Titanium vacancies

Titanium Monoxide. Titanium monoxide [12137-20-17, TiO, has a rock-salt stmcture but can exist with both oxygen and titanium vacancies. For stoichiometric TiO, the lattice parameter is 417 pm, but varies from ca 418 pm at 46 atom % to 4I62 pm at 54 atom % oxygen. Apparendy, stoichiometric TiO has ca 15% of the Ti and O sites vacant. At high temperatures (>900° C), these vacancies are randomly distributed at low temperatures, they become ordered. Titanium monoxide may be made by heating a stoichiometric mixture of titanium metal and titanium dioxide powders at 1600°C... 

Chloride layers are shown as lines while titanium ions are indicated by black dots. The white boxes represent titanium vacancies. These occur because, in order to achieve electrical neutrality, only two-thirds of the octahedral interstices between chloride layers contain titanium (III) ions. Although it is not indicated in Fig. 23, chloride vacancies exist at the edge. Analysis revealed that the number of vacancies is practically equal to the number of titanium ions at the edge. This is so despite differences in size and shape of crystals. 

The strong dependence between site occupancy and thermal displacement parameters led us to improve the stmctural model by adding titanium vacancies and fixing B factors to acceptable values. Finally both these parameters were refined simultaneously. The best fit... 

Experimental evidence for such a stmcture containing titanium vacancies can be correlated with an experimental density value smaller than the one of pure TiOF2 (i.e. 3.06 g.cm ). Indeed, using two different density measurement methods bromobenzene pycnometry and helium pycnometry, similar values were obtained 2.628(3) and 2.631(2), respectively. 

In order to take into account 25% of titanium vacancies, a formula close to Tio7s(OH)i 5F15 should be considered. This formula was confirmed by Microprobe analysis measurements showing the presence of 39.6 % of titanium atoms and 30.6 % of fluorine atoms, which are in good agreement with the 39.9 % and 31.6 % calculated values. 

Coalescence results from the transport of hydroxyl ions from the grain toward the neck with creation of titanium vacancies at the neck, which migrate toward the grain. The following elementaiy steps describe the matter transport ... 

Crystal structure of solids. The a-crystal form of TiCla is an excellent catalyst and has been investigated extensively. In this particular crystal form of TiCla, the titanium ions are located in an octahedral environment of chloride ions. It is believed that the stereoactive titanium ions in this crystal are located at the edges of the crystal, where chloride ion vacancies in the coordination sphere allow coordination with the monomer molecules. 

Fig. 7.13, this shifts the vacancy—represented by the square-in the coordination sphere of the titanium to a different site. Syndiotactic regulation occurs if the next addition takes place via this newly created vacancy. In this case the monomer and the growing chain occupy alternating coordination sites in successive steps. For the more common isotactic growth the polymer chain must migrate back to its original position. 

Figure 7.14a illustrates the insertion of a propylene monomer into an edge vacancy in a crystal adjacent to an alkylated titanium atom. In Fig. 7.14b a cross-sectional view of the same site shows how the preferential orientation of the coordinated monomer is dictated by constraints imposed by the protuberances on the crystal surface. 

Both anatase and mtile are broad band gap semiconductors iu which a fiUed valence band, derived from the O 2p orbitals, is separated from an empty conduction band, derived from the Ti >d orbitals, by a band gap of ca 3 eV. Consequendy the electrical conductivity depends critically on the presence of impurities and defects such as oxygen vacancies (7). For very pure thin films, prepared by vacuum evaporation of titanium metal and then oxidation, conductivities of 10 S/cm have been reported. For both siugle-crystal and ceramic samples, the electrical conductivity depends on both the state of reduction of the and on dopant levels. At 300 K, a maximum conductivity of 1 S/cm has been reported at an oxygen deficiency of... 

Figure 6. Structure of the perovskite-type lithium-ion conductor Li 2yLa057TiO3. The lithium ions (small, gray) and the lanthanum ions (large, gray) are randomly distributed over the A sites, of which 14 percent are vacancies, enabling the lithium ions to be mobile. Titanium forms TiOh octahedra, as shown in yellow. The unit cell is indicated.

In crystals of more complex formula, such as titanium dioxide, TiC>2, a Schottky defect will consist of two anion vacancies and one cation vacancy. This is because it is necessary to counterbalance the loss of one Ti4+ ion from the crystal by the absence of two O2- ions in order to maintain composition and electroneutrality. This ratio of two anion vacancies per one cation vacancy will hold in all ionic compounds of formula MX2. In crystals like A1203, two Al3+ vacancies must be balanced by three O2- vacancies. Thus, in crystals with a formula M2X3, a Schottky defect will consist of two vacancies on the cation sublattice and three vacancies on the anion sublattice. These vacancies are not considered to be clustered together but are distributed... 

Defect clusters are similarly prominent in hydrated phases. For example, anatase nanocrystals prepared by sol-gel methods contain high numbers of vacancies on titanium sites, counterbalanced by four protons surrounding the vacancy, making a (Vxi 4H ) cluster. In effect the protons are associated with oxygen ions to form OH- ions, and a vacancy-hydroxyl cluster is an equally valid description. Similar clusters are known in other hydrated systems, the best characterized being Mn4+ vacancies plus 4H in y-Mn02, known as Reutschi defects. 

The conductivity of titanium dioxide, TiC>2, at 1166 K was found to depend upon the oxygen partial pressure as in the following table. Assuming that oxygen vacancies are introduced into the oxide as the main point defect, is the vacancy more likely to be doubly or singly charged ... 

However, density measurements suggest that the formation of chromium vacancies occurs, and atomistic simulations indicate that the vacancies are likely to cluster near to titanium substituents to form neutral entities ... 

At the stoichiometric composition of TiOi oo the crystal structure can be thought of as a NaCl-type structure with vacancies in both the metal and the oxygen sublattices one-sixth of the titaniums and one-sixth of the oxygens are missing. Above 900 "C, these vacancies are randomly distributed, but below this temperature, they are ordered as shown in Figure 5.28. 

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