Germanium dioxide

Germanium dioxide, also called germanium(IV) oxide, germania, and salt of germanium, is an inorganic compound with the chemical formula GeO2. It is the main commercial source of germanium. It also forms as a passivation layer on pure germanium in contact with atmospheric oxygen. The two predominant polymorphs of GeO2 are hexagonal and tetragonal. Hexagonal GeO2 has the same structure as β-quartz, with germanium having coordination number 4. Tetragonal GeO2 (the mineral argutite) has the rutile-like structure seen in stishovite. In this motif, germanium has the coordination number 6. An amorphous (glassy) form of GeO2 is similar to fused silica. Germanium dioxide can be prepared in both crystalline and amorphous forms. At ambient pressure the amorphous structure is formed by a network of GeO4 tetrahedra. At elevated pressure up to approximately 9 GPa the germanium average coordination number steadily increases from 4 to around 5 with a corresponding increase in the Ge–O bond distance. At higher pressures, up to approximately 15 GPa, the germanium coordination number increases to 6, and the dense network structure is composed of GeO6 octahedra. When the pressure is subsequently reduced, the structure reverts to the tetrahedral form. At high pressure, the rutile form converts to an orthorhombic CaCl2 form. Heating germanium dioxide with powdered germanium at 1000 °C forms germanium monoxide (GeO). The hexagonal (d = 4.29 g/cm3) form of germanium dioxide is more soluble than the rutile (d = 6.27 g/cm3) form and dissolves to form acid, H4GeO4, or Ge(OH)4. GeO2 is only slightly soluble in acid but dissolves more readily in alkali to give germanates. The germanic acid forms stable complexes with di- and polyfunctional carboxylic acids, poly-alcohols, and o-diphenols. In contact with hydrochloric acid, it releases the volatile and corrosive germanium tetrachloride. The refractive index (1.7) and optical dispersion properties of germanium dioxide make it useful as an optical material for wide-angle lenses, in optical microscope objective lenses, and for the core of fiber-optic lines.