Titanium lattice defects

Wang, J.A., R. Limas-Ballesteros, T. Lopez, A. Moreno, R. Gomez, O. Novaro and X. Bokhimi (2001b). Quantitative determination of titanium lattice defects and solid-state reaction mechanism in iron-doped Ti02 photocatalysts. Journal of Physical Chemistry B, 105(40), 9692-9698. 

Results of the lattice constants and density measurements are given in Table III their variations as a function of composition are shown in Figures 8 and 9. At the present time, two crystal structures only slightly different from one another have been proposed (4, 9). They differ only in the origin of z, the distances between planes of titanium and of sulfur, and in the distribution of titanium among the available sites (Figure 10). In the present case, density measurements do not permit making a decision they simply show that lattice defects are related to the variable number of titanium atoms per unit cell, and that the number of sulfur atoms per unit cell is constant and equal to 4. 

In addition to the binary catalysts from transition metal compounds and metal alkyls there 2ire an increasing number which are clearly of the same general type but which have very different structures. Several of these are crystalline in character, and have been subjected to an activation process which gives rise to lattice defects and catalytic activity. Thus, nickel and cobalt chlorides, which untreated are not catalysts, lose chlorine on irradiation and become active for the polymerization of butadiene to high cis 1,4-polymer [59]. Titanium dichloride, likewise not a catalyst, is transformed into an active catalyst (the activity of which is proportional to the Ti content) for the polymerization of ethylene [60]. In these the active sites evidently react with monomer to form organo-transition metal compounds which coordinate further monomer and initiate polymerization. 

Ikeda, J.A.S. and Chiang, Y.-M. (1993) Space charge segregation at grain boundaries in titanium dioxide 1, Relationship between lattice defect chemistry and space charge potential. f Am. Ceram. Soc., 76, 2437—2446. 

The formation of surface defects of a crystal lattice. It was observed while using crystal compounds of transition metals as catalysts [e.g. as was shown by Arlman (171, 173), for a TiCl3 surface defects appear on the lateral faces of the crystal]. In this case low surface concentration of the propagation centers should be expected, as is illustrated in the case of polymerization by titanium dichloride (158). The observed... 

Defects in which both a cation and sufficient anions to balance the charge (or vice versa) are completely missing from the lattice are called Schottky defects. Schottky defects result in a density that is lower than that calculated on the basis of unit cell dimensions, whereas Frenkel defects do not affect this density. Titanium(II) oxide, for example, also has the NaCl structure, but, even when its composition is TiOi.oo (which it rarely is see Section 5.4), about one-sixth of the Ti2+ and 02 sites are vacant. 

It has been established that under intense milling in a planetary mill in a nitrogen atmosphere, titanium hydride may decompose with formation of defected structure characterized by partly non-occupied tetrahedron voids in a face-centered cubic sublattice of titanium atoms. This effect is accompanied by decrease in the crystal lattice parameter of titanium hydride subjected to intense milling for 60 min. 

For instance, the Til atom has a three-fold co-ordination by oxygen on the facet ridges, as opposed to bulk co-ordination. In the final model, a surface octahedral interstitial site of the O lattice was found to be occupied by a surface titanium atom, with 40% occupancy per (1x3) cell. Partial occupancies of 60% were also found for the 01 and Ti5 surface atoms. The resulting stoiehiometry for this surface structure is TiOi.es, which is equivalent to a 15.5% oxygen deficiency on the surface relative to the bulk. Hence, the refined model can be described as the formation of strongly distorted 110 micro facets on the surface with oxygen defects and a partial occupancy of an interstitial site. Relaxations are found down to 9 A below the topmost layer. The different coordinations found for Ti might explain part of the photo-catalytic properties of this surface. 

Electrons and holes that are generated in particulate semiconductors are localized at different defect sites on the surface and in the lattice of the particles. Electron paramagnetic resonance (EPR) results have shown that electrons are trapped as two reduced metal centers—Ti(III) sites—eoordinated either [38, 39] 1) with anatase lattice oxygen atoms only, or 2) with OH or H2O the holes are trapped as oxygen-centered radicals covalently linked to surface titanium atoms [40] (Figure 7). This is summarized by Eqs. (7)-(9). 

A number of mechanisms have been offered to explain isotaeticity. This author favors the approach advanced by Cossee (37-33), and extended by others (2,34), which attributes stereoregularity to the special geometry arising from active sites which are adjacent to crystal defects in a solid substrate. Titanium trichloride is a layer-lattice structure (35) consisting of layers of titanium ions interspersed with layers of chloride ions, with the specific structure shown in Fig. 8b. 

Figure 1.6 Schematic illustration of a fully coordinated tetrahedrally bonded titanium atom substituted for a tin atom at one of the lattice positions of TS-1 (A) and the same titanium site located near a silicon vacancy filled with hydrogen atoms to form a silanol nest (B). The Ti/ defect mechanism for the partial silanol nest model showing the preadsorbed complex of propylene on the hydroperoxy intermediate (C) and the Ti/defect mechanism for the full silanol nest model showing the preadsorbed complex of H2O2 on the titanium site. Distances in A. Color coding small white spheres, H atoms red spheres, O atoms gray spheres, C atoms large white spheres, Ti atoms green spheres. Si atoms. Adapted from Ref (191 b), with permission from The American Chemical Society.

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