Properties

The second structure is called fluorite, after the mineral calcium fluoride (CaF2), which possesses this structure. Here the oxygen anions are bonded to only four cations. Oxides with this structure are well known for the ease with which oxygen vacancies can be formed. In zirconia (zirconium dioxide, ZrO2), which also possesses this structure, a great number of vacancies can be formed by doping, or carefully inserting ions of a different element into the composition. These vacancies become mobile at high temperatures, imparting oxygen-ion conductivity to the material and making it useful in certain electrical applications. The fluorite structure also exhibits considerable open space, especially at the centre of the unit cell. In urania, which is used as a fuel element in nuclear reactors, this openness is believed to help accommodate fission products and reduce unwanted swelling.

The third structure is called perovskite. In most cases the perovskite structure is cubic; i.e., all sides of the unit cell are the same. However, in barium titanate (BaTiO3) the central Ti4+ cation can be induced to move off-centre, leading to a noncubic symmetry and to an electrostatic dipole, or alignment of positive and negative charges toward opposite ends of the structure. This dipole is responsible for the ferroelectric properties of barium titanate, in which domains of neighbouring dipoles line up in the same direction. The enormous dielectric constants achievable with perovskite materials are the basis of many ceramic capacitor devices.

The noncubic variations found in perovskite ceramics introduce the concept of anistropy; i.e., an ionic arrangement that is not identical in all directions. In severely anisotropic materials there can be great variation of properties. These cases are illustrated by yttrium barium copper oxide (YBCO; chemical formula YBa2Cu3O7). YBCO is a superconducting ceramic; that is, it loses all resistance to electric current at extremely low temperatures. Its structure consists of three cubes, with yttrium or barium at the centre, copper at the corners, and oxygen at the middle of each edge--with the exception of the middle cube, which has oxygen vacancies at the outer edges. The critical feature in this structure is the presence of two sheets of copper-oxygen ions, located above and below the oxygen vacancies, along which superconduction takes place. The transport of electrons perpendicular to these sheets is not favoured, making the YBCO structure severely anisotropic. (One of the challenges in fabricating crystalline YBCO ceramics capable of passing large currents is to align all the grains in such a manner that their copper-oxygen sheets line up.)

Ordinarily, ceramics are poor conductors of electricity and therefore make excellent insulators. Nonconductivity arises from the lack of "free" electrons such as those found in metals. In ionically bonded ceramics, bonding electrons are accepted by the electronegative elements, such as oxygen, and donated by the electropositive elements, usually a metal. The result is that all electrons are tightly bound to the ions in the structure, leaving no free electrons to conduct electricity. In covalent bonding, bonding electrons are similarly localized in the directional orbitals between the atoms, and there are no free electrons to conduct electricity.

There are two ways that ceramics can be made electrically conductive. At sufficiently high temperatures point defects such as oxygen vacancies can arise, leading to ionic conductivity. (This is pointed out in the case of zirconia, above.) In addition, the introduction of certain transition-metal elements (such as iron, copper, manganese, or cobalt), lanthanide elements (such as cerium), or actinide elements (such as uranium) can produce special electronic states in which mobile electrons or electron holes arise. The copper-based superconductors are a good example of conductive transition-metal oxide ceramics - in this case, conductivity arising at extremely low temperatures.