Enthalpy of transformation of a high-pressure polymorph of titanium dioxide to the rutile modification

The enthalpy of transformation of a high-pressure form of titanium dioxide, which has the alpha lead dioxide structure, to the rutile modification was measured by the method of "transposed temperature-drop calorimetry." For the reaction, titanium dioxide (alpha lead dioxide form) transforming to rutile, the change in the heat content is -0.76 +/- 0.17 kilocalorie per mole. From this value and the volume change (+ 2.8 percent) associated with the transformation, we estimate the equilibrium pressure at 294 degrees K to be 60 +/- 20 kilobars.     

Nanometer-size alpha-PbO(2)-type TiO(2) in garnet: A thermobarometer for ultrahigh-pressure metamorphism

A high-pressure phase of titanium dioxide (TiO(2)) with an alpha-PbO(2)-type structure has been identified in garnet of diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany. Analytical electron microscopy indicates that this alpha-PbO(2)-type TiO(2) occurred as an epitaxial nanometer-thick slab between twinned rutile bicrystals. Given a V-shaped curve for the equilibrium phase boundary of alpha-PbO(2)-type TiO(2) to rutile, the stabilization pressure of alpha-PbO(2)-type TiO(2) should be 4 to 5 gigapascals at 900 degrees to 1000 degrees C. This suggests a burial of continental crustal rocks to depths of at least 130 kilometers. The alpha-PbO(2)-type TiO(2) may be a useful pressure and temperature indicator in the diamond stability field.     

Baddeleyite-Type High-Pressure Phase of TiO2

A high-pressure phase of TiO(2), which had been observed by shock-wave experiments and remained unresolved, has been studied by in situ x-ray diffraction. The single phase was formed at 20 gigapascals and 770 degrees C with the use of sintered-diamond multianvils; it has the same structure as baddeleyite, the stable phase of ZrO(2) at ambient conditions. The coordination number of Ti increases from six to seven across the rutile to baddeleyite transition, and the volume is reduced by approximately 9 percent.     

High-pressure polymorphism of titanium dioxide

X-ray diffraction studies made in situ under conditions of high pressure and high temperature revealed the direct transition of rutile to the alpha lead dioxide form in titanium dioxide. Compressibility studies of this alpha lead dioxide form at room temperature showed anomalous behavior in that its molar volume converges close to, but not equal to, that of the rutile form. Under this circumstance an unexpectedly large error appears in the calculations of the equilibrium pressure for the two forms at 298 degrees K.     

An ultradense polymorph of rutile with seven-coordinated titanium from the Ries crater

We report the discovery of an ultradense post-rutile polymorph of titanium dioxide in shocked gneisses of the Ries crater in Germany. The microscopic diagnostic feature is intense blue internal reflections in crossed polarizers in reflected light. X-ray diffraction studies revealed a monoclinic lattice, isostructural with the baddeleyite ZrO2 polymorph, and the titanium cation is coordinated with seven oxygen anions. The cell parameters are as follows: a = 4.606(2) angstroms, b = 4.986(3) angstroms, c = 4.933(3) angstroms, beta (angle between c and a axes) = 99.17(6) degrees; space group P2(1)/c; density = 4.72 grams per cubic centimeter, where the numbers in parentheses are standard deviations in the last significant digits. This phase is 11% denser than rutile. The mineral is sensitive to x-ray irradiation and tends to invert to rutile. The presence of baddeleyite-type TiO2 in the shocked rocks indicates that the peak shock pressure was between 16 and 20 gigapascals, and the post-shock temperature was much lower than 500 degrees C.     

Quartzlike carbon dioxide: An optically nonlinear extended solid at high pressures and temperatures

An extended-solid phase, carbon dioxide phase V (CO2-V), was synthesized in a diamond anvil cell by laser heating the molecular orthorhombic phase, carbon dioxide phase III, above 40 gigapascals and 1800 kelvin. This new material can be quenched to ambient temperature above 1 gigapascal. The vibration spectrum of CO2-V is similar to that of the quartz polymorph of silicon dioxide, indicating that it is an extended covalent solid with carbon-oxygen single bonds. This material is also optically nonlinear, generating the second harmonic of a neodymium-yttrium-lithium-fluoride laser at a wavelength of 527 nanometers with a conversion efficiency that is near 0.1 percent.     

铕掺杂二氧化钛纳米晶的制备和发光性质

利用溶胶-凝胶技术制备了TiO2纳米晶和掺杂Eu3+的TiO2纳米晶,测试了样品的X射线谱(XRD)和荧光光谱(PL),研究了不同煅烧温度下,TiO2:Eu3+的发光性质.Eu3+离子的掺入能抑制TiO2由锐钛矿相向金红石相的转变,464nm的激发波长对发射最有利,578nm处存在自吸收,随着退火温度增高,TiO2:Eu3+样品稀土发光强度下降.     

Reaction of plutonium dioxide with water: formation and properties of PuO(2+x)

Results show that PuO(2+x), a high-composition (x 相似文献   

A new allotropic form of carbon from the ries crater

A new allotropic form of carbon occurs in shock-fused graphite gneisses in the Ries Crater, Bavaria. The assemblage in which it occurs consists of hexagonal graphite, rutile, pseudobrookite, magnetite, nickeliferous pyrrhotite, and baddeleyite. Electron-probe analyses indicate that the new phase is pure carbon. It is opaque and much more strongly reflecting than hexagonal graphite. Measurement of x-ray diffraction powder patterns leads to cell dimensions a = 8.948 +/- 0.009, c = 14.078 +/- 0.017 angstroms, with a primitive hexagonal lattice.     

In situ studies of chemistry and structure of materials in reactive environments

Most materials and devices typically operate under specific environmental conditions, many of them highly reactive. Heterogeneous catalysts, for example, work under high pressure of reactants or in acidic solutions. The relationship between surface structure and composition of materials during operation and their chemical properties needs to be established in order to understand the mechanisms at work and to enable the design of new and better materials. Although studies of the structure, composition, chemical state, and phase transformation under working conditions are challenging, progress has been made in recent years in the development of new techniques that operate under a variety of realistic environments. With them, new chemistry and new structures of materials that are only present under reaction conditions have been uncovered.     

Efficient photochemical water splitting by a chemically modified n-TiO2

Although n-type titanium dioxide (TiO2) is a promising substrate for photogeneration of hydrogen from water, most attempts at doping this material so that it absorbs light in the visible region of the solar spectrum have met with limited success. We synthesized a chemically modified n-type TiO2 by controlled combustion of Ti metal in a natural gas flame. This material, in which carbon substitutes for some of the lattice oxygen atoms, absorbs light at wavelengths below 535 nanometers and has a lower band-gap energy than rutile (2.32 versus 3.00 electron volts). At an applied potential of 0.3 volt, chemically modified n-type TiO2 performs water splitting with a total conversion efficiency of 11% and a maximum photoconversion efficiency of 8.35% when illuminated at 40 milliwatts per square centimeter. The latter value compares favorably with a maximum photoconversion efficiency of 1% for n-type TiO2 biased at 0.6 volt.     

Ultrafast interfacial proton-coupled electron transfer

The coupling of electron and nuclear motions in ultrafast charge transfer at molecule-semiconductor interfaces is central to many phenomena, including catalysis, photocatalysis, and molecular electronics. By using femtosecond laser excitation, we transferred electrons from a rutile titanium dioxide (110) surface into a CH3OH overlayer state that is 2.3 +/- 0.2 electron volts above the Fermi level. The redistributed charge was stabilized within 30 femtoseconds by the inertial motion of substrate ions (polaron formation) and, more slowly, by adsorbate molecules (solvation). According to a pronounced deuterium isotope effect (CH3OD), this motion of heavy atoms transforms the reverse charge transfer from a purely electronic process (nonadiabatic) to a correlated response of electrons and protons.     

Time-Resolved X-ray Diffraction Study of Solid Combustion Reactions

Real-time synchrotron diffraction has been used to monitor the phase transformations of highly exothermic, fast self-propagating solid combustion reactions on a subsecond time scale down to 100 milliseconds and in some instances to 10 milliseconds. Three systems were investigated: Ti + C --> TiC; Ti + C + xNi --> TiC + Ni-Ti alloy; and Al + Ni --> AlNi. In all three reactions, the first step was the melting of the metal reactants. Formation of TiC in the first two reactions was completed within 400 milliseconds of the melting of the Ti metal, indicating that the formation of TiC took place during the passage of the combustion wave front. In the Al + Ni reaction, however, passage of the wave front was followed by the appearance and disappearance of at least one intermediate in the afterburn region. The final AlNi was formed some 5 seconds later and exhibited a delayed appearance of the (210) reflection, which tends to support a phase transformation from a disordered AlNi phase at high temperature to an ordered CsCl structure some 20 seconds later. This new experimental approach can be used to study the chemical dynamics of high-temperature solid-state phenomena and to provide the needed database to test various models for solid combustion.     

Electron-Optical Observations on the agr-Transformation in Troilite

The transformation from high to low troilite occurs via a transitional structure. The transformation to this transitional state is continuous, whereas that to the low-temperature form is discontinuous. This illustrates alternative metastable behavior in a system when nucleation of the low-temperature phase may be impeded.     

Structural Memory in Pressure-Amorphized AlPO4

Molecular dynamics simulations reveal the mechanism of the structural memory effect in alpha-berlinite (AlPO(4)), where a single crystal transforms to an amorphous solid by compression and then reverts to the original crystalline structure upon release of pressure. The enhanced oxygen coordination around the aluminum atoms in AlPO(4) at high pressure leads to a mechanical instability that causes the phase transformation. The difference in the structural memory behavior between AlPO(4) and isostructural alpha-quartz, for which the pressure-induced amorphized phase is recoverable, can be attributed to the presence of the PO(4) units, which remain essentially four-coordinated even when severely distorted.     

Superconductivity in hydrogen dominant materials: silane

The metallization of hydrogen directly would require pressure in excess of 400 gigapascals (GPa), out of the reach of present experimental techniques. The dense group IVa hydrides attract considerable attention because hydrogen in these compounds is chemically precompressed and a metallic state is expected to be achievable at experimentally accessible pressures. We report the transformation of insulating molecular silane to a metal at 50 GPa, becoming superconducting at a transition temperature of Tc = 17 kelvin at 96 and 120 GPa. The metallic phase has a hexagonal close-packed structure with a high density of atomic hydrogen, creating a three-dimensional conducting network. These experimental findings support the idea of modeling metallic hydrogen with hydrogen-rich alloy.     

Io: longitudinal distribution of sulfur dioxide frost

Twenty spectra of Io (0.26 to 0.33 micrometer), acquired with the International Ultraviolet Explorer spacecraft, have been studied. There is a strong ultraviolet absorption shortward of 0.33 micrometer that is consistent with earlier ground-based spectrophotometry; its strength is strongly dependent on Io's rotational phase angle at the time of observation. This spectral feature and its variation are interpreted as indicative of a longitudinal variation in the distribution of sulfur dioxide frost on Io. The frost is most abundant at orbital longitudes 72 degrees to 137 degrees and least abundant at longitudes 250 degrees to 323 degrees . Variations in spectral reflectivity between 0.4 and 0.5 micrometer, reported in earlier ground-based spectral studies, correlate inversely with variations in reflectivity between 0.26 and 0.33 micrometer. It is concluded that this is because the Io surface component with the highest visible reflectivity (sulfur dioxide frost) has the lowest ultraviolet reflectivity. At least one other component is present and may be sulfur allotropes or alkali sulfides. This model is consistent with ground-based ultraviolet, visible, and infrared spectrophotometry. Comparison with Voyager color photographs indicates that the sulfur dioxide frost is in greatest concentration in the "white" areas on Io and the other sulfurous components are in greatest concentration in the "red" areas.     

Compression of Ice to 210 Gigapascals: Infrared Evidence for a Symmetric Hydrogen-Bonded Phase

Protonated and deuterated ices (H2O and D2O) compressed to a maximum pressure of 210 gigapascals at 85 to 300 kelvin exhibit a phase transition at 60 gigapascals in H2O ice (70 gigapascals in D2O ice) on the basis of their infrared reflectance spectra determined with synchrotron radiation. The transition is characterized by soft-mode behavior of the nu3 O-H or O-D stretch below the transition, followed by a hardening (positive pressure shift) above it. This behavior is interpreted as the transformation of ice phase VII to a structure with symmetric hydrogen bonds. The spectroscopic features of the phase persisted to the maximum pressures (210 gigapascals) of the measurements, although changes in vibrational mode coupling were observed at 150 to 160 gigapascals.     

Structure of Mars' Atmosphere up to 100 Kilometers from the Entry Measurements of Viking 2

The Viking 2 entry science data on the structure of Mars' atmosphere up to 100 kilometers define a morning atmosphere with an isothermal region near the surface; a surface pressure 10 percent greater than that recorded simultaneously at the Viking 1 site, which implies a landing site elevation lower by 2.7 kilometers than the reference ellipsoid; and a thermal structure to 100 kilometers at least qualitatively consistent with pre-Viking modeling of thermal tides. The temperature profile exhibits waves whose amplitude grows with altitude, to approximately 25 degrees K at 90 kilometers. These waves are believed to be a consequence of layered vertical oscillations and associated heating and cooling by compression and expansion, excited by the daily thermal cycling of the planet surface. As is necessary for gravity wave propagation, the atmosphere is stable against convection, except possibly in some very local regions. Temperature is everywhere appreciably above the carbon dioxide condensation boundary at both landing sites, precluding the occurrence of carbon dioxide hazes in northern summer at latitudes to at least 50 degrees N. Thus, ground level mists seen in these latitudes would appear to be condensed water vapor.     

Ytterbium: Transition at High Pressure from Face-Centered Cubic to Body-Centered Cubic Structure

Pressure of 40,000 atmospheres at 25 degrees C induces a phase transformation in ytterbium metal; the face-centered cubic structure changes to body-centered cubic. The radius of the atom changes from 1.82 to 1.75 A. At the same time the atom's volume decreases by 11 percent and the volume, observed macroscopically, decreases 3.2 percent.