Germanium dioxide, also called germanium oxide and germania, is an inorganic compound with the chemical formula Ge O₂. 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 germanic acid, H₄GeO₄ or Ge(OH)₄. GeO₂ is only slightly soluble in acid but dissolves more readily in alkali to give germanates.

...
Wikipedia