The term is usually applied to solid ionic compounds that contain the oxide anionO2− and two or more element cations. Typical examples are ilmenite (FeTiO3), a mixed oxide of iron (Fe2+) and titanium (Ti4+) cations, perovskite and garnet.The cations may be the same element in different ionization states: a notable example is magnetiteFe3O4, which is also known as ferrosoferric oxide , contains the cations Fe2+ ("ferrous" iron) and Fe3+ ("ferric" iron) in 1:2 ratio. Other notable examples include red leadPb3O4, the ferrites,[2] and the yttrium aluminum garnetY3Al5O12,[3] used in lasers.
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 oxyanions. In sodium zincateNa2ZnO2, for example, the oxygens are bound to the zinc atoms forming zincate anions.[4] (On the other hand, strontium titanateSrTiO3, despite its name, contains Ti4+ cations and not the TiO2−3 anion.)
Sometimes the term is applied loosely to solid solutions of metal oxides rather than chemical compounds, or to fine mixtures of two or more oxides.