Titanate

Potassium titanate (IV) chemical structure

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.

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.

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. 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. 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.

Perovskite titanates

Perovskite titanates are a family of titanates that crystallize in the perovskite structure, with the general formula ABO3, where the B-site is occupied by Ti4+. They constitute one of the most extensively studied classes of oxide materials owing to their wide range of electrical, magnetic, optical, and mechanical properties. The physical properties of these compounds can be strongly modified by chemical substitution, epitaxial strain, pressure, and temperature.

The mineral calcium titanate (CaTiO3) is the archetypal perovskite and gives its name to the structure. Other common perovskite titanates include strontium titanate (SrTiO3), barium titanate (BaTiO3), lead titanate (PbTiO3), and europium titanate (EuTiO3).

Ferroelectric titanates

Several perovskite titanates exhibit ferroelectricity, in which a spontaneous electric polarization can be reversed by an applied electric field. Barium titanate undergoes a series of structural phase transitions upon cooling and is one of the most widely used ferroelectric ceramics in multilayer ceramic capacitors, actuators, and piezoelectric devices. Lead titanate possesses a larger spontaneous polarization and forms the basis of many technologically important solid solutions, including lead zirconate titanate (PZT).

Quantum paraelectrics

Strontium titanate is a quantum paraelectric that remains close to a ferroelectric instability down to absolute zero because of quantum fluctuations. It is widely used as a substrate for the epitaxial growth of oxide thin films and exhibits superconductivity when chemically doped or when oxygen deficient. Interfaces between SrTiO3 and other oxides, particularly lanthanum aluminate, have become model systems for studying two-dimensional electron gases and other emergent electronic phenomena.

Magnetic and correlated titanates

Rare-earth titanates such as gadolinium titanate, samarium titanate, and yttrium titanate are Mott insulators whose magnetic and electronic properties arise from strong electron correlations. Other titanates, including europium titanate, display strong coupling between their magnetic, dielectric, and structural properties and are studied as candidate multiferroic materials.

Applications

Perovskite titanates are used in capacitors, piezoelectric transducers, microwave resonators, electro-optic devices, thermistors, oxide electronics, and as substrates for the epitaxial growth of functional oxide thin films. They also serve as model materials in condensed matter physics for investigating ferroelectricity, superconductivity, metal–insulator transitions, and strongly correlated electron systems.

Complex titanates

More complex titanates are also known, such as bismuth titanate, Bi4Ti3O12.

References

  1. 1 2 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.