AB2X4

AB2X4 is a general formula for a family of compounds known as spinels. In many cases X is oxygen, yielding oxide spinels, but the formula also encompasses sulfides, selenides, and related chalcogenides. The spinel structure is a cubic, close-packed array of X anions with A cations occupying one-eighth of the tetrahedral interstices and B cations occupying half of the octahedral interstices. This arrangement gives the characteristic AB2X4 composition and a typical unit cell that contains eight formula units.

Normal and inverse spinels describe how the A and B cations are distributed among the interstitial sites. In a normal spinel, A2+ cations sit in tetrahedral sites and B3+ cations fill octahedral sites. In an inverse spinel, B cations occupy the tetrahedral sites and the octahedral sites are shared by A and B cations in a 1:1 ratio. The degree of inversion, describing the proportion of cations in tetrahedral versus octahedral positions, influences physical properties such as magnetism and conductivity.

Common oxide spinels include MgAl2O4 and ZnAl2O4 (often used as refractory ceramics), and Fe3O4 (magnetite), MnFe2O4, or CoFe2O4 (ferrites) which exhibit ferrimagnetic behavior. Sulfide and selenide spinels also exist and can display distinct electronic or optical properties.

Properties of AB2X4 materials vary with composition and cation distribution but often feature high chemical and thermal stability, wide band gaps in oxides, and, in ferrites, notable magnetic behavior. Applications span ceramics, catalysts, pigments, and electronic or magnetic devices. Synthesis typically involves high-temperature solid-state reactions, with alternatives including sol-gel, hydrothermal methods, or flux growth to tailor particle size and inversion degree.