Tantalum Pentoxide
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Tantalum pentoxide, also known as
tantalum Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as ''tantalium'', it is named after Tantalus, a villain in Greek mythology. Tantalum is a very hard, ductile, lustrous, blue-gray transition metal that is ...
(V) oxide, is the
inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...
with the formula . It is a white solid that is insoluble in all solvents but is attacked by strong bases and hydrofluoric acid. is an inert material with a high
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, o ...
and low absorption (i.e. colourless), which makes it useful for coatings. It is also extensively used in the production of capacitors, due to its high
dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insulat ...
.


Preparation


Occurrence

Tantalum occurs in the minerals
tantalite The mineral group tantalite manganese.html" ;"title="iron.html" ;"title="iron">Fe, manganese">Mn)Tantalum">Ta2oxygen">O6is the primary source of the chemical element tantalum, a corrosion (heat and acid) resistant metal. It is chemically simila ...
and
columbite Columbite, also called niobite, niobite-tantalite and columbate [], is a black mineral group that is an ore of niobium. It has a submetallic Lustre (mineralogy), luster and a high density and is a niobate of iron and manganese. This mineral group w ...
(columbium being an archaic name for niobium), which occur in
pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic co ...
s, an igneous rock formation. Mixtures of columbite and tantalite are called
coltan Coltan (short for columbite–tantalites and known industrially as tantalite) is a dull black metallic ore from which the elements niobium and tantalum are extracted. The niobium-dominant mineral in coltan is columbite (after niobium's original ...
. Tantalite was discovered by Anders Gustaf Ekeberg at
Ytterby Ytterby () is a village on the Swedish island of Resarö, in Vaxholm Municipality in the Stockholm archipelago. Today the residential area is dominated by suburban homes. The name of the village translates to "outer village". Ytterby is pe ...
, Sweden, and Kimoto, Finland. The minerals microlite and
pyrochlore Pyrochlore () is a mineral group of the niobium end member of the pyrochlore supergroup. The general formula, (where A and B are metals), represent a family of phases isostructural to the mineral pyrochlore. Pyrochlores are an important class o ...
contain approximately 70% and 10% Ta, respectively.


Refining

Tantalum ores often contain significant amounts of
niobium Niobium is a chemical element with chemical symbol Nb (formerly columbium, Cb) and atomic number 41. It is a light grey, crystalline, and ductile transition metal. Pure niobium has a Mohs hardness rating similar to pure titanium, and it ha ...
, which is itself a valuable metal. As such, both metals are extracted so that they may be sold. The overall process is one of hydrometallurgy and begins with a leaching step; in which the ore is treated with
hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepr ...
and sulfuric acid to produce water-soluble hydrogen fluorides, such as the heptafluorotantalate. This allows the metals to be separated from the various non-metallic impurities in the rock. :(FeMn)(NbTa)2O6 + 16 HF → H2 aF7+ H2 bOF5+ FeF2 + MnF2 + 6 H2O The tantalum and niobium hydrogenflorides are then removed from the
aqueous An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would ...
solution by liquid-liquid extraction using
organic solvents A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
, such as
cyclohexanone Cyclohexanone is the organic compound with the formula (CH2)5CO. The molecule consists of six-carbon cyclic molecule with a ketone functional group. This colorless oily liquid has an odor reminiscent of acetone. Over time, samples of cyclohex ...
or methyl isobutyl ketone. This step allows the simple removal of various metal impurities (e.g. iron and manganese) which remain in the aqueous phase in the form of
fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an inorganic, monatomic anion of fluorine, with the chemical formula (also written ), whose salts are typically white or colorless. Fluoride salts ...
s. Separation of the tantalum and niobium is then achieved by pH adjustment. Niobium requires a higher level of acidity to remain soluble in the organic phase and can hence be selectively removed by extraction into less acidic water. The pure tantalum hydrogen fluoride solution is then neutralised with aqueous
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...
to give hydrated tantalum oxide (Ta2O5(H2O)x), which is calcinated to tantalum pentoxide (Ta2O5) as described in these idealized equations: : H2 aF7+ 5 H2O + 7 NH3 → Ta2O5(H2O)5 + 7 NH4F : Ta2O5(H2O)5 → Ta2O5 + 5 H2O Natural pure tantalum oxide is known as the mineral
tantite Tantite is a rare tantalum oxide mineral with formula: Ta2 O5. Tantite forms transparent microscopic colorless triclinic - pedial crystals with an adamantine luster. It has a Mohs hardness of 7 and a high specific gravity of 8.45. Chemical analys ...
, although it is exceedingly rare.


From alkoxides

Tantalum oxide is frequently used in electronics, often in the form of
thin film A thin film is a layer of material ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many a ...
s. For these applications it can be produced by
MOCVD Metalorganic vapour-phase epitaxy (MOVPE), also known as organometallic vapour-phase epitaxy (OMVPE) or metalorganic chemical vapour deposition (MOCVD), is a chemical vapour deposition method used to produce single- or polycrystalline thin films. ...
(or related techniques), which involves the
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...
of its volatile
halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a f ...
s or alkoxides: : Ta2(OEt)10 + 5 H2O → Ta2O5 + 10 EtOH : 2 TaCl5 + 5 H2O → Ta2O5 + 10 HCl


Structure and properties

The crystal structure of tantalum pentoxide has been the matter of some debate. The bulk material is disordered, being either
amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek language ...
or
polycrystalline A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains. Bacillite is a type of crystallite. It is rodlike with parallel longulites. Stru ...
; with
single crystal In materials science, a single crystal (or single-crystal solid or monocrystalline solid) is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries.RIWD. "R ...
s being difficult to grow. As such
Xray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angl ...
has largely been limited to
powder diffraction Powder diffraction is a scientific technique using X-ray, neutron, or electron diffraction on powder or microcrystalline samples for structural characterization of materials. An instrument dedicated to performing such powder measurements is cal ...
, which provides less structural information. At least 2 polymorphs are known to exist. A low temperature form, known as L- or β-Ta2O5, and the high temperature form known as H- or α-Ta2O5. The transition between these two forms is slow and reversible; taking place between 1000 and 1360 °C, with a mixture of structures existing at intermediate temperatures. The structures of both polymorphs consist of chains built from octahedral TaO6 and pentagonal bipyramidal TaO7 polyhedra sharing opposite vertices; which are further joined by edge-sharing. The overall crystal system is
orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with ...
in both cases, with the
space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it ...
of β-Ta2O5 being identified as ''Pna2'' by single crystal X-ray diffraction. A high pressure form (''Z''-Ta2O5) has also been reported, in which the Ta atoms adopt a 7 coordinate geometry to give a
monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic ...
structure (space group C2). Purely amorphous tantalum pentoxide has a similar local structure to the crystalline polymorphs, built from TaO6 and TaO7 polyhedra, while the molten liquid phase has a distinct structure based on lower coordination polyhedra, mainly TaO5 and TaO6. The difficulty in forming material with a uniform structure has led to variations in its reported properties. Like many metal oxides Ta2O5 is an insulator and its
band gap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference ( ...
has variously been reported as being between 3.8 and 5.3 eV, depending on the method of manufacture. In general the more
amorphous In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal. Etymology The term comes from the Greek language ...
the material the greater its observed band gap. These observed values are significantly higher than those predicted by
computational chemistry Computational chemistry is a branch of chemistry that uses computer simulation to assist in solving chemical problems. It uses methods of theoretical chemistry, incorporated into computer programs, to calculate the structures and properties of mo ...
(2.3 - 3.8 eV). Its
dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insulat ...
is typically about 25 although values of over 50 have been reported. In general tantalum pentoxide is considered to be a high-k dielectric material.


Reactions

Ta2O5 does not react appreciably with either HCl or HBr, however it will dissolve in
hydrofluoric acid Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly corrosive. It is used to make most fluorine-containing compounds; examples include the commonly used pharmaceutical antidepr ...
, and reacts with
potassium bifluoride Potassium bifluoride is the inorganic compound with the formula . This colourless salt consists of the potassium cation () and the bifluoride anion (). The salt is used as an etchant for glass. Sodium bifluoride is related and is also of commerc ...
and HF according to the following equation: :Ta2O5 + 4 KHF2 + 6 HF → 2 K2 aF7 + 5 H2O Ta2O5 can be reduced to metallic Ta via the use of metallic reductants such as calcium and aluminium. :Ta2O5 + 5 Ca → 2 Ta + 5 CaO


Uses


In electronics

Owing to its high
band gap In solid-state physics, a band gap, also called an energy gap, is an energy range in a solid where no electronic states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference ( ...
and
dielectric constant The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insulat ...
, tantalum pentoxide has found a variety of uses in electronics, particularly in
tantalum capacitor A tantalum electrolytic capacitor is an electrolytic capacitor, a passive component of electronic circuits. It consists of a pellet of porous tantalum metal as an anode, covered by an insulating oxide layer that forms the dielectric, surrounde ...
s. These are used in
automotive electronics Automotive electronics are electronic systems used in vehicles, including engine management, ignition, radio, carputers, telematics, in-car entertainment systems, and others. Ignition, engine and transmission electronics are also found in truc ...
, cell phones, and pagers, electronic circuitry; thin-film components; and high-speed tools. In the 1990s, interest grew in the use of tantalum oxide as a high-k dielectric for
DRAM Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxi ...
capacitor applications. It is used in on-chip metal-insulator-metal capacitors for high frequency CMOS integrated circuits. Tantalum oxide may have applications as the charge trapping layer for non-volatile memories. There are applications of tantalum oxide in resistive switching memories.


Other uses

Due to its high
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, o ...
, Ta2O5 has been utilized in the fabrication of the
glass Glass is a non-Crystallinity, crystalline, often transparency and translucency, transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most ...
of
photographic lens A camera lens (also known as photographic lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capa ...
es. It can also be deposited as an
optical coating An optical coating is one or more thin layers of material deposited on an optical component such as a lens, prism or mirror, which alters the way in which the optic reflects and transmits light. These coatings have become a key technology in t ...
with typical applications being antireflection and multilayer filter coatings in near UV to near
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...
.


References

{{DEFAULTSORT:Tantalum Pentoxide Tantalum compounds High-κ dielectrics Transition metal oxides