**3. TiO2 crystal structure**

Titanium oxide exists in nature as minerals and it has various structures under ambient conditions: rutile, anatase, brookite and srilankite (this last structure is also called PbO2-type TiO2 or TiO2-II) [12–14]. Rutile is a relatively abundant material and its structure is the most stable [13] and also the most studied. Anatase and brookite are extremely rare in nature [10]. TiO2 thin films are generally amorphous for deposition temperatures ≤350°C, above which anatase is formed. The most stable crystalline phase, rutile, is formed at temperatures greater than about 800°C. The brookite phase is rarely observed in deposited thin films. The functional properties of TiO2 films, powders and ceramics are strongly dependent on the phase of the material. Thus, TiO2-x is an n-type semiconductor, in contrast with p-type semiconductors, which contain electron acceptors and where the charge carriers are holes rather than electrons [15]. Substoichiometric TiO2-x is both a poor insulator and a modest semiconductor. Therefore, several attempts have been made either to control the oxygen vacancy concentration or to introduce charge carriers (doping) inside TiO2 in order to enhance the properties, depending on the needed application. After 50 years, the periodic table was updated by incorporation of new atoms such as Ti and so on [16]. The structural, optical and electrical properties of TiO2 are reported in **Table 1**.


**45**

**Figure 2.**

**Figure 1.**

*Elementary cell of TiO2 polymorphs.*

*Phase transition of titanium dioxide.*

*Titanium Dioxide Versatile Solid Crystalline: An Overview*

The elementary cells of the TiO2 crystal structures, phase transition and crystallographic structures of TiO2 are presented in **Figures 1–3**, respectively. Rutile and anatase, which are tetragonal, are more ordered than the orthorhombic structure. Anatase, which is the least dense structure, has empty channels along the a and b axes.

*DOI: http://dx.doi.org/10.5772/intechopen.92056*

**Table 1.** *TiO2 properties.* *Assorted Dimensional Reconfigurable Materials*

**3. TiO2 crystal structure**

a white pigment in paint [2], replacing lead oxide that is toxic, and in toothpaste. In the form of solid thin films, transparent single crystals or its thin films have a high refractive index that makes TiO2 suitable for optical applications [3, 4]. Multi-layers composed of TiO2 and SiO2 are designed to make antireflection coatings in the whole visible range [5, 6]. TiO2 is widely used for photocatalysis [6]; for example, electrodes made of TiO2 are used in electrochromic devices [7] and dye-sensitized solar cells [8] etc., and solid-state photovoltaic solar cells with porous TiO2 layer show promising results [2, 3]. Pd-TiO2 diodes are used as hydrogen gas sensors [9, 10], and nowadays, TiO2 can replace ZrO2 in lambda probes used in the car industry [11]. Most of these applications are due to its n-type semiconducting property and realized with micro- or nano-structured TiO2 nanopowders or nano thin films.

Titanium oxide exists in nature as minerals and it has various structures under ambient conditions: rutile, anatase, brookite and srilankite (this last structure is also called PbO2-type TiO2 or TiO2-II) [12–14]. Rutile is a relatively abundant material and its structure is the most stable [13] and also the most studied. Anatase and brookite are extremely rare in nature [10]. TiO2 thin films are generally amorphous for deposition temperatures ≤350°C, above which anatase is formed. The most stable crystalline phase, rutile, is formed at temperatures greater than about 800°C. The brookite phase is rarely observed in deposited thin films. The functional properties of TiO2 films, powders and ceramics are strongly dependent on the phase of the material. Thus, TiO2-x is an n-type semiconductor, in contrast with p-type semiconductors, which contain electron acceptors and where the charge carriers are holes rather than electrons [15]. Substoichiometric TiO2-x is both a poor insulator and a modest semiconductor. Therefore, several attempts have been made either to control the oxygen vacancy concentration or to introduce charge carriers (doping) inside TiO2 in order to enhance the properties, depending on the needed application. After 50 years, the periodic table was updated by incorporation of new atoms such as Ti and so on [16]. The structural, optical and electrical properties of TiO2 are reported in **Table 1**.

**Polymorph Anatase Rutile Brookite**

I 41/amd

c *=* 9.5548 (T *=* 350°C)

⊥ to c axis 2.55 // to c axis 2.48

// to c axis 48

Direct 3.42 // to c axis indirect 3.46

crystal: 15–550 thin film: 0.1–4

) [1] 3.84 4.26 4.12

Tetragonal P 42/mnm

a = 4.5930 c = 2.9590

⊥ to c axis 2.60 // to c axis 2.89

⊥ to c axis 89 // to c axis 173

⊥ to c axis Direct 3.04 // to c axis indirect 3.05

crystal: 0.1–10 thin film: 0.1

Orthorhombic Pbca

> a = 5.4558 b = 9.1819 c = 5.142

⊥ to a or b axis 2.57 // to c axis 2.69

78

3.14

—

Structure and space group [1] Tetragonal

Lattice parameter a *=* 3.7923

Dielectric constant [8–10] ⊥ to c axis 31

Band gap (eV) [12, 13] ⊥ to c axis

 m2 /Vs)

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**Table 1.** *TiO2 properties.*

Density (g cm<sup>−</sup><sup>3</sup>

Refractive index λ = 600 nm [7]

Electron mobility (10<sup>−</sup><sup>4</sup>

[14, 15]

The elementary cells of the TiO2 crystal structures, phase transition and crystallographic structures of TiO2 are presented in **Figures 1–3**, respectively. Rutile and anatase, which are tetragonal, are more ordered than the orthorhombic structure. Anatase, which is the least dense structure, has empty channels along the a and b axes.

**Figure 2.** *Phase transition of titanium dioxide.*

**Figure 1.**

**Figure 3.** *Crystallographic structures of TiO2 (a) anatase, (b) brookite, and (c) rutile.*
