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Germanium
Chemical element with atomic number 32

Germanium is a lustrous, grayish-white chemical element with the symbol Ge and atomic number 32, classified as a metalloid in the carbon group. Chemically similar to silicon, it naturally reacts with oxygen. Predicted by Dmitri Mendeleev and discovered in 1886 by Clemens Winkler, germanium is mainly mined from sphalerite. Its semiconductor properties made it vital in early transistors, and it is now used in fibre-optic systems, solar cells, and LEDs. Although not essential to living organisms, some synthetic germanium compounds can be toxic or irritating.

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History

In his report on The Periodic Law of the Chemical Elements in 1869, the Russian chemist Dmitri Mendeleev predicted the existence of several unknown chemical elements, including one that would fill a gap in the carbon family, located between silicon and tin.1 Because of its position in his periodic table, Mendeleev called it ekasilicon (Es), and he estimated its atomic weight to be 70 (later 72).

In mid-1885, at a mine near Freiberg, Saxony, a new mineral was discovered and named argyrodite because of its high silver content.2 The chemist Clemens Winkler analyzed this new mineral, which proved to be a combination of silver, sulfur, and a new element. Winkler was able to isolate the new element in 1886 and found it similar to antimony. He initially considered the new element to be eka-antimony, but was soon convinced that it was instead eka-silicon.34 Before Winkler published his results on the new element, he decided that he would name his element neptunium, since the recent discovery of planet Neptune in 1846 had similarly been preceded by mathematical predictions of its existence.5 However, the name "neptunium" had already been given to another proposed chemical element (though not the element that today bears the name neptunium, which was discovered in 1940).6 So instead, Winkler named the new element germanium (from the Latin word, Germania, for Germany) in honor of his homeland.7 Argyrodite proved empirically to be Ag8GeS6. Because this new element showed some similarities with the elements arsenic and antimony, its proper place in the periodic table was under consideration, but its similarities with Dmitri Mendeleev's predicted element "ekasilicon" confirmed that place on the periodic table.89 With further material from 500 kg of ore from the mines in Saxony, Winkler confirmed the chemical properties of the new element in 1887.101112 He also determined an atomic weight of 72.32 by analyzing pure germanium tetrachloride (GeCl4), while Lecoq de Boisbaudran deduced 72.3 by a comparison of the lines in the spark spectrum of the element.13

Winkler was able to prepare several new compounds of germanium, including fluorides, chlorides, sulfides, dioxide, and tetraethylgermane (Ge(C2H5)4), the first organogermane.14 The physical data from those compounds—which corresponded well with Mendeleev's predictions—made the discovery an important confirmation of Mendeleev's idea of element periodicity. Here is a comparison between the prediction and Winkler's data:15

PropertyEkasiliconMendeleevprediction (1871)GermaniumWinklerdiscovery (1887)
atomic mass72.6472.63
density (g/cm3)5.55.35
melting point (°C)high947
colorgraygray
oxide typerefractory dioxiderefractory dioxide
oxide density (g/cm3)4.74.7
oxide activityfeebly basicfeebly basic
chloride boiling point (°C)under 10086 (GeCl4)
chloride density (g/cm3)1.91.9

Until the late 1930s, germanium was thought to be a poorly conducting metal.16 Germanium did not become economically significant until after 1945 when its properties as an electronic semiconductor were recognized. During World War II, small amounts of germanium were used in some special electronic devices, mostly diodes.1718 The first major use was the point-contact Schottky diodes for radar pulse detection during the War.19 The first silicon–germanium alloys were obtained in 1955.20 Before 1945, only a few hundred kilograms of germanium were produced in smelters each year, but by the end of the 1950s, the annual worldwide production had reached 40 metric tons (44 short tons).21

The development of the germanium transistor in 194822 opened the door to countless applications of solid state electronics.23 From 1950 through the early 1970s, this area provided an increasing market for germanium, but then high-purity silicon began replacing germanium in transistors, diodes, and rectifiers.24 For example, the company that became Fairchild Semiconductor was founded in 1957 with the express purpose of producing silicon transistors. Silicon has superior electrical properties, but it requires much greater purity that could not be commercially achieved in the early years of semiconductor electronics.25

Meanwhile, the demand for germanium for fiber optic communication networks, infrared night vision systems, and polymerization catalysts increased dramatically.26 These end uses represented 85% of worldwide germanium consumption in 2000.27 The US government even designated germanium as a strategic and critical material, calling for a 146 ton (132 tonne) supply in the national defense stockpile in 1987.28

Germanium differs from silicon in that the supply is limited by the availability of exploitable sources, while the supply of silicon is limited only by production capacity since silicon comes from ordinary sand and quartz. While silicon could be bought in 1998 for less than $10 per kg,29 the price of germanium was almost $800 per kg.30

Characteristics

Under standard conditions, germanium is a brittle, silvery-white,31 semiconductor. This form constitutes an allotrope known as α-germanium, which has a metallic luster and a diamond cubic crystal structure, the same structure as silicon and diamond.32 In this form, germanium has a threshold displacement energy of 19.7 − 0.5 + 0.6   eV {\displaystyle 19.7_{-0.5}^{+0.6}~{\text{eV}}} .33 At pressures above 120 kbar, germanium becomes the metallic allotrope β-germanium with the same structure as β-tin.34 Like silicon, gallium, bismuth, antimony, and water, germanium is one of the few substances that expands as it solidifies (i.e. freezes) from the molten state.35

Germanium is a semiconductor having an indirect bandgap, as is crystalline silicon. Zone refining techniques have led to the production of crystalline germanium for semiconductors that has an impurity of only one part in 1010,36 making it one of the purest materials ever obtained.37 The first semi-metallic material discovered (in 2005) to become a superconductor in the presence of an extremely strong electromagnetic field was an alloy of germanium, uranium, and rhodium.38

Pure germanium is known to spontaneously extrude very long screw dislocations, referred to as germanium whiskers. The growth of these whiskers is one of the primary reasons for the failure of older diodes and transistors made from germanium, as, depending on what they eventually touch, they may lead to an electrical short.39

Chemistry

Main article: Germanium compounds

Elemental germanium starts to oxidize slowly in air at around 250 °C, forming GeO2 .40 Germanium is insoluble in dilute acids and alkalis but dissolves slowly in hot concentrated sulfuric and nitric acids and reacts violently with molten alkalis to produce germanates ([GeO3]2−). Germanium occurs mostly in the oxidation state +4 although many +2 compounds are known.41 Other oxidation states are rare: +3 is found in compounds such as Ge2Cl6, and +3 and +1 are found on the surface of oxides,42 or negative oxidation states in germanides, such as −4 in Mg2Ge. Germanium cluster anions (Zintl ions) such as Ge42−, Ge94−, Ge92−, [(Ge9)2]6− have been prepared by the extraction from alloys containing alkali metals and germanium in liquid ammonia in the presence of ethylenediamine or a cryptand.4344 The oxidation states of the element in these ions are not integers—similar to the ozonides O3−.

Two oxides of germanium are known: germanium dioxide (GeO2, germania) and germanium monoxide, (GeO).45 The dioxide, GeO2, can be obtained by roasting germanium disulfide (GeS2), and is a white powder that is only slightly soluble in water but reacts with alkalis to form germanates.46 The monoxide, germanous oxide, can be obtained by the high temperature reaction of GeO2 with elemental Ge.47 The dioxide (and the related oxides and germanates) exhibits the unusual property of having a high refractive index for visible light, but transparency to infrared light.4849 Bismuth germanate, Bi4Ge3O12 (BGO), is used as a scintillator.50

Binary compounds with other chalcogens are also known, such as the disulfide (GeS2) and diselenide (GeSe2), and the monosulfide (GeS), monoselenide (GeSe), and monotelluride (GeTe).51 GeS2 forms as a white precipitate when hydrogen sulfide is passed through strongly acid solutions containing Ge(IV).52 The disulfide is appreciably soluble in water and in solutions of caustic alkalis or alkaline sulfides. Nevertheless, it is not soluble in acidic water, which allowed Winkler to discover the element.53 By heating the disulfide in a current of hydrogen, the monosulfide (GeS) is formed, which sublimes in thin plates of a dark color and metallic luster, and is soluble in solutions of the caustic alkalis.54 Upon melting with alkaline carbonates and sulfur, germanium compounds form salts known as thiogermanates.55

Four tetrahalides are known. Under normal conditions germanium tetraiodide (GeI4) is a solid, germanium tetrafluoride (GeF4) a gas and the others volatile liquids. For example, germanium tetrachloride, GeCl4, is obtained as a colorless fuming liquid boiling at 83.1 °C by heating the metal with chlorine.56 All the tetrahalides are readily hydrolyzed to hydrated germanium dioxide.57 GeCl4 is used in the production of organogermanium compounds.58 All four dihalides are known and in contrast to the tetrahalides are polymeric solids.59 Additionally Ge2Cl6 and some higher compounds of formula GenCl2n+2 are known.60 The unusual compound Ge6Cl16 has been prepared that contains the Ge5Cl12 unit with a neopentane structure.61

Germane (GeH4) is a compound similar in structure to methane. Polygermanes—compounds that are similar to alkanes—with formula GenH2n+2 containing up to five germanium atoms are known.62 The germanes are less volatile and less reactive than their corresponding silicon analogues.63 GeH4 reacts with alkali metals in liquid ammonia to form white crystalline MGeH3 which contain the GeH3− anion.64 The germanium hydrohalides with one, two and three halogen atoms are colorless reactive liquids.65

The first organogermanium compound was synthesized by Winkler in 1887; the reaction of germanium tetrachloride with diethylzinc yielded tetraethylgermane (Ge(C2H5)4).66 Organogermanes of the type R4Ge (where R is an alkyl) such as tetramethylgermane (Ge(CH3)4) and tetraethylgermane are accessed through the cheapest available germanium precursor germanium tetrachloride and alkyl nucleophiles. Organic germanium hydrides such as isobutylgermane ((CH3)2CHCH2GeH3) were found to be less hazardous and may be used as a liquid substitute for toxic germane gas in semiconductor applications. Many germanium reactive intermediates are known: germyl free radicals, germylenes (similar to carbenes), and germynes (similar to carbynes).6768 The organogermanium compound 2-carboxyethylgermasesquioxane was first reported in the 1970s, and for a while was used as a dietary supplement and thought to possibly have anti-tumor qualities.69

Using a ligand called Eind (1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl) germanium is able to form a double bond with oxygen (germanone). Germanium hydride and germanium tetrahydride are very flammable and even explosive when mixed with air.70

Isotopes

Main article: Isotopes of germanium

Germanium occurs in five natural isotopes: 70Ge, 72Ge, 73Ge, 74Ge, and 76Ge. Of these, 76Ge is very slightly radioactive, decaying by double beta decay with a half-life of 1.78×1021 years. 74Ge is the most common isotope, having a natural abundance of approximately 36%. 76Ge is the least common with a natural abundance of approximately 7%.71 When bombarded with alpha particles, the isotope 72Ge will generate stable 77Se, releasing high energy electrons in the process.72 Because of this, it is used in combination with radon for nuclear batteries.73

At least 27 radioisotopes have also been synthesized, ranging in atomic mass from 58 to 89. The most stable of these is 68Ge, decaying by electron capture with a half-life of 270.95 days. The least stable is 60Ge, with a half-life of 30 ms. While most of germanium's radioisotopes decay by beta decay, 61Ge and 64Ge decay by β+ delayed proton emission.74 84Ge through 87Ge isotopes also exhibit minor β− delayed neutron emission decay paths.75

Occurrence

See also: Category:Germanium minerals

Germanium is created by stellar nucleosynthesis, mostly by the s-process in asymptotic giant branch stars. The s-process is a slow neutron capture of lighter elements inside pulsating red giant stars.76 Germanium has been detected in some of the most distant stars77 and in the atmosphere of Jupiter.78

Germanium's abundance in the Earth's crust is approximately 1.6 ppm.79 Only a few minerals like argyrodite, briartite, germanite, renierite and sphalerite contain appreciable amounts of germanium.8081 Only few of them (especially germanite) are, very rarely, found in mineable amounts.828384 Some zinc–copper–lead ore bodies contain enough germanium to justify extraction from the final ore concentrate.85 An unusual natural enrichment process causes a high content of germanium in some coal seams, discovered by Victor Moritz Goldschmidt during a broad survey for germanium deposits.8687 The highest concentration ever found was in Hartley coal ash with as much as 1.6% germanium.8889 The coal deposits near Xilinhaote, Inner Mongolia, contain an estimated 1600 tonnes of germanium.90

Production

About 118 tonnes of germanium were produced in 2011 worldwide, mostly in China (80 t), Russia (5 t) and United States (3 t).91 Germanium is recovered as a by-product from sphalerite zinc ores where it is concentrated in amounts as great as 0.3%,92 especially from low-temperature sediment-hosted, massive Zn–Pb–Cu(–Ba) deposits and carbonate-hosted Zn–Pb deposits.93 A recent study found that at least 10,000 t of extractable germanium is contained in known zinc reserves, particularly those hosted by Mississippi-Valley type deposits, while at least 112,000 t will be found in coal reserves.94 In 2007 35% of the demand was met by recycled germanium.95

YearCost($/kg)96
19991,400
20001,250
2001890
2002620
2003380
2004600
2005660
2006880
20071,240
20081,490
2009950
2010940
20111,625
20121,680
20131,875
20141,900
20151,760
2016950
20171,358
20181,300
20191,240
20201,000

While it is produced mainly from sphalerite, it is also found in silver, lead, and copper ores. Another source of germanium is fly ash of power plants fueled from coal deposits that contain germanium. Russia and China used this as a source for germanium.97 Russia's deposits are located in the far east of Sakhalin Island, and northeast of Vladivostok. The deposits in China are located mainly in the lignite mines near Lincang, Yunnan; coal is also mined near Xilinhaote, Inner Mongolia.98

The ore concentrates are mostly sulfidic; they are converted to the oxides by heating under air in a process known as roasting:

GeS2 + 3 O2 → GeO2 + 2 SO2

Some of the germanium is left in the dust produced, while the rest is converted to germanates, which are then leached (together with zinc) from the cinder by sulfuric acid. After neutralization, only the zinc stays in solution while germanium and other metals precipitate. After removing some of the zinc in the precipitate by the Waelz process, the residing Waelz oxide is leached a second time. The dioxide is obtained as precipitate and converted with chlorine gas or hydrochloric acid to germanium tetrachloride, which has a low boiling point and can be isolated by distillation:99

GeO2 + 4 HCl → GeCl4 + 2 H2O GeO2 + 2 Cl2 → GeCl4 + O2

Germanium tetrachloride is either hydrolyzed to the oxide (GeO2) or purified by fractional distillation and then hydrolyzed.100 The highly pure GeO2 is now suitable for the production of germanium glass. It is reduced to the element by reacting it with hydrogen, producing germanium suitable for infrared optics and semiconductor production:

GeO2 + 2 H2 → Ge + 2 H2O

The germanium for steel production and other industrial processes is normally reduced using carbon:101

GeO2 + C → Ge + CO2

Applications

The major global end uses for germanium were electronics and solar applications, fiber-optic systems, infrared optics, and polymerization catalysts. Other uses included chemotherapy, metallurgy, and phosphors.102

Optics

The notable properties of germania (GeO2) are its high index of refraction and its low optical dispersion. These make it especially useful for wide-angle camera lenses, microscopy, and the core part of optical fibers.103104 It has replaced titania as the dopant for silica fiber, eliminating the subsequent heat treatment that made the fibers brittle.105 At the end of 2002, the fiber optics industry consumed 60% of the annual germanium use in the United States, but this is less than 10% of worldwide consumption.106 GeSbTe is a phase change material used for its optic properties, such as that used in rewritable DVDs.107

Because germanium is transparent in the infrared wavelengths, it is an important infrared optical material that can be readily cut and polished into lenses and windows. It is especially used as the front optic in thermal imaging cameras working in the 8 to 14 micron range for passive thermal imaging and for hot-spot detection in military, mobile night vision, and fire fighting applications.108 It is used in infrared spectroscopes and other optical equipment that require extremely sensitive infrared detectors.109 It has a very high refractive index (4.0) and must be coated with anti-reflection agents. Particularly, a very hard special antireflection coating of diamond-like carbon (DLC), refractive index 2.0, is a good match and produces a diamond-hard surface that can withstand much environmental abuse.110111

Electronics

Germanium can be alloyed with silicon, and silicon–germanium alloys are rapidly becoming an important semiconductor material for high-speed integrated circuits. Circuits using the properties of Si-SiGe heterojunctions can be much faster than those using silicon alone.112 The SiGe chips, with high-speed properties, can be made with low-cost, well-established production techniques of the silicon chip industry.113

High efficiency solar panels are a major use of germanium. Because germanium and gallium arsenide have nearly identical lattice constant, germanium substrates can be used to make gallium-arsenide solar cells.114 Germanium is the substrate of the wafers for high-efficiency multijunction photovoltaic cells for space applications, such as the Mars Exploration Rovers, which use triple-junction gallium arsenide on germanium cells.115 High-brightness LEDs, used for automobile headlights and to backlight LCD screens, are also an important application.116

Germanium-on-insulator (GeOI) substrates are seen as a potential replacement for silicon on miniaturized chips.117 CMOS circuit based on GeOI substrates has been reported recently.118 Other uses in electronics include phosphors in fluorescent lamps119 and solid-state light-emitting diodes (LEDs).120 Germanium transistors are still used in some effects pedals by musicians who wish to reproduce the distinctive tonal character of the "fuzz"-tone from the early rock and roll era, most notably the Dallas Arbiter Fuzz Face.121

Germanium has been studied as a potential material for implantable bioelectronic sensors that are resorbed in the body without generating harmful hydrogen gas, replacing zinc oxide- and indium gallium zinc oxide-based implementations.122

Germanium was also used to create many of the circuits found in some of the very first pieces of electronic musical gear, initially 1950s, primarily in early transistor-based circuits. The first guitar effects pedals in the 1960s – Fuzz pedals like the Maestro FZ-1 (1962), Dallas-Arbiter Fuzz Face (1966), and Tone Bender (1965) - used germanium transistors.123 Silicon diodes are more frequently used in more modern equipment, but germanium diodes are still used in some applications as they have lower barrier potential and smoother transconductance curves, leading to less harsh clipping.124

Other uses

Germanium dioxide is also used in catalysts for polymerization in the production of polyethylene terephthalate (PET).125 The high brilliance of this polyester is especially favored for PET bottles marketed in Japan.126 In the United States, germanium is not used for polymerization catalysts.127

Due to the similarity between silica (SiO2) and germanium dioxide (GeO2), the silica stationary phase in some gas chromatography columns can be replaced by GeO2.128

In recent years germanium has seen increasing use in precious metal alloys. In sterling silver alloys, for instance, it reduces firescale, increases tarnish resistance, and improves precipitation hardening. A tarnish-proof silver alloy trademarked Argentium contains 1.2% germanium.129

Semiconductor detectors made of single crystal high-purity germanium can precisely identify radiation sources—for example in airport security.130 Germanium is useful for monochromators for beamlines used in single crystal neutron scattering and synchrotron X-ray diffraction. The reflectivity has advantages over silicon in neutron and high energy X-ray applications.131 Crystals of high purity germanium are used in detectors for gamma spectroscopy and the search for dark matter.132 Germanium crystals are also used in X-ray spectrometers for the determination of phosphorus, chlorine and sulfur.133

Germanium is emerging as an important material for spintronics and spin-based quantum computing applications. In 2010, researchers demonstrated room temperature spin transport134 and more recently donor electron spins in germanium has been shown to have very long coherence times.135

Germanium and health

Germanium is not considered essential to the health of plants or animals.136 Germanium in the environment has little or no health impact. This is primarily because it usually occurs only as a trace element in ores and carbonaceous materials, and the various industrial and electronic applications involve very small quantities that are not likely to be ingested.137 For similar reasons, end-use germanium has little impact on the environment as a biohazard. Some reactive intermediate compounds of germanium are poisonous (see precautions, below).138

Germanium supplements, made from both organic and inorganic germanium, have been marketed as an alternative medicine capable of treating leukemia and lung cancer.139 There is, however, no medical evidence of benefit; some evidence suggests that such supplements are actively harmful.140 U.S. Food and Drug Administration (FDA) research has concluded that inorganic germanium, when used as a nutritional supplement, "presents potential human health hazard".141

Some germanium compounds have been administered by alternative medical practitioners as non-FDA-allowed injectable solutions. Soluble inorganic forms of germanium used at first, notably the citrate-lactate salt, resulted in some cases of renal dysfunction, hepatic steatosis, and peripheral neuropathy in individuals using them over a long term. Plasma and urine germanium concentrations in these individuals, several of whom died, were several orders of magnitude greater than endogenous levels. A more recent organic form, beta-carboxyethylgermanium sesquioxide (propagermanium), has not exhibited the same spectrum of toxic effects.142

Certain compounds of germanium have low toxicity to mammals, but have toxic effects against certain bacteria.143

Precautions for chemically reactive germanium compounds

While use of germanium itself does not require precautions, some of germanium's artificially produced compounds are quite reactive and present an immediate hazard to human health on exposure. For example, Germanium tetrachloride and germane (GeH4) are a liquid and gas, respectively, that can be very irritating to the eyes, skin, lungs, and throat.144

See also

Notes

Wikisource has the text of the 1911 Encyclopædia Britannica article "Germanium".

References

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  2. From Greek, argyrodite means silver-containing.[12] ↩

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  5. Just as the existence of the new element had been predicted, the existence of the planet Neptune had been predicted in about 1843 by the two mathematicians John Couch Adams and Urbain Le Verrier, using the calculation methods of celestial mechanics. They did this in attempts to explain the fact that the planet Uranus, upon very close observation, appeared to be being pulled slightly out of position in the sky.[15] James Challis started searching for it in July 1846, and he sighted this planet on September 23, 1846.[16] /wiki/Neptune ↩

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  11. Winkler, Clemens (1887). "Germanium, Ge, a New Nonmetal Element". Berichte der Deutschen Chemischen Gesellschaft (in German). 19 (1): 210–211. doi:10.1002/cber.18860190156. Archived from the original on December 7, 2008. /wiki/Clemens_Winkler ↩

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