From WolfWikis

Rutile

TiO₂

Figure 1: TiO₂ structure shown here, with space group P4₂/mnm.

Figure 2: Viewed from the (110) axis, TiO₂ contains alternating chains of octahedra.

The general rutile (TiO₂) structure takes the form AB₂ and has tetragonal symmetry (a=b=4.954 A, c=2.958 A) [1]. It is composed of alternating TiO₆ octrahedra. The octahedra link by sharing edges and corners that become apparent as the boundaries of the unit cell are extended outwards (Figure 2). This gives coordination number of 6 for titanium and 3 for oxygen. If viewed down the (110) axis, rutile appears to have fourfold symmetry, but in fact contains a screw axis, located along the interstitial cavities [2]. Among heterocationic rutile structures, distortions of the octahedra are primarily from second order Jahn-Teller distortion, where the axial bond lengths are slightly longer than along the equitorial [3]. Octahedral twists or cationic displacements are minimal. Other compounds that adopt the rutile structure are SnO₂, MnO₂, MgF₂, and some transition metal fluorides. Compounds containing larger metal ions that have a different radius ratio (> .732) prefer to adopt the fluorite (CaF₂) structure [4].

TiO₂ contains interesting physical properties. Its intense whiteness (anatase) has made it a valuable substitute for toxic lead compounds in paints as a coloring agent. The mineral is manufactured chemically from its principle ore, ilmenite (FeTiO₃) [5]. Energy conversion from solar light also has given TiO₂ some extensive focus in recent years, particularly due to its ability to absorb light in the UV range. The original Gratzel Cell [6] incorporates thin-film TiO₂ layers as a low cost and highly efficient photovoltaic cell (12% in diffuse daylight). A chemical dye (trimeric ruthenium complex) injected into the TiO₂ solution shifts the absorption wavelength to 750 nm, well in the visible range. A photon absorbed by the dye releases an electron into the conduction band of the metal oxide semiconductor by charge transfer. Two conducting electrodes detect (and collect) the current generated by the cell [6]. Subsequent improvements on the Gratzel Cell has led to some very interesting and inexpensive solar cells. Dyes from various berries have been incorporated into the cell and show a similar shift in the absorption spectra and comparable current and voltage generation [7].

TiO₂ also is found in Anatase and Brookite structures. See the link for a further discussion in varied structures and properties of this compound

FeSbO₄

Figure 3: Slice along the [101] of the structure of FeSbO₄ where green atoms are antimony, pink atoms are iron and blue atoms are oxygen. The unit cell of FeSbO₄ in an ordered domain is also shown.

FeSbO₄ is a derivative of the rutile structure with the formula ABO₄. The crystal structure of FeSbO₄ is similar to that of TiO₂ and is composed of Fe³⁺ and Sb⁵⁺ cations distributed in the octahedral sites of the oxygen lattice[8]. Each cation is coordinated to six oxygens and each oxygen is coordinated to three cations. FeSbO₄ has P42/mnm symmetry and the unit cell of FeSbO₄ is shown in Figure 3.

How the Fe and Sb cations are ordered in the structure was originally thought to be completely random, but the magnetic properties of the compound indicate a more ordered distribution of cations.

FeSbO₄ displays Antiferromagnetic correlations in the two dimensions perpendicular to the c axis of the crystal[10]. These correlations extend to maximum distances of about 30A[10]. DFT calculation studies were preformed to determine the extent of ordering of cations that would provide the observed magnetic properties and be the most energetically favorable[9].

If the cations are ordered in only one dimension, a structure similar to Figure 4 would be observed. This structure however would not provide two dimensional magnetic ordering because the spins can only couple in one direction vertically. The second proposed structure shown in Figure 5 displays two dimensional ordering in which the cations alternate along the c axis of the crystal. This ordering would produce magnetic ordering in the two directions perpendicular to the c axis. DFT calculations also found the two dimensional ordering to be more energetically favorable than the one dimension ordering within the super cell structure[9].

Figure 4: An example of a one dimensional ordered structure of FeSbO₄

Figure 5: An example of a two dimensional ordered structure of FeSbO₄.

Figure 6: An example of spin frustration that occurs in FeSbO₄ when an ordered domain encounters disorder. Circled region indicates a site of spin frustration.

The magnetic correlations only extend maximum distances of 30A which means a significant amount of disorder is still present in the structure. The structure is composed of areas of ordered domains extending about 30A[10]. An experimentally observed spin glass like transition has been observed at 70K[10]. This is observed because a large amount of spin frustration exists at the edges of the domains as shown in Figure 6.

The current applications of FeSbO₄, however do not utilize its magnetic properties. FeSbO₄ is a catalyst for the selective oxidation of propene to acrolein, which is a precursor to acrylonitrile. During the catalytic process, oxygen is taken from the surface of FeSbO₄ and in then reoxidized by oxygen in the gas phase[11].

RuO₂

Figure 7: RuO₂ unit cell. Red atoms are ruthenium, and green atoms are oxygen.

Ruthenium dioxide (unit cell structure shown in Figure 7) is one of many metal dioxide compounds which crystallize with the tetragonal rutile structure. The structure is very similar to that of TiO₂. Distortions from the ideal rutile are often measured in terms of the ratio of axial to equatorial bond lengths within the octahedra. An ideal octahedron has a ratio of 1; RuO₂ has a ratio of 0.979, which means the axial bonds are shorter [12]. Ruthenium dioxide can act as a barrier to diffusion.

Figure 8: Views of the ruthenium dioxide structure looking down the c-axis on the left and the a-axis on the right.

Under high pressure RuO₂ forms the more dense CaCl₂ type of deformed rutile [14, 15]. There are two types of "ideal" rutile. One "ideal" rutile is based on the ideal octahedron previously discussed. Another "ideal" rutile is the more dense CaCl₂ type structure, in which the anions are idealized to yield a hexagonal closest packed anion lattice with cations filling one half of the octahedral holes. The phase transition from RuO₂ to CaCl₂-like structure occurs via a rotation about the c-axis of the RuO₂ octahedra, and a small lattice distortion. The relative change in octahedral orientations can be seen in Figure 9. At even higher pressures the cubic fluorite structure is formed [15, 16].

Figure 9: Views of the rutile ruthenium dioxide structure octahedra on the left and the CaCl₂-like structure octahedra on the right.

ε-Ti₂N

Discovered by Holmberg in 1962, titanium heminitride ε-Ti₂N crystallizes in the P4₂/mnm antirutile structure, which can be viewed as a rutile-type structure (considering the anion as central atom) consisting of edge and corner sharing NTi₆ distorted octahedra (Figure 10). As it is frequently useful to consider the connectivity and environments of the cations in a crystal structure, it can be seen that TiN₃ distorted trigonal planar subunits connect in a rather unusual fashion (Figure 11) in the antirutile structure. The TiN₃ connectivity is described as 1-dimensional "ribbons" of alternating edge and corner sharing triangles.

Figure 10: A ~(001) view of the unit cell of antirutile ε-Ti₂N. Nitrogen (light blue) has distorted octahedral coordination to six titaniums (dark grey).

Figure 11: A ~(001) view of 2x2x2 supercell of ε-Ti₂N. One can see the connectivity of distorted triangular TiN₃ subunits. TiN₃ ribbons propogate in the [1,1,0] and [-1,1,0] directions.

Common sense dictates that this system should be classified as a nitride, and it is clearly Ti-N bonding which holds this structure together. In examining the stability of competing heminitride phases, differing levels of theory [17, 18] suggest that the stability of the antirutile ε heminitride phase is provided by d-d splitting, of both σ-type and π-type, between neighboring titaniums (Figures 12, 13). The competing δ' heminitride phase (an ordered vacancy derivitive of δ phase TiN -- rocksalt) does not have significant σ-type splitting in the d-manifold, and consequently the fermi energy does not lie in a desirable local minima of the density of states.

Figure 12: A 3x3 (001) cross-section at z=0 of ε-Ti₂N. Instead of Ti-N connectivity, here Ti-Ti connectivity is shown arising from e_g σ-type splitting in the (1,-1,0) direction and t_2g π-type splitting in the (1,1,0) direction. The corresponding z=0.5c cross-section would be a 90° rotation of this view via P4₂/mnm symmetry.

Figure 13: A view of bonding to nitrogen, and weak splitting with neighboring titaniums, that a single cation in this structure will experience. By considering figure 13 and the space group, it is clear this orientation is not unique.

The TiN_x system has a rich phase diagram (for an early example, see [19]), although it should be noted that there is disagreement in the literature as to many finer details of the system.

NaTi₂O₄

NaTi₂O₄ crystallized in the orthorhombic calcium ferrite-type structure with space group Pnam. It can be viewed as a rutile derivative structure. This structure is built up from “double rutile” [20] chains (shown in Figure 14) composed of edge-sharing TiO₆ Octahedral extending along the c-axis. Each double rutile-type chain connects with four neighboring chains by corner-sharing, which forms one-dimensional tunnels along c-axis. Sodium cations reside in the one-dimensional tunnels. A unit cell view of the structure is shown in Figure 15, also demonstrating the rutile-like connectivity of these chains.

The average oxidation state of Ti in NaTi₂O₄ is 3.5+, which requires either mixed valence Ti3+/Ti4+ or metallic delocalization. From Ti-O bond length (shown in Figure 16 and Figure 17), we can see that there are two kind of Ti atoms in NaTi₂O₄. So the double rutile-type chains in NaTi₂O₄ are in mixed vanlence assignment.

Ti3+ has one d electron, filling in the π* Ti-O orbital, which decreases the bond order of Ti3+-O and then increases the Ti3+-O bond length. Therefore, Ti in Figure 16 is Ti3+ and Ti in Figure 17 is Ti4+. The mixed valence Ti3+/Ti4+ will cause a charge ordering in NaTi₂O₄.

Figure 14: "Double rutile" chain in NaTi₂O₄. Ti is in blue, O is in red.

Figure 15: The structure of NaTi₂O₄ viewd along the c-axis. The Ti(1)O₆ octahedral are shown in light blue; the Ti(2)O₆ octahedral are medium blue. Sodium ions are represented as yellow spheres. Ti is in blue, O is in red.[20]

Figure 16: Oxygen-oxygen distances in the light blue double rutile chain of NaTi₂O₄.

Figure 17: Oxygen-oxygen distances in the medium blue double rutile chain of NaTi₂O₄.

N-doped TiO₂

Figure 16: 3D picture of TiN structure, arrow points at N

Figure 16: 2D picture of TiN structure, arrow points at N

Figure 16: 2D picture of TiO2, N replaced with O as arrow points

TiO2 is a typical rutile structure compound. It is a good photocatalyst under UV-light(Photocatalyst is the acceleration of a photoreaction in the presence of a catalyst.

Recently, in order to improve the photocatalytic activity, N-doped titanium oxide (TiO₂) films were obtained by thermal oxidation of TiN films, which were prepared on Ti substrates by ion beam assisted deposition (IBAD). The dominating rutile TiO₂ phase was found in films after thermal oxidation. According to the results of XPS, the residual N atoms occupied O-atom sites in TiO₂ lattice to form Ti-O-N bonds. It gives a better photocatalyst compared with undoped TiO₂.

The effect of nitrogen was responsible for the enhancement of photoactivity of N-doped TiO₂ films in the range of visible light. TiO_2-xN_x shows a large shift of the absorption edge from 420 nm to 600 nm.

This result can be explained by the introduction of an isolated narrow N 2p band above the valence band of TiO_2-xN_x, with which visible light could excite electrons from the narrow band to the conduction band.

References

[1] A. R. West, "Basic Solid State Chemistry", J.W.& D. Ltd, 1999; 48-50.

[2] Dutch, Steven. UW-GB Natural & Applied Sciences. Rutile Structure, http://www.uwgb.edu/dutchs/MPNOTES.HTM, 2006.

[3] Mo, S.; Ching, W. Phys Rev. B. 1995, 51, 13023.

[4] Miessler, G.; Tarr, D. "Inorganic Chemistry", Pearson Educatiion, Inc, 217.

[5] G. Wulfsburg, "Inorganic Chemistry", University Science Books, 686

[6] Brian O'Regan and Michael Gratzel, Nature (London), 353, 737 (1991)

[7] Christian Garcia, et. al., Journal of Photochemistry and Photobiology A. 2003, 160, 87.

[8]Grau-Crespo, R.; Leeuw, N. H.; Catlow, R. A. J. Mater. Chem. 2004, 16, 1954-1960.

[9] Grau-Crespo, R., Leeuw, N. H., Catlow, R. A., J. Mater. Chem. 2003, 13, 2848–2850.

[10] Labarta, R., etal. J. Phys. Review. 1991 44, 2, 691-698.

[11] Grau-Crespo, R., etal. J. Matter. Chem. 2006, 16, 1943–1949.

[12] Glassford, Keith M. and James R. Chelikowsy. Physical Review B. 1993, 47, 1732.

[13]Krusin-Elbaum, L. and M. Wittmer. J. Electrochem. Soc: Solid-State Science and Technology. 1988, 135, 2610.

[14]Wu, Ruqian, and W.H. Weber. J.Phys.: Condens. Matter 2000, 12, 6725.

[15] Haines, Julian and J. M. Léger. Phys. Rev. B. 1993, 48, 13344.

[16]Over, H. Appl. Phys. A. 2002, 75, 37.

[17] R Eibler, J. Phys. Cond. Matter 5 (1993) 5261-5276

[18] R Eibler, J. Phys. Cond. Matter 10 (1998) 10223-10240

[19] N McDonald, G. Wallwork, Oxidation of Metals 2 3 (1970) 263-283

[20] J.Akimoto and H. Takei, Journal of Solid State Chemistry 1989, 79, 212

[21] A. Fujishima and K. Honda, Nature 238 (1972), p. 37.

[22] Wan, L.; Li, J. F.; Feng, J. Y.; Sun, W.; Mao, Z. Q.Applied Surface Science (2007), 253(10), 4764-4767. Ë