab Initio Study

Ab initio quantum chemical calculations at the SCF level using a split valence plus polarization basis set have been performed on molecules containing Ti, Si, O, and H, which represent substructures of titanozeolites and ETS-10. Their purely siliceous analogues are also studied. Among them is octahydrooctasilasesquioxane. The calculations show that titanium prefers a 4-fold coordination in zeolites. The predicted relative energies and vibrational spectra make it very unlikely that TiO4 tetrahedra share edges with SiO4 tetrahedra. The typical 960-cm-1 vibrational band found for titanium-containing zeolites can be assigned to antisymmetric TiOSi stretching modes of corner-sharing tetrahedra. The frequency shift is due to the fact that the Ti−O bonds are about 20 pm longer than the Si−O bonds. The use of the crystal unit cell size as a measure for the titanium content of zeolites is supported.

The Luminescence of Ba2TiO4, a Compound with Titanate Tetrahedra

The luminescence of Ba2TiO4, the only compound with TiO4 tetrahedra, is reported and discussed. The quantum efficiency is low. The reason for this is discussed in relation to the luminescence of isoelectronic complexes.

Structure and Physical Properties of Non-Alkali Multicomponent Glasses

Non-alkali multicomponent glasses in the system SiO2-Al2O3-CaO-ZnO-TiO2-B2O3 were prepared by the melt quenching method. The composition range was (in wt%), SiO2=32.0-38.0, Al2O3=9.0-18.0, CaO=17.0-23.0, ZnO=12.0-18.0, TiO2=10.0-19.0, and B2O3=2.0-7.0. Composition dependencies of glass transition temperature Tg, softening temperature Td, thermal expansion coefficient α, and density ρ were investigated. It was found that an increase in SiO2, Al2O3 or CaO content increases the Tg and Td but anincrease in ZnO, TiO2 or B2O3 content decreases these characteristic temperatures. The thermal expansion coefficient decreased with an increase in SiO2, Al2O3, ZnO, TiO2 or B2O3 content, but increased by the addition of CaO. It was speculated that alumina and titania are combined with CaO to produce AlO4 or TiO4 tetrahedra, which help SiO2 poor compositions to form a rather stable glass network.

多成分系無アルカリガラスの粘性特性と結晶化挙動

Critical cooling rate Q* for glass formation was determined by means of DTA method for a non-alkali multicomponent system of 39.8SiO2·10.1Al2O3·24.4CaO·12.6ZnO·11.1TiO2·2.0B2O3mol% composition. Q* was found to be 1.5×10-2°C/sec. Viscosity of the system was measured by the beambending and the counter balancing method. The liquidus temperature T1 of the system was determined by DTA method. From the viscosity and T1 data, the Liquidus viscosity η1 was obtained (logη1/PaS=0.94). The Q*-η1 relationship of the non-alkali system was compared with those of alkali silicate and alkali borate systems.It was found that the present non-alkali system has a critical cooling rate equivalent to 94SiO2·6Li2O, 75B2O3·25Li2O or 85B2O3·15Na2O glass. The fusion entropy ΔSf was calculated from eq. (2) using the values of liquid parameters of the non-alkali system. It became apparent that the non-alkali system has a considerably high ΔSf value compared with the alkali borate or alkali silicate system. This was regarded to indicate that the present system is composed of various polyhedra, including SiO4, AlO4 or TiO4 tetrahedra, thus easing a glass formation of a SiO2 poor melt.

ChemInform Abstract: Structure of CsAlTiO4 ‐ A Compound with TiO4 Tetrahedra.

AbstractSingle crystals of the title compound, which appears to be only the second Ti(IV) compound in which TiO4 tetrahedra are present, are selected from a preparation in which relevant mole ratios of CsNO3, TiO2, and Al2O3 are heated for 24 h at 1200 K followed by 48 h at 1400 K. The compound crystallizes in the orthorhombic system, space group Imma, Z=4.

Structure of CsAlTiO4 – a compound with TiO4 tetrahedra

Structure of CsAlTiO4 – a compound with TiO4 tetrahedra

ＳｉＯ２‐ＭｇＯ‐ＣａＯ‐Ａｌ２Ｏ３‐ＴｉＯ２系合成スラグ融液のガラス化条件

As a primary condition for vitrification procedure of molten slag in the SiO2-MgO-CaO-Al2O3-TiO2 system, the vitrification temperature TV and the devitrification temperature TC were determined at three different cooling rates as a function of slag composition. Both TV and TC were found to decrease with increasing cooling rate. DTA analysis on heating and cooling runs of the slag specimen showed that the TV was well above the liquidus temperature and the TC was just above it. In order to explain this phenomenon, a new type of liquid model was proposed by which a perceptible amount of nuclei are preserved even above the liquidus. Systematic analysis on the compositional dependences of TV and TC revealed that constituents SiO2, Al2O3 and TiO2 lower both TV and TC, while MgO and CaO raise them. This is considered to mean that Al2O3 and TiO2 would produce AlO4 and TiO4 tetrahedra in the presence of MgO and CaO, and these tetrahedra will work as glass network formers as will do SiO4 tetrahedra. Excess MgO or CaO, on the other hand, would produce non-bridging oxygens in the AlO4, TiO4 or SiO4 tetrahedra and will weaken vitrification tendency.

Infrared and raman spectra of vitreous silica and sodium silicates containing titanium

Infrared and Raman spectra of vitreous silica and sodium silicates containing various concentrations of Ti were studied. Vibrational bands associated with the titanium groups, TiO4 tetrahedra and TiO6 octahedra, were identified. From structural symmetry and selection rule arguments, the TiO2 tetrahedra were found to exist in vitreous silica and the TiO2 tetrahedra and TiO6 octahedra to coexist in vitreous sodium silicates as a titanium group. With increasing titanium content in TiO2-SiO2 glasses, the concentration of TiO4 tetrahedra increased. With increasing sodium content in TiO2-Na2O-SiO2 glasses, the concentration of the TiO4 tetrahedra decreased and that of the TiO6 octahedra increased.

Structure of Glasses and Melts in the Na<sub>2</sub>O-SiO<sub>2</sub>-TiO<sub>2</sub> System

In order to elucidate the influence of TiO2 on the structure of sodium silicate glasses and melts, a MD (molecular dynamics) simulation using the ionic pair potential function, Born-Mayer-Huggins form, has been performed for 0.25Na2O⋅0.47SiO2⋅0.28TiO2 glass and the corresponding melt. The calculation pair correlation functions were in good agreement with a RDF (radial distribution function) curve obtained from the previous structural analysis by TOF (time-of-flight) pulsed neutron scattering. The computer-simulated configuration for the glass indicated that most of Ti ions were in 4-fold coordination, whilst the remainder were 6-fold coordination. The frequency spectrum of the glass obtained from MD simulation was in good agreement with the Raman spectra measured on glasses in the Na2O-SiO2-TiO2 system. As a result, the weak Raman band near 700cm-1 can be assigned to Ti ion in 6-fold coordination, and the strong Raman band near 900cm-1 reflects a Ti-O stretching band, which is characteristic of TiO4 tetrahedra corner-linked with SiO4 tetrahedra. The Raman spectra for glasses and melts in Na2O-SiO2-TiO2 system were measured by the high temperature Raman spectroscopic technique. A slight difference in the obtained Raman spectra was observed between the glasses and the corresponding melts, implying that the structure of glass should be similar to that of the corresponding melt.

Effects of Al<sub>2</sub>O<sub>3</sub> Addition on Glassy Phase Separation and Crystallization of a PbO-TiO<sub>2</sub>-SiO<sub>2</sub> Glass

Some of PbO-TiO2-SiO2 glasses are converted into polycrystalline materials consisting mainly of perovskite-type PbTiO3 crystals by heat treatment. However, the kind of crystals first precipitated in the glasses on heating was found to be markedly affected by the amount of Al2O3 in the glasses. For its reasoning, the crystallization processes of the PbO 40, TiO2 25, SiO2 35mol% glass (glass 1) and PbO 40, TiO2 25, Al2O3 10, SiO2 25mol% glass (glass 2) were investigated by means of DTA, electrical resistivity measurement, X-ray diffraction analysis, electron microscopic observation, infrared spectroscopic analysis and fluorescent X-ray spectroscopic analysis. The results are summarized as follows:1) It is concluded that glass 1 as-annealed has a homogeneous structure being composed of a network of uniformly distributed SiO4, TiO4 and PbO4 tetrahedra and modifier ions of Ti4+ and Pb2+ sitting in holes of the network. On heat treatment, the glass crystallizes directly forming a metastable pyrochlore-type lead titanate dissolving a small amount of SiO2, without going through any precursory structural change such as glassy two-phase separation. At an elevated temperature, the pyrochlore-type crystals transform to the perovskite-type PbTiO3 crystals.Glass 2 as-annealed has a two-phase structure being composed of a large number of minute glassy droplets rich in PbO and TiO2 and a continuous glassy matrix rich in SiO2. On heat treatment, the droplets first grow in size at the expence of the matrix, so rapidly as a sharp exotherm peak can be observed in DTA, and then the perovskite-type PbTiO3 crystals precipitate in the droplets.2) It is concluded that addition of the Al2O3 to a PbO-TiO2-SiO2 glass induces a glassy two-phase separation, and as the result promotes precipitation of the perovskite-type PbTiO3 crystals in the PbO- and TiO2-rich phase, thus suppressing formation of the metastable crystal. The effect of the Al2O3 on the glassy two-phase separation is interpreted in terms of “tricluster” model proposed by Lacy and field strengths of the constituents ions.