Mixed oxide

In chemistry, a mixed oxide is a somewhat informal name for an oxide that contains more than one chemical element in its cation, or of a single element cation that has atoms in several states of oxidation.

The term is usually applied to solid ionic compounds that contain the oxide anion O²⁻ and two or more element cations. A typical example is the mineral ilmenite (FeTiO₃), a mixed oxide of iron (Fe²⁺) and titanium (Ti⁴⁺) cations. The cations may be the same element in different ionization states: a notable example is magnetite Fe₃O₄, which contains the cations Fe²⁺ ("ferrous" iron) and Fe³⁺ ("ferric" iron) in 1:2 ratio. Other notable examples include the ferrites, strontium titanate SrTiO₃ (which, despite its name, contains Ti⁴⁺ cations and not the TiO₃^2- anion), yttrium aluminum garnet Y₃Al₅O₁₂, and many more.

Mixed oxides are intermediate between a metal oxide and a metal salt. Sometimes mixed oxides are the salts of weak metallic acids. Most other times they are just two different oxides that bond together strongly.

However, the term is sometimes also applied to compounds of oxygen and two or more other elements, where some or all of the oxygen atoms are covalently bound into oxoanions.; or to fine mixtures of two or more oxides. An example would be the zincates. Zinc hydroxide can react with concentrated sodium hydroxide to become sodium zincate. This contains zincate ions.

Mixed oxide minerals are plentiful in nature. Synthetic mixed oxides are components of many ceramics with remarkable properties and important advanced technological applications, such as strong magnets, fine optics, lasers, semiconductors, piezoelectrics, superconductors, catalysts, refractories, gas mantles, nuclear fuels, and more. Piezoelectric mixed oxides, in particular, are extensively used in pressure and strain gauges, microphones, ultrasound transducers, micromanipulators, delay lines, etc..