Germanium




Germanium

MELTING POINT: 937.4°C
BOILING POINT: 2,830°C
DENSITY: 5.323 g/cm 3
MOST COMMON ION: Ge 4+ Ge 2+

Germanium has chemical and physical properties similar to those of silicon. It was predicted as an element ("eka-silicon") by Dimitri Mendeleev in 1871 from calculations made during the construction of his periodic table, and it was discovered by Clemens Winkler in 1886. It is 10,000 times less abundant than silicon, and it occurs as a trace element in silicate minerals (1–2 ppm). The element is usually obtained via the acid leaching of mineral dusts, especially certain coals and zinc ores (using H 2 SO 4 ) formed during ceramic processing, followed by addition of hydrochloric acid, to form the volatile germanium tetrachloride (GeCl 4 ), which is easily separated from coprecipitated zinc compounds. Its low abundance and complicated processing make germanium an expensive material: Although some of its semiconducting properties are superior to those of silicon, its use in the electronics industry is limited. Like silicon, germanium forms gaseous hydrides (e.g., GeH 4 ) and volatile halides (e.g., GeF 4 and GeCl 4 ).

Germanium is more reactive than silicon, and the chemistry of its 2+ valence state is better developed: Binary solids such as GeF 2 and GeCl 2 are known, in addition to the tetrahalides. The monoxide GeO (a dark brown solid) occurs, although the dioxide, GeO 2 , is more stable. Germanium dioxide has octahedrally coordinated germanium atoms; the tetrahedrally coordinated quartz phase is stable only at high temperature, although it exists metastably at ambient conditions. This contrasts with SiO 2 : The corresponding octahedrally coordinated compound isostructural with the mineral rutile (TiO 2 ) form is prepared only at high pressure (>90,000 atmospheres). High pressure treatment of germanium nitride, Ge 3 N 4 , gives (as with like treatment of silicon nitride) a new high-density form with the same structure as the mineral spinel (MgAl 2 O 4 ), containing octahedrally coordinated (GeN 6 ) groups. Like silicon, germanium forms "Zintl" phases with electropositive metals (e.g., KGe, BaGe 2 ) that contain unusual polyanions with Ge in negative oxidation states, and "semiconductor clathrates" (e.g., Na 8 Ge 44 ; Na 16 Cs 8 Ge 136 ) with open framework structures constructed from Ge atoms in tetrahedral coordination that are structurally analogous with H 2 O-ice clathrates, and with alkali or alkaline earth atoms occupying "cages" in the structure.

SEE ALSO Mendeleev, Dimitri ; Silicon .

Paul F. McMillan

Bibliography

Greenwood, Norman N., and Earnshaw, A. (1984). Chemistry of the Elements. New York: Pergamon Press.

Kauzlarich, Susan M., ed. (1996). Chemistry: Structure and Bonding of Zintl Phases and Ions. New York: VCH.

Stwertka, Albert (1998). A Guide to the Elements, revised edition. New York: Oxford University Press.



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