Titanium polymorphs

Titanium oxide nanostructures have versatile applications, for example, in photocatalysis, solar-energy conversion, sensors, and ductile ceramics. The synthesis of derivatives with all kinds of size and shape (spherical particles, nanotubes, and nanorods) has been described in numerous studies. Out of the three main titanium polymorphs (anatase, brookite, rutile), research so far has been centred on the synthesis of anatase nanoparticles. However, recently the generation of nanometer-sized rutile has received growing attention due to its promising potential as a photocatalyst and as an electrode material. 

A significant advantage of the PLM is in the differentiation and recognition of various forms of the same chemical substance polymorphic forms, eg, brookite, mtile, and anatase, three forms of titanium dioxide calcite, aragonite and vaterite, all forms of calcium carbonate Eorms I, II, III, and IV of HMX (a high explosive), etc. This is an important appHcation because most elements and compounds possess different crystal forms with very different physical properties. PLM is the only instmment mandated by the U.S. Environmental Protection Agency (EPA) for the detection and identification of the six forms of asbestos (qv) and other fibers in bulk samples. 

Anatase and mtile are produced commercially, whereas brookite has been produced by heating amorphous titanium dioxide, which is prepared from an alkyl titanate or sodium titanate [12034-34-3] with sodium or potassium hydroxide in. an autoclave at 200—600°C for several days. Only mtile has been synthesized from melts in the form of large single crystals. More recentiy (57), a new polymorph of titanium dioxide, Ti02(B), has been demonstrated, which is formed by hydrolysis of K Ti O to form 20, followed by subsequent calcination/dehydration at 500°C. The relatively open stmcture... 

Titanium Trichloride. Titanium trichloride [7705-07-9] exists in four different soHd polymorphs that have been much studied because of the importance of TiCl as a catalyst for the stereospecific polymerization of olefins (120,124). The a-, y-, and 5-forms are all violet and have close-packed layers of chlorines. The titaniums occupy the octahedral interstices between the layers. The three forms differ in the arrangement of the titaniums among the available octahedral sites. In a-TiCl, the chlorine sheets are hexagonaHy close-packed in y-TiCl, they are cubic close-packed. The brown P-form does not have a layer stmcture but, instead, consists of linear strands of titaniums, where each titanium is coordinated by three chlorines that act as a bridge to the next Ti The stmctural parameters are as follows ... 

Titanium Dioxide. The recrystallization of titanium dioxide in a cover-coat glass is very important to the development of thin, highly opaque finish coats. Titania, Ti02, is the primary opacifying agent for white finish coats. Two polymorphic forms of titania, anatase and mtile, may be present in... 

Fig. 2.1. Some metals hove more than one crystal structure. The most important examples of this polymorphism ore in iron and titanium.

Titanium dioxide exists in nature as three different polymorphs rutile, anatase and brookite. This material has been extensively studied during the last few decades due to its interesting physical properties and numerous technological applications. Rutile and anatase (a popular white pigment) are widely used in photocataly s and as sensors. Both of them have had new structural and electronic applications suggested recently (see for a review). 

Numerous theoretical and experimental studies have dealt with the properties of rutile (see 3 and references therein), while the other polymorphs have attracted less attention. This is due to the fact that rutile is easier to grow and characterize, and has a simpler structure that can be readily investigated using a variety of theoretical techniques. The fact that the experimentally reported bulk modulus of anatase is in the range from 59 GPa to 360 GPa illustrates the need for further studies of titanium dioxide polymorphs. 

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. 

Ground State Properties of Titanium Dioxide Polymorphs... 

The fluorite phase is found to be extremely high in energy (it falls outside the energy range of Figure 1). Its equilibrium volume at P=0 would be 27.648 A /mol, and calculated equation of state gives B=287 GPa and B"=4.18. These values make fluorite structure the least compressible of all titanium dioxide polymorphs studied here, but still leaves the observation of a phase with B>500 GPa unexplained. ... 

Table 7 shows the calculated weight percent of calcium carbonate and titanium dioxide in the white-colored paint sample. These levels are based on the calcium and titanium levels shown in Table 6. Calcium carbonate was evident by the FTIR spectrum acquired from the dried paint sample, shown in Figure 13. (Flad it been available, Raman spectroscopy, which gives ready access to the low wavenumber region, could have been used to confirm the presence (and polymorphic form) of titanium dioxide [4].) Given the white color of the paint, it is likely that the titanium present was present as titanium dioxide, and this was assumed in the calculations. The calculated weight percentage of calcium carbonate in the dried paint is 21.7 wt%, and 12.6 wt% in the paint containing the solvents. The titanium dioxide levels were calculated to be 30.6 and 17.7 wt% in the dried and solvent-containing paint sample, respectively.

This chapter has been concerned mainly with the chemistry of making pigments but has also stressed the importance of preparing them in the correct physical form. In the case of those pigments which can be made in more than one crystal form, such as the anatase and rutile forms of titanium dioxide, this may mean that all the pigment should be in one form only. In others, a mixture of polymorphic forms may be required, and it is then necessary to ensure that the desired ratio of these forms is present. 

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. 

Test results at 25°C and 90°C have been interpreted at Pennsylvania State University as follows. The higher the purity of the Zr02, the more likely it is to behave well as a pH sensor. Exceptions to this rule are titanium, which is beneficial for thermal shock resistance, and yttria, which is necessary for polymorphic stabilization. The optimum Y20, content was investigated by testing Zr02 tubes with Y203 contents of 4, 6, 8, 10, and 12 mol%. 

Ti02 is an important oxide with a broad range of applications in catalysis (as a catalyst or a support) (6), photocatalysis (35, 36), and sensor technology it is also used as a pigment. Of the three titanium dioxide polymorphs (rutile, anatase, and brookite), rutile and anatase have been most widely investigated they are the only ones reviewed here. 

Anatase, brookite and rutile are three polymorphs of titanium dioxide. Anatase is a kind of thermodynamically metastable form while rutile is a kind of stable one. Anatase can transform irreversibly to rutile at elevated temperatures ranged from 400 to 1200 °C according to particle size, morphology and additives. The solid-state phase transformation behavior has been widely investigated while the phase evolution between anatase and rutile under hydrothermal condition has been little paid attention to so far [5]. In this work, the structural evolution from anatase to rutile under milder hydrothermal conditions is proposed as well [7, 10]. 

Titanium is commonly found in nature as the Ti02 polymorph rutile, containing the Ti cation in octahedral coordination. Although rutile is tetragonal, the distortion of the coordination polyhedra is small (two Ti-O distances are 1.988 A, four are 1.944 A Grant, 1959). The TT+ cation in octahedral coordination is the most common naturally occurring form (also found in more complex oxides and silicates), although the TP+ species does also occur, sometimes in fourfold coordination. 

Part of the difficulty encountered in searching and interpreting the literature on polymorphic behaviour of materials is due to the inconsistent labelling of polymorphs. In many cases, the inconsistency arises from lack of an accepted standard notation. However, often, and perhaps more important, it is due to the lack of various authors awareness of previous work or lack of attempts to reconcile their own work with earlier studies (see, for instance. Bar and Bernstein 1985). While many polymorphic minerals and inorganic compounds acmally have different names (e.g. calcite, aragonite and vaterite for calcium carbonate or rutile, brookite, and anatase for titanium dioxide) this has not been the practice for molecular crystals, which have been labelled with Arabic (1, 2, 3,...) or Roman (I, II, III,...) numerals, lower or upper case Latin (a, b, c,... or A, B, C,...) or lower case Greek a, P,y, ) letters, or by names descriptive of properties (red form, low-temperature polymorph, metastable modification, etc.). 

Three polymorphs of titanium dioxide occur naturally, namely rutile, anatase and brookite, but surface scientists have concentrated almost entirely on the... 

Rosoff M, Sheen P-C. Pan abrasion and polymorphism of titanium dioxide in coating suspensions. ] Pharm Sci 1983 72 1485. Lehtola VM, Heinamaki JT, Nikupaavo P, Yliruusi JK. Effect of titanium dioxide on mechanical, permeability and adhesion properties of aqueous-based hydroxypropyl methylcellulose films. Boll Chim Farm 1994 133(Dec) 709-714. 

Another ternary system involving titanium which may also yield a complex set of phases is the Cu-Ti-O system. There is some evidence that the phase Cu3Ti04 exists in at least two polymorphic modifications. The compound does not form at temperatures below about 1200 K, where mixtures of CuO and Ti02 seem to be stable, and it may be that it forms directly from the melt or exists in a narrow temperature band just below the solidus line in the phase diagram. It would be of interest to... 

Navrotsky A, Kleppa OJ (1967) Enthalpy of the anatase-rutile transformation. J Am Ceram Soc 50 626 Navrotsky A, Jamieson JC, Kleppa OJ (1967) Enthalpy of transformation of a high-pressure polymorph of titanium dioxide to the ratile modification. Science 158 388-389 Navrotsky A, Petrovic 1, Hu Y, Chen CY, Davis ME (1995) Little energetic limitation to microporous and mesoporous materials. Microporous Mater 4 95-98... 

C.M. Freeman, Inorganic Crystal-Structure Prediction Using Simplified Potentials and Experimental Unit Cells - Application to the Polymorphs of Titanium-Dioxide. J. Mater. Chem., 1993, 3, 531-535. 

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