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Germanium
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Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, greyish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. Germanium has five naturally occurring isotopes ranging in atomic mass number from 70 to 76. It forms a large number of organometallic compounds, including tetraethylgermane and isobutylgermane.
Because few minerals contain it in large concentration, germanium was discovered relatively late despite the fact that it is relatively abundant in the Earth's crust.
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Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, greyish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. Germanium has five naturally occurring isotopes ranging in atomic mass number from 70 to 76. It forms a large number of organometallic compounds, including tetraethylgermane and isobutylgermane.
Because few minerals contain it in large concentration, germanium was discovered relatively late despite the fact that it is relatively abundant in the Earth's crust. In 1869, Dmitri Mendeleev predicted its existence and some of its properties based on its position on his periodic table and called the element ekasilicon. Nearly two decades later, in 1886, Clemens Winkler found it in the mineral argyrodite. Winkler found that experimental observations agreed with Mendeleev's predictions and named the element after his country, Germany.
Germanium is an important semiconductor material used in transistors and various other electronic devices. Its major end uses are fiber-optic systems and infrared optics, but is also used for polymerization catalysts, in electronics and in solar electric applications. Germanium is mined primarily from sphalerite, though it is also recovered from silver, lead, and copper ores. Some germanium compounds, such as germanium chloride and germane, can irritate the eyes, skin, lungs, and throat.
History
In his report on The Periodic Law of the Chemical Elements, in 1869, Dmitri Mendeleev predicted the existence of several unknown elements, including one filling a gap in the carbon family, between silicon and tin. Because of its position in the table, he called it ekasilicon (Es) and assigned it an atomic weight of 72.
In the summer of 1885, in a mine near Freiberg, Saxony, a new mineral was found. It was named argyrodite, because of its high silver content. Clemens Winkler examined this new mineral and was able to isolate an element similar to antimony in 1886. Before he published his results on the new element Winkler intended to name the element neptunium, as the actual discovery of planet Neptune in 1846 had been preceded by mathematical prediction of its existence. However, the name neptunium had already been given to an element (though not the element that today bears the name neptunium, discovered in 1940), and instead, Winkler named the new metal germanium (from the Latin Germania for Germany) in honor of his fatherland. Because the element showed similarities with the elements arsenic and antimony, its place in the periodic table was under discussion, but the similarities between Mendeleev's ekasilicon and germanium confirmed its place. With further material from 500 kg of ore from the mines in Saxony, Winkler could confirm the chemical properties of the new element in 1887. He also determined an atomic weight of 72.32 by analyzing pure germanium tetrachloride , whilst Lecoq de Boisbaudran deduced 72.3 by a comparison of the lines in the spark spectrum of the element.
Winkler was able to prepare several new compounds of germanium, including the fluorides, chlorides, sulfides, germanium dioxide, and tetraethylgermanium, the first organogermane. With the physical data from these compounds—which corresponded with Mendeleev's predictions—it made the discovery an important confirmation of Mendeleev's idea of element periodicity. Here is a comparison between the prediction and Winkler's data:
Until the late 1930s, germanium was believed to be a poorly conducting metal. It did not become economically significant until after 1945, when its properties as a semiconductor were recognized as being valuable in electronics. It was only during World War II, in 1941, that germanium diodes began to supplant vacuum tubes in electronic devices. Its first major use was the point contact Schottky diodes for radar reception during WWII.
The first silicon-germanium alloys were obtained in 1955. Before 1945, only a few hundred kilograms of the element were produced each year, but by the end of the 1950s, annual worldwide production had reached 40 metric tons.
The development of the germanium transistor in 1948 opened the door to countless applications of solid state electronics. 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. Silicon has superior electrical properties, but requires much higher purity—a purity which could not be commercially achieved in the early days.
Meanwhile, demand for germanium in fiber optics communication networks, infrared night vision systems, and polymerization catalysts increased dramatically. These end uses represented 85% of worldwide germanium consumption for 2000. The U.S. government even designated germanium as a strategic and critical material, calling for a 146 tons (132 t) supply in the national defense stockpile in 1987. Germanium differs from silicon in that the supply of silicon is only limited by production capacity, while that for germanium is limited by the shortage of exploitable sources. As a result, while silicon could be bought in 1998 for less than $10 per kg, the price of 1 kg of germanium was then almost $800.
Characteristics
Under standard conditions germanium is a brittle, silvery-white, semi-metallic element. This form constitutes an allotrope technically known as a-germanium, which has a metallic luster and a diamond cubic crystal structure, the same as diamond. At pressures above 120 kbar, a different allotrope known as ß-germanium forms, which has the same structure as ß-tin. Along with silicon, gallium, bismuth, antimony, and water, it is one of the few substances that expands as it solidifies (i.e. freezes) from its molten state.
Germanium is a semiconductor. Zone refining techniques have led to the production of crystalline germanium for semiconductors that has an impurity of only one part in 1010, making it one of the purest materials ever obtained. The first metallic material becoming a superconductor in the presence of an extremely strong electromagnetic field was a alloy of germanium with uranium and rhodium. The discovery was made in 2005.
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.
Chemistry
Elemental germanium oxidizes slowly to GeO2 at 250 °C. Germanium is insoluble in dilute acids and alkalis but dissolves slowly in concentrated sulfuric acid and reacts violently with molten alkalis to produce germanates . Germanium occurs mostly in the oxidation state +4 although many compounds are known with the oxidation state of +2. Other oxidation states are rare, such as +3 found in compounds such as Ge2Cl6, and +3 and +1 observed on the surface of oxides, or negative oxidation states in germanes, such as -4 in . Germanium cluster anions, (Zintl ions), such as Ge42-, Ge94-, Ge92-, [(Ge9)2]6- have been prepared by the extraction from e.g. alloys containing alkali metals and germanium in liquid ammonia in the presence of ethylenediamine or a cryptand. 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 (germania) and germanium monoxide, . The dioxide, GeO2 can be obtained by roasting germanium sulfide , and is a white powder that is only slightly soluble in water but reacts with alkalis to form germanates. The monoxide, (germanous oxide), can be obtained by the high temperature reaction of GeO2 with Ge metal. 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. Bismuth germanate, Bi4Ge3O12, (BGO) is used as a scintillator.
Binary compounds with other chalcogens are also known, such as the disulfide , diselenide , and the monosulfide (GeS), selenide (GeSe), and Telluride (GeTe). GeS2 forms as a white precipitate when hydrogen sulfide is passed through strongly acid solutions containing Ge(IV). 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 for Winkler to make the discovery of the element. 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. Upon melting with alkaline carbonates and sulfur, germanium compounds form salts known as thiogermanates.
Four tetrahalides are known. Under normal conditions GeI4 is a solid, GeF4 a gas and the others volatile liquids. For example germanium tetrachloride, GeCl4, is obtained as a colourless fuming liquid boiling at 83.1 °C by heating the metal with chlorine. All the tetrahalides are readily hydrolysed to hydrated germanium dioxide. GeCl4 is used in the production of organogermanium compounds. All four dihalides are known and in contrast to the tetrahalides are polymeric solids. Additionally Ge2Cl6 and some higher compounds of formula GenCl2n+2 are known. The unusual compound Ge6Cl16 has been prepared that contains the Ge5Cl12 unit with a neopentane structure.
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. The germanes are less volatile and less reactive than their corresponding silicon analogues. GeH4 reacts with alkali metals in liquid ammonia to form white crystalline MGeH3 which contain the GeH3- anion. The germanium hydrohalides with one, two and three halogen atoms are colorless reactive liquids.
The first organogermanium compound was synthesised by Winkler in 1887; the reaction of germanium tetrachloride with diethylzinc yielded tetraethylgermane . Organogermanes of the type R4Ge (where R is an alkyl) such as tetramethylgermane and tetraethylgermane are accessed through the cheapest available germanium precursor germanium tetrachloride and alkyl nucleophiles. Organic germanium hydrides such as isobutylgermane 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). 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.
Isotopes
Germanium has five naturally-occurring isotopes, 70Ge, 72Ge, 73Ge, 74Ge, and 76Ge. Of these, 76Ge is very slightly radioactive, decaying by double beta decay with a half-life of 1.58 × 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%. When bombarded with alpha particles, the isotope 72Ge will generate stable 77Se, releasing high energy electrons in the process. Because of this, it is used in combination with radon for nuclear batteries.
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 d. 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 ß+ delayed proton emission. 84Ge through 87Ge also have minor ß- delayed neutron emission decay paths.
Natural abundance
Germanium is created through 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. Germanium has been detected in the atmosphere of Jupiter and in some of the most distant stars. Its abundance in the Earth's crust is approximately 1.6 ppm. There are only a few minerals like argyrodite, briartite, germanite, and renierite that contain appreciable amounts of germanium, but no minable deposits exist for any of them. Nonetheless, none is mined for its germanium content. Some zinc-copper-lead ore bodies contain enough germanium that it can be extracted from the final ore concentrate.
An unusual enrichment process causes a high content of germanium in some coal seams, which was discovered by Victor Mordechai Goldschmidt during a broad survey for germanium deposits. The highest concentration ever found was in the Hartley coal ash with up to 1.6% of germanium. The coal deposits near Xilinhaote Inner Mongolia contains estimated 1600 tonnes of germanium.
Production
Worldwide production in 2006 was roughly 100 tonnes of germanium. Currently, it is recovered as a by-product from sphalerite ores where it is concentrated in amounts of up to 0.3%, especially from sediment-hosted, massive Zn–Pb–Cu(–Ba) deposits and carbonate-hosted Zn–Pb deposits. Figures for worldwide Ge reserves are not available, but in the US it is estimated to be around 500 tonnes. In 2007 35% of the demand was met by recycled germanium.
It is produced mainly from sphalerite, a zinc ore, but is also found in silver, lead, and copper ores. A source of germanium is fly ash of coal power plants which use coal from certain coal deposits with a large concentration of germanium. Russia and China used this as a source for germanium. Russia's deposits are located in the far east of the country on Sakhalin island. The coal mines northeast of Vladivostok have also been used as a germanium source. The deposits in China are mainly located in the lignite mines near Lincang, Yunnan and coal mines near Xilinhaote, Inner Mongolia and are also used as germanium sources.
The ore concentrates are mostly sulfidic; they are converted to the oxides by heating under air, in a process known as roasting:
- GeS2 + 3O2 ? GeO2 + 2SO2
Part of the germanium ends up in the dust produced during this process, while the rest is converted to germanates which are leached together with the zinc from the cinder by sulfuric acid. After neutralisation only the zinc stays in solution and the precipitate contains the germanium and other metals. After reducing the amount of 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 distilled off:
- GeO2 + 4HCl ? GeCl4 + 2H2O
- GeO2 + 2Cl2 ? GeCl4 + O2
Germanium tetrachloride is either hydrolysed to the oxide (GeO2) or purified by fractionated distillation and then hydrolysed.
The highly pure GeO2 is now suitable for the production of germanium glass. The pure germanium oxide is reduced by the reaction with hydrogen to obtain germanium suitable for the infrared optics or semiconductor industry:
- GeO2 + 4H2 ? Ge + 2H2O
The germanium for steel production and other industrial processes is normally reduced using carbon:
- GeO2 + C ? Ge + CO2
Applications
The major end uses for germanium in 2007, worldwide, were estimated to be: 35% for fiber-optic systems, 30% infrared optics, 15% for polymerization catalysts, and 15% for electronics and solar electric applications. The remaining 5% went into other uses such as phosphors, metallurgy, and chemotherapy.
Optics
The most notable physical characteristics 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 for the core part of optical fibers. It also replaced titania as the silica dopant for silica fiber, eliminating the need for subsequent heat treatment, which made the fibers brittle. At the end of 2002 the fiber optics industry accounted for 60% of the annual germanium use in the United States, but this use accounts for less than 10% of world wide consumption. GeSbTe is a phase change alloy used for its optic properties, such as in rewritable DVDs.
Because germanium is transparent in the infrared it is a very 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 wavelength range for passive thermal imaging and for hot-spot detection in military, night vision system in cars, and fire fighting applications. It is therefore used in infrared spectroscopes and other optical equipment which require extremely sensitive infrared detectors.
Electronics
The alloy silicon germanide (commonly referred to as "silicon-germanium", or SiGe) is rapidly becoming an important semiconductor material, for use in high speed integrated circuits. Circuits utilizing the properties of Si-SiGe junctions can be much faster than those using silicon alone. Silicon-germanium is beginning to replace gallium arsenide (GaAs) in wireless communications devices. The SiGe chips, with high-speed properties, can be made with low-cost, well-established production techniques of the silicon chip industry.
The recent rise in energy cost has improved the economics of solar panels, a potential major new use of germanium. Germanium is the substrate of the wafers for high-efficiency multijunction photovoltaic cells for space applications.
Gallium arsenide germanium solar cell
Because germanium and gallium arsenide have very similar lattice constants, germanium substrates can be used to make gallium arsenide solar cells. The Mars Exploration Rovers and several satellites use triple junction gallium arsenide on germanium cells.
Germanium-on-insulator substrates are seen as a potential replacement for silicon on miniaturized chips. Other uses in electronics include phosphors in fluorescent lamps, and germanium-base solid-state light-emitting diodes (LEDs). 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.
Other uses
Germanium dioxide is also used in catalysts for polymerisation in the production of polyethylene terephthalate (PET). The high brilliance of the produced polyester is especially used for PET bottles marketed in Japan. However, in the United States, no germanium is used for polymerization catalysts. 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.
In recent years germanium has seen increasing use in precious metal alloys. In sterling silver alloys, for instance, it has been found to reduce firescale, increase tarnish resistance, and increase the alloy's response to precipitation hardening. A tarnish-proof sterling silver alloy, trademarked Argentium, requires 1.2% germanium. The material has a very high refractive index (4.0) and so needs to be anti-reflection coated. 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 rough treatment.
High purity germanium single crystal detectors can precisely identify radiation sources—for example in airport security. 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. Crystals of high purity germanium are used in detectors for gamma spectroscopy and the search for dark matter.
Certain compounds of germanium have low toxicity to mammals, but have toxic effects against certain bacteria. This property makes these compounds useful as chemotherapeutic agents.
Precautions
As early as 1922, doctors in the United States used the inorganic form of germanium to treat patients with anemia. It was used in other forms of treatments, but its efficiency has been dubious. Its role in cancer treatments has been debated. U.S. Food and Drug Administration research has concluded that germanium, when used as a nutritional supplement, "presents potential human health hazard".
Germanium is not thought to be essential to the health of plants or animals. Some of its compounds present a hazard to human health, however. For example, germanium chloride and germane (GeH4) are a liquid and gas, respectively, that can be very irritating to the eyes, skin, lungs, and throat. Germanium has little or no effect upon the environment because it usually occurs only as a trace element in ores and carbonaceous materials and is used in very small quantities in commercial applications.
Footnotes
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