Stoichiometric rutile

Some earlier thermodynamic studies on rutile reported expressions involving simple idealized quasi-chemical equilibrium constants for point defect equilibria (see, e.g., Kofstad 1972) by correlating the composition x in TiOx with a function of AGm (O2), which is the partial molar free energy of oxygen. However, the structural effects were not accounted for in these considerations. Careful measurements of AGm (O2) in the TiOjc system (Bursill and Hyde 1971) have indicated that complete equilibrium is rarely achieved in non-stoichiometric rutile. 

Figure 1 Bulk-terminated stoichiometric rutile (1 10) ball-and-stick geometry (side view, black=Ti, white=0) and simulated constant-p/variable- STM image r roduced from [48].

Mikkelsen J (1984) Self-diffusivity of network oxygen in vitreous Si02. Appl Phys Lett 45 1187-1189 Millot F, Niu Y (1997) Discussion of O in Fc304 An experimental approach to study the behavior of minority defects in oxides. J Phys Chem Solids 58 63-72 Millot F, Picard C (1988) Oxygen self-diffusion in non-stoichiometric rutile Ti02-x at high temperature. Solid State Ionics 28-30 1344-1348... 

The interaction of water with stoichiometric rutile Ti02 (110) surface has been extensively studied. It is both a probe system for fundamental research and a system of great interest for many technological issues. Experiments show the existence of a stable phase of molecular adsorbed water at low temperature [94]. Dissociation occurs at higher temperature and is mediated by surface defects. Most theoretical studies conclude to facile dissociation on the perfect surface [95,96] or to a mixed mode with both molecular and dissociated molecules [97]. 

The fraction reaction completed versus time curves at 900°C for the first two reactions listed in Table I are given in Fig. 1. Fraction of reaction completed versus time curves for the reactions between strontium carbonate and nonstoichiometric titania are not included because the distribution of these curves with respect to the strontium carbonate-stoichiometric rutile reaction curve is highly dependent on the temperature chosen. 

The observed change in activation energy with stoichiometry suggests that the phase boundary model is untenable for the reaction between strontium carbonate and rutile. If the reaction were phase-boundary controlled in the case of stoichiometric rutile, then when the activation energy for the... 

The kinetics of the reaction between strontium carbonate and stoichiometric rutile were found to be described by the kt == [—ln(l — x)Y nuclei growth equation. The activation energy for this reaction is 97.8 kcal/mole and the frequency factor is 3.2 x 10. ... 

Figure 3. 5p3/2,l/2 photoemission spectra of xenon adsorbed on a) stoichiometric rutile (100) plane and b) the same rutile plane annealed at 1123 K in UHV. 

Peteket al. [148, 149] provided some indirect evidence that part of CH3OH was adsorbed at Tisc sites dissociatively. Using two-photon photoemission spectroscopy (2PPE), an empty wet electron state at about 2.3 0.2 eV above Fermi level (Ep) was detected on both reduced and stoichiometric rutile TiO2(110). However, in the case of H2O, this excited state could only be observed on reduced rutile Ti02(l 10) surfaces with simultaneous presence of monolayer water and bridging hydroxyls... 

OHbr) (resulting from spontaneous dissociation of water at Ov s [49]), and the electron distribution in this state was reported to encompass several adsorbate H sites [150]. While on the water covered stoichiometric rutile Ti02 surface, this state was totally absent. By analogy with the properties of the excited state at H20/rutile TiO2(110) interface, those authors argued the partial dissociation of methanol oti rutile Ti02(l 10) surface. 

The coordination theory, which has been applied successfully2 to brook-ite, the orthorhombic form of titanium dioxide, is based on the assumption that the anions in a crystal are constrained to assume positions about the cations such that they indicate the corners of polyhedra of which the cations mark the centers. These polyhedra are the fundamentally important constituents of the crystal retaining their form essentially, they are combined by sharing corners, edges, and faces in such a way as to build up a crystal with the correct stoichiometrical composition. Thus in rutile, anatase, and brookite there occur octahedra of oxygen ions about titanium ions in rutile each octahedron shares two edges with adjoining octahedra, in anatase four, and in brookite three. 

Rutile and anatase are used in photocatalysis and as catalyst supports and we describe some important structural features later. The titanium-oxygen system plays a key role in understanding stoichiometric variations. The highest oxide, Ti02, is polymorphic and the four known crystal structures are rutile, anatase, brookite and a high-pressure form similar to a-Pb02. 

It crystallizes in the tetragonal rutile structure (see Fig. 5.27) with cell dimensions a = 474 pm and c = 319 pm in the single-crystal form it is known by its mineralogical name, cassiterite. It is a wide band gap semiconductor, with the full valence band derived from the 02p level and the empty conduction band from the Sn 5s level. The band gap at OK is approximately 3.7eY, and therefore pure stoichiometric Sn02 is a good insulator at room temperature when its resistivity is probably of the order of 106Qm. 

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