Titanate

In chemistry, titanate usually refers to inorganic compounds composed of titanium oxides, or oxides containing the titanium element. Together with niobate, titanate salts form the Perovskite group.

In some cases, the term is used more generally for any titanium-containing anion, e.g. [TiCl₆]²⁻ and [Ti(CO)₆]²⁻. This article focuses on the oxides.

Many kinds of titanium oxides are known, and some are commercially important. Typically these materials are white, diamagnetic, high-melting, and insoluble in water. They are often prepared at high temperatures, e.g. using tube furnaces, from titanium dioxide. In virtually all cases, titanium achieves octahedral coordination geometry.^[1]

Orthotitanates

Orthotitanates have the formula M₂TiO₄, where M is divalent. An example of such a material is magnesium titanate (Mg₂TiO₄), which adopts the spinel structure. Li₂TiO₃ is not considered an orthotitanate since it adopts the rock-salt structure and does not feature an identifiable titanium anion. Orthotitanates almost never feature identifiable TiO₄⁴⁻ centres, an exception being Ba₂TiO₄.^[2]

Titanic acid and its esters

Also called orthotitanic acid or titanium hydroxide, the substance H₄TiO₄ is called titanic acid. This material, which is not well defined, is obtained by hydrolysis of TiCl₄.^[3] The solid is unstable with respect to loss of water and formation of titanium dioxide. Esters of orthotitanic acid are known, however; one example being titanium isopropoxide. Esters derived from smaller alcohols adopt more complex structures wherein titanium does achieve octahedral coordination, e.g. Ti₄(OCH₃)₁₆ or titanium tetramethoxide.^[4] It is a weak acid, if it can be isolated.

Metatitanates

The metatitanates have the formula MTiO₃, where again M is divalent. They do not feature discrete TiO₃²⁻ centres. Some, like the commercially important mineral ilmenite (FeTiO₃), crystallize in the hexagonal close packing motif seen in corundum. Alternatively, some materials with the formula MTiO₃ crystallize in the motif known as perovskite, which is also the name of the mineral form of calcium titanate (CaTiO₃). Barium titanate is one such perovskite-structured titanate with ferroelectric properties.^[1]

Complex titanates

More complex titanates are also known, such as bismuth titanate, Bi₄Ti₃O₁₂.^[5]

References

1. Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 1121–23. ISBN 978-0-08-022057-4..
2. Wu, Kang Kun; Brown, I. D. (10 April 1973). "The Crystal Structure of β-Barium Orthotitanate, β-Ba₂TiO₄, and the Bond Strength-Bond Length Curve of Ti-O". Acta Crystallographica. B29: 2009–2012. doi:10.1107/S0567740873005959.
3. Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 421.
4. D. A. Wright and D. A. Williams "The crystal and molecular structure of titanium tetramethoxide" Acta Crystallogr. B 1968, volume 24, pp. 1107-1114. doi:10.1107/S0567740868003766.
5. Galasso, F. S.; Kestigan, M. (2007), "Bismuth Titanate, Bi₄Ti₃O₁₂", Inorg. Synth., 30: 121, doi:10.1002/9780470132616.ch24.