Rutile transition

XRD analysis of the xerogels obtained by drying pure titanium dioxide sol at 70°C showed the presence of the nanocrystalline anatase phase [109]. Thermal treatment of this xerogel resulted in the growth of anatase crystallites up to 400°C. The anatase-to-rutile transformation began to occur at 450-500°C. This process was practically completed at 700°C, and only rutile phase existed at rcaic > 700°C. This feature of Ti02 xerogels is typical and well known (see, for example, [109]). Thus, it can be concluded that anatase-rutile transition temperature of nanosized particles is considerably lower than that of the... 

Deactivation of catalysts is a major problem in o-xylene oxidation [6,7]. For this reaction, deactivation has been mainly attributed to the irreversible anatase - rutile transformation [2,3]. In fact, anatase was found to be the best support for vanadium pentoxide catalysts leading the presence of rutile to lower activities and selectivities [8,9], The anatase-rutile transition can take place at temperatures above 973 K [10] but it is known that the presence of vanadia promotes such transformation [11-14] which, in these conditions, can start at 773 K [14], Such temperatures are easily attained in industrial reactors due to the high exothermicity of o-xylene oxidation that can lead to the formation of temperature profiles lengthwise with pronounced maxima (hot spot) [1]. 

Effects of metal salts, DMF, and DMSO on the anatase-rutile transition... 

Titania has three naturally occurring polymorphs anatase, brookite, and rutile. Anatase and brookite are considered to be kinetic products despite the fact that depending on the particle size, anatase becomes more stable than rutile. The anatase-rutile transition is exothermic and irreversible, occurring in the range 400-1200°C [25,26]. 

The anatase-rutile transition is not observed in supported samples. This transition is generally catalyzed by oxide mixture we conclude that y-alumina support hinders the crystallization of the TiOj anatase structure, in agreement with DRX data. 

Among other oxides, it has already been reported that the addition of zirconia to titania can contribute to remarkably increase the final surface area of the catalyst [192-197], the anatase-to-rutile transition temperature, [193, 195, 198] the surface acidity and the overall adsorption and hydrophilic properties. All these changes are expected to enhance also the photocatalytic activity. However, the fact that Zr02 has a bandgap much larger ( 5 eV) than Ti02 (3.0-3.2 eV) prevents its use as a photocatalyst under UVA illumination. 

A simple approach to prepare doped Ti02 powders relies on mixing a titanium alkoxide with a solution of the dopant precursor (namely, vanadium oxychloride) in dichloromethane. Then the solvent is evaporated, and either Ti isopropoxide or Ti tetrachloride is added. After reacting the final mixture with water and acetone, the powders are dried and calcined. Thick film sensors are fabricated by screen printing a paste of annealed powders surprisingly, the vanadium doping facilitates the anatase-rutile transition, in contrast to what has... 

Figure 1. TGA curve of the oxidation of TiS2 to Ti02. The unusual inflection at 620 6° is apparently associated with the anatase-to-rutile (Ti02) phase transition. XPD combined with TGA suggests the rapid oxidation at about 350° results in the presence of similar amounts of anatase and rutile, with the residual sulfur trapped in amorphous grain boundaries. The enhanced oxidation of this residual sulfur at about 620° may be associated with interfacial changes initiated by the onset of the anatase-to-rutile transition, which is complete by the end of the analysis. ...

The measures of solid state reactivity to be described include experiments on solid-gas, solid-liquid, and solid-solid chemical reaction, solid-solid structural transitions, and hot pressing-sintering in the solid state. These conditions are achieved in catalytic activity measurements of rutile and zinc oxide, in studies of the dissolution of silicon nitride and rutile, the reaction of lead oxide and zirconia to form lead zirconate, the monoclinic to tetragonal transformation in zirconia, the theta-to-alpha transformation in alumina, and the hot pressing of aluminum nitride and aluminum oxide. 

The heavier metal tantalum is distinctly less inclined than niobium to form oxides in lower oxidation states. The rutile phase TaOz is known but has not been studied, and a cubic rock-salt-type phase TaO with a narrow homogeneity range has also been reported but not yet fully characterized. TazOs has two well-established polymorphs which have a reversible transition temperature at 1355°C but the detailed structure of these phases is too complex to be discussed here. 

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

The room temperature transformation of the columbite phase to baddeleyite commences at 13-17 GPa 6, with transition pressure increasing linearly with temperature Direct transition from rutile to baddeleyite phase at room temperature and 12 GPa has also been reported 7. The baddeleyite phase undergoes further transition to an as yet undefined high-symmetry structure at 70-80 GPa. The most likely candidate for the high-pressure phase is fluorite, which is consistent with the general pattern of increasing Ti coordination number from 6 in rutile, to 7 in baddeleyite (a distorted fluorite structure), and to 8 in fluorite. 

This paper presents the results of ab initio calculation investigating the pressure dependence of properties of rutile, anatase and brookite, as well as of columbite and hypothetical fluorite phases. The main emphasis is on lattice properties since it was possible to locate transitions and investigate transformation precursors by using constant-pressure optimization algorithm. 

Present in the next sections are the LDA results for equilibrium structure, pressure-induced transitions and electronic properties of various polymorphs, and the comparative analysis of the results for rutile and anatase that were obtained using LDA and GGA forms of the exchange-correlation potential. 

L.-G. Liu and T.P. Memagh, Phase transitions and Raman spectra of anatase and rutile at high pressures... 

It was shown by Hund (Ref. 23) that for small values of n (less than 6 or 9, depending upon the assumptions made) the rutile structure can bgpome stable. However, our discussion makes it probable that the transition is actually due to the radius ratio. 

This theoretical result is completely substantiated by experiment. Goldschmidt,31 from a study of crystal structure data, observed that the radius ratio is large for fluorite type crystals, and small for those of the rutile type, and concluded as an empirical rule that this ratio is the determining factor in the choice between these structures. Using Wasastjerna s radii he decided on 0.67 as the transition ratio. He also stated that this can be explained as due to anion contact for a radius ratio smaller than about 0.74. With our radii we are able to show an even more satisfactory verification of the theoretical limit. In Table XVII are given values of the radius ratio for a large number of compounds. It is seen that the max-... 

In this discussion, two mutually canceling simplifications have been made. For the transition value of the radius ratio the phenomenon of double repulsion causes the inter-atomic distances in fluorite type crystals to be increased somewhat, so that R is equal to /3Rx-5, where i has a value of about 1.05 (found experimentally in strontium chloride). Double repulsion is not operative in rutile type crystals, for which R = i M + Rx- From these equations the transition ratio is found to be (4.80/5.04)- /3i — 1 = 0.73, for t = 1.05 that is, it is increased 12%. But Ru and Rx in these equations are not the crystal radii, which we have used above, but are the univalent crystal radii multiplied by the constant of Equation 13 with z placed equal to /2, for M++X2. Hence the univalent crystal radius ratio should be used instead of the crystal radius ratio, which is about 17% smaller (for strontium chloride). Because of its simpler nature the treatment in the text has been presented it is to be emphasized that the complete agreement with the theoretical transition ratio found in Table XVII is possibly to some extent accidental, for perturbing influences might cause the transition to occur for values a few per cent, higher or lower. 

XANES spectroscopy shows that a narrow and intense pre-edge peak at 4967 eV, due to the Is 3pd electronic transition involving Ti atoms in tetrahedral coordination, is present in well-manufactured TS-1 (Fig. 2c). Conversely this electronic transition of Ti(IV) species in Ti02 (anatase or rutile) is characterized by a very low intensity due to the small pd hybridization in octahedral symmetry. Indeed the transitions l2g are symmetrically forbidden in the case of octahedral coordination of Ti (IV), but the transition Ai T2 is allowed in the case of tetrahedral coordination of Ti(IV), as in the case of [Ti04] units [52,58-61,63,68]. 

The photolytic reduction of N2 at TiO -suspensions was at first reported by Schrauzer et al. Small amounts of NH3 and N2H4 were obtained as products. The highest activity was found with anatase containing 20-30 % rutile. Very low yields were also obtained with p-GaP electrodes under illumination It is much easier to produce NH3 from NO -solutions at CdS- and Ti02-particles using S -ions as hole scavengers . Efficiencies are not reported yet. Recently the formation of NH3 from NO was observed at p-GaAs electrodes under illumination. In this case NH3-formation was only found in the presence of transition metal ions or their complex with EDTA. 

As a contradistinction to the relatively simple case of AI2O3 Cr(III) where the color is due to a metal-centred electronic transition, we mention now on one hand the fact that the Cr(III) ion colors many transition-metal oxides brown (e.g. rutile Ti02 or the perovskite SrTi03 [15]), and on the other hand the fact that the color of blue sapphire (AI2O3 Fe, Ti [16]) is not simply due to a metal-centred transition. By way of illustration Fig. 1 shows the diffuse reflection spectrum of SrTiOj and SrTi03 Cr(III) [17], and Fig. 2 the absorption spectrum of Al203 Ti(III) and Al203 Ti(III), Fe(III) [18]. It has been shown that these colors are due to MMCT transitions and cannot simply be described by metal-centred transitions [19],... 

The interstitial hydrides of transition metals differ from the salt-like hydrides of the alkali and alkaline-earth metals MH and MH2, as can be seen from their densities. While the latter have higher densities than the metals, the transition metal hydrides have expanded metal lattices. Furthermore, the transition metal hydrides exhibit metallic luster and are semiconducting. Alkali metal hydrides have NaCl structure MgH2 has rutile structure. 

Ferroelasticity is the mechanical analogon to ferroelectricity. A crystal is ferroelastic if it exhibits two (or more) differently oriented states in the absence of mechanical strain, and if one of these states can be shifted to the other one by mechanical strain. CaCl2 offers an example (Fig. 4.1, p. 33). During the phase transition from the rutile type to the CaCl2 type, the octahedra can be rotated in one or the other direction. If either rotation takes place in different regions of the crystal, the crystal will consist of domains having the one or the other orientation. By exerting pressure all domains can be forced to adopt only one orientation. 

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