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Nickel forms a series of mixed oxide compounds which are commonly called nickelates. A nickelate is an anion containing nickel or a salt containing a nickelate anion, or a double compound containing nickel bound to oxygen and other elements. Nickel can be in different or even mixed oxidation states, ranging from +1, +2, +3 to +4. The anions can contain a single nickel ion, or multiple to form a cluster ion. The solid mixed oxide compounds are often ceramics, but can also be metallic. They have a variety of electrical and magnetic properties.
Rare-earth element The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...
s form a range of perovskite nickelates, in which the properties vary systematically as the rare-earth element changes. Fine tuning of properties is achievable with mixtures of elements, applying stress or pressure, or varying the physical form. Inorganic chemists call many compounds that contain nickel centred anions "nickelates". These include the chloronickelates, fluoronickelates,
tetrabromonickelate The tetrabromonickelate anion contains a doubly-charged nickel atom (Ni2+) surrounded by four bromide ions in a tetrahedral arrangement. The formula is iBr4sup>2−. The anion combines with cations to form a series of salts called tetrabromonicke ...
s,
tetraiodonickelate Tetraiodonickelate is a complex ion of nickel with four iodide atoms iI4− arranged in a tetrahedron. iI4− is red in solution. This colour is due to absorption around 530 nm and below 450 nm. Maximum light transmission is aroun ...
s,
cyanonickelate The cyanonickelates are a class of chemical compound containing anions consisting of nickel atoms, and cyanide groups. The most important of these are the tetracyanonickelates containing four cyanide groups per nickel. The tetracyanonickelates c ...
s, nitronickelates and other nickel-organic acid complexes such as oxalatonickelates.


Alkali nickelates

The lithium nickelates are of interest to researchers as cathodes in
lithium cell Lithium battery may refer to: * Lithium metal battery, a non-rechargeable battery with lithium as an anode ** Rechargeable lithium metal battery, a rechargeable counterpart to the lithium metal battery * Lithium-ion battery, a rechargeable batte ...
s, as these substance can hold a variable amount of lithium, with the nickel varying in oxidation state.


Rare-earth nickelates

Rare-earth nickelates with nickel in a +1 oxidation state have an electronic configuration to same as for
cuprate Cuprate loosely refers to a material that can be viewed as containing anionic copper complexes. Examples include tetrachloridocuprate ( uCl4sup>2−), the superconductor YBa2Cu3O7, and the organocuprates (e.g., dimethylcuprate u(CH3)2sup>∠...
s and so are of interest to
high-temperature superconductor High-temperature superconductors (abbreviated high-c or HTS) are defined as materials that behave as superconductors at temperatures above , the boiling point of liquid nitrogen. The adjective "high temperature" is only in respect to previ ...
researchers. Other rare-earth nickelates can function as
fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...
catalysts. The ability to switch between an insulating and a conducting state in some of these materials is of interest in the development of new transistors, that have higher on to off current ratios. The rare-earth nickelates were first made by Demazeau et al. in 1971, by heating a mixture of oxides under high pressure oxygen, or potassium perchlorate. However they were unable to make the cerium, praseodymium, and terbium nickelates. This may be because Ce, Pr and Tb oxidises to 4+ions in those conditions. For two decades after that no one paid attention to them. Many rare-earth nickelates have the Ruddlesden–Popper phase structure.


List of oxides


See also

* Nickel manganese oxides for what are considered nickel
manganate In inorganic nomenclature, a manganate is any negatively charged molecular entity with manganese as the central atom.. However, the name is usually used to refer to the tetraoxidomanganate(2−) anion, MnO, also known as manganate(VI) because it c ...
s


References

{{nickel compounds Nickel compounds Transition metal oxyanions Oxometallates