Mixed oxide

For nuclear reactor fuel, see MOX fuel.

In chemistry, a mixed oxide is a somewhat informal name for an oxide that contains cations of more than one chemical element or cations of a single element 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. Typical examples are ilmenite (FeTiO₃), a mixed oxide of iron (Fe²⁺) and titanium (Ti⁴⁺) cations, the mineral perovskite and oxides sharing the perovskite structure and garnet. 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 TiO₃²⁻ anion), yttrium aluminum garnet Y₃Al₅O₁₂, and many more. Sometimes the term is applied loosely to solid solutions of metal oxides rather than chemical compounds

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..