Barium
Barium is a chemical element with symbol Ba and atomic number 56. It is a soft, silvery alkaline earth metal; because of its high chemical reactivity, barium is never found in nature as a free element. Its hydroxide, known in pre-modern times as baryta, does not occur as a mineral, but can be prepared by heating barium carbonate; the most common occurring minerals of barium are barite and witherite, both insoluble in water. The name barium originates from the alchemical derivative "baryta", from Greek βαρύς, meaning "heavy." Baric is the adjectival form of barium. Barium was identified as a new element in 1774, but not reduced to a metal until 1808 with the advent of electrolysis. Barium has few industrial applications, it was used as a getter for vacuum tubes and in oxide form as the emissive coating on indirectly heated cathodes. It is a component of YBCO and electroceramics, is added to steel and cast iron to reduce the size of carbon grains within the microstructure. Barium compounds are added to fireworks to impart a green color.
Barium sulfate is used as an insoluble additive to oil well drilling fluid, as well as in a purer form, as X-ray radiocontrast agents for imaging the human gastrointestinal tract. The soluble barium ion and soluble compounds are poisonous, have been used as rodenticides. Barium is a silvery-white metal, with a slight golden shade when ultrapure; the silvery-white color of barium metal vanishes upon oxidation in air yielding a dark gray oxide layer. Barium has good electrical conductivity. Ultrapure barium is difficult to prepare, therefore many properties of barium have not been measured yet. At room temperature and pressure, barium has a body-centered cubic structure, with a barium–barium distance of 503 picometers, expanding with heating at a rate of 1.8×10−5/°C. It is a soft metal with a Mohs hardness of 1.25. Its melting temperature of 1,000 K is intermediate between those of the lighter strontium and heavier radium; the density is again intermediate between those of radium. Barium is chemically similar to magnesium and strontium, but more reactive.
It always exhibits the oxidation state of +2, except in a few rare and unstable molecular species that are only characterised in the gas phase such as BaF. Reactions with chalcogens are exothermic. Reactions with other nonmetals, such as carbon, phosphorus and hydrogen, are exothermic and proceed upon heating. Reactions with water and alcohols are exothermic and release hydrogen gas: Ba + 2 ROH → Ba2 + H2↑ Barium reacts with ammonia to form complexes such as Ba6; the metal is attacked by most acids. Sulfuric acid is a notable exception because passivation stops the reaction by forming the insoluble barium sulfate on the surface. Barium combines with several metals, including aluminium, zinc and tin, forming intermetallic phases and alloys. Barium salts are white when solid and colorless when dissolved, barium ions provide no specific coloring, they are denser than the calcium analogs, except for the halides. Barium hydroxide was known to alchemists. Unlike calcium hydroxide, it absorbs little CO2 in aqueous solutions and is therefore insensitive to atmospheric fluctuations.
This property is used in calibrating pH equipment. Volatile barium compounds burn with a green to pale green flame, an efficient test to detect a barium compound; the color results from spectral lines at 455.4, 493.4, 553.6, 611.1 nm. Organobarium compounds are a growing field of knowledge: discovered are dialkylbariums and alkylhalobariums. Barium found in the Earth's crust is a mixture of seven primordial nuclides, barium-130, 132, 134 through 138. Barium-130 undergoes slow radioactive decay to xenon-130 by double beta plus decay, barium-132 theoretically decays to xenon-132, with half-lives a thousand times greater than the age of the Universe; the abundance is ≈ 0.1 %. The radioactivity of these isotopes is so weak. Of the stable isotopes, barium-138 composes 71.7% of all barium. In total, barium has about 40 known isotopes, ranging in mass between 114 and 153; the most stable artificial radioisotope is barium-133 with a half-life of 10.51 years. Five other isotopes have half-lives longer than a day.
Barium has 10 meta states, of which barium-133m1 is the most stable with a half-life of about 39 hours. Alchemists in the early Middle Ages knew about some barium minerals. Smooth pebble-like stones of mineral baryte were found in volcanic rock near Bologna, so were called "Bologna stones." Alchemists were attracted to them. The phosphorescent properties of baryte heated with organics were described by V. Casciorolus in 1602. Carl Scheele determined that baryte contained a new element in 1774, but could not isolate barium, only barium oxide. Johan Gottlieb Gahn isolated barium oxide two year
Electron gun
An electron gun is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy. The largest use is in cathode ray tubes, used in nearly all television sets, computer displays and oscilloscopes that are not flat-panel displays, they are used in field emission displays, which are flat-panel displays made out of rows of small cathode ray tubes. They are used in microwave linear beam vacuum tubes such as klystrons, inductive output tubes, travelling wave tubes, gyrotrons, as well as in scientific instruments such as electron microscopes and particle accelerators. Electron guns may be classified by the type of electric field generation, by emission mechanism, by focusing, or by the number of electrodes. A direct current, electrostatic thermionic electron gun is formed from several parts: a hot cathode, heated to create a stream of electrons via thermionic emission, electrodes generating an electric field which focus the beam, one or more anode electrodes which accelerate and further focus the electrons.
A large voltage between the cathode and anode accelerates the electrons. A repulsive ring placed between them focuses the electrons onto a small spot on the anode at the expense of a lower extraction field strength on the cathode surface. At this spot is a hole so that the electrons that pass through the anode form a collimated beam and reach a second anode called a collector; this arrangement is similar to an Einzel lens. Most color cathode ray tubes – such as those used in color televisions – incorporate three electron guns, each one producing a different stream of electrons; each stream travels through a shadow mask where the electrons will impinge upon either a red, green or blue phosphor to light up a color pixel on the screen. The resultant color, seen by the viewer will be a combination of these three primary colors; the most common use of electron guns is in cathode ray tubes, which were used in computer and television monitors. An electron gun can be used to ionize particles by adding or removing electrons from an atom.
This technology is sometimes used in mass spectrometry in a process called electron ionization to ionize vaporized or gaseous particles. More powerful electron guns are used for welding, metal coating, 3D metal printers, metal powder production and vacuum furnaces. Electron guns are used in medical linac, where high energy electron beams, hit a target and emit X-rays. A nanocoulombmeter in combination with a Faraday cup can be used to detect and measure the beams emitted from electron gun and ion guns. Another way to detect electron beams from an electron gun is by using a phosphor screen which will glow when struck by an electron. Optics Electron beam technology Simulation of an Electron Gun Interactive tutorial from LMU Munich
Vacuum tube
In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as a thermionic tube or thermionic valve uses the phenomenon of thermionic emission of electrons from a heated cathode and is used for a number of fundamental electronic functions such as signal amplification and current rectification. Non-thermionic types, such as a vacuum phototube however, achieve electron emission through the photoelectric effect, are used for such as the detection of light levels. In both types, the electrons are accelerated from the cathode to the anode by the electric field in the tube; the simplest vacuum tube, the diode invented in 1904 by John Ambrose Fleming, contains only a heated electron-emitting cathode and an anode. Current can only flow in one direction through the device—from the cathode to the anode. Adding one or more control grids within the tube allows the current between the cathode and anode to be controlled by the voltage on the grid or grids.
These devices became a key component of electronic circuits for the first half of the twentieth century. They were crucial to the development of radio, radar, sound recording and reproduction, long distance telephone networks, analogue and early digital computers. Although some applications had used earlier technologies such as the spark gap transmitter for radio or mechanical computers for computing, it was the invention of the thermionic vacuum tube that made these technologies widespread and practical, created the discipline of electronics. In the 1940s the invention of semiconductor devices made it possible to produce solid-state devices, which are smaller, more efficient and durable, cheaper than thermionic tubes. From the mid-1960s, thermionic tubes were being replaced with the transistor. However, the cathode-ray tube remained the basis for television monitors and oscilloscopes until the early 21st century. Thermionic tubes still have some applications, such as the magnetron used in microwave ovens, certain high-frequency amplifiers, amplifiers that audio enthusiasts prefer for their tube sound.
Not all electronic circuit valves/electron tubes are vacuum tubes. Gas-filled tubes are similar devices, but containing a gas at low pressure, which exploit phenomena related to electric discharge in gases without a heater. One classification of thermionic vacuum tubes is by the number of active electrodes. A device with two active elements is a diode used for rectification. Devices with three elements are triodes used for switching. Additional electrodes create tetrodes, so forth, which have multiple additional functions made possible by the additional controllable electrodes. Other classifications are: by frequency range by power rating by cathode/filament type and Warm-up time by characteristic curves design by application specialized parameters specialized functions tubes used to display information Tubes have different functions, such as cathode ray tubes which create a beam of electrons for display purposes in addition to more specialized functions such as electron microscopy and electron beam lithography.
X-ray tubes are vacuum tubes. Phototubes and photomultipliers rely on electron flow through a vacuum, though in those cases electron emission from the cathode depends on energy from photons rather than thermionic emission. Since these sorts of "vacuum tubes" have functions other than electronic amplification and rectification they are described in their own articles. A vacuum tube consists of two or more electrodes in a vacuum inside an airtight envelope. Most tubes have glass envelopes with a glass-to-metal seal based on kovar sealable borosilicate glasses, though ceramic and metal envelopes have been used; the electrodes are attached to leads. Most vacuum tubes have a limited lifetime, due to the filament or heater burning out or other failure modes, so they are made as replaceable units. Tubes were a frequent cause of failure in electronic equipment, consumers were expected to be able to replace tubes themselves. In addition to the base terminals, some tubes had an electrode terminating at a top cap.
The principal reason for doing this was to avoid leakage resistance through the tube base for the high impedance grid input. The bases were made with phenolic insulation which performs poorly as an insulator in humid conditions. Other reasons for using a top cap include improving stability by reducing grid-to-anode capacitance, improved high-frequency performance, keeping a high plate voltage away from lower voltages, accommodating one more electrode than allowed by the base. There was an occasional design that had two top cap connections; the earliest vacuum tubes evolved from incandescent light bulbs, containing a filament sealed in an evacuated glass envelope. When hot, the filament releases electrons into the vacuum, a process called thermio
Calcium oxide
Calcium oxide known as quicklime or burnt lime, is a used chemical compound. It is a white, alkaline, crystalline solid at room temperature; the broadly used term lime connotes calcium-containing inorganic materials, in which carbonates and hydroxides of calcium, magnesium and iron predominate. By contrast, quicklime applies to the single chemical compound calcium oxide. Calcium oxide that survives processing without reacting in building products such as cement is called free lime. Quicklime is inexpensive. Both it and a chemical derivative are important commodity chemicals. Calcium oxide is made by the thermal decomposition of materials, such as limestone or seashells, that contain calcium carbonate in a lime kiln; this is accomplished by heating the material to above 825 °C, a process called calcination or lime-burning, to liberate a molecule of carbon dioxide, leaving quicklime. CaCO3 → CaO + CO2The quicklime is not stable and, when cooled, will spontaneously react with CO2 from the air until, after enough time, it will be converted back to calcium carbonate unless slaked with water to set as lime plaster or lime mortar.
Annual worldwide production of quicklime is around 283 million tonnes. China is by far the world's largest producer, with a total of around 170 million tonnes per year; the United States is the next largest, with around 20 million tonnes per year. 1.8 t of limestone is required per 1.0 t of quicklime. Quicklime is a more efficient desiccant than silica gel; the reaction of quicklime with water is associated with an increase in volume by a factor of at least 2.5. The major use of quicklime is in the basic oxygen steelmaking process, its usage varies from about 30 to 50 kilograms per ton of steel. The quicklime neutralizes the acidic oxides, SiO2, Al2O3, Fe2O3, to produce a basic molten slag. Ground quicklime is used with densities of ca. 0.6–1.0 g/cm3. Quicklime and hydrated lime can increase the load carrying capacity of clay-containing soils, they do this by reacting with finely divided silica and alumina to produce calcium silicates and aluminates, which possess cementing properties. Small quantities of quicklime are used in other processes.
Heat: Quicklime releases Thermal energy by the formation of the hydrate, calcium hydroxide, by the following equation:CaO + H2O ⇌ Ca2 As it hydrates, an exothermic reaction results and the solid puffs up. The hydrate can be reconverted to quicklime by removing the water by heating it to redness to reverse the hydration reaction. One litre of water combines with 3.1 kilograms of quicklime to give calcium hydroxide plus 3.54 MJ of energy. This process can be used to provide a convenient portable source of heat, as for on-the-spot food warming in a self-heating can and heating water without open flames. Several companies sell cooking kits using this heating method, it is known as a food additive to the FAO as an acidity regulator, a flour treatment agent and as a leavener. It has E number E529. Light: When quicklime is heated to 2,400 °C, it emits an intense glow; this form of illumination is known as a limelight, was used broadly in theatrical productions before the invention of electric lighting.
Cement: Calcium oxide is a key ingredient for the process of making cement. As a cheap and available alkali. About 50% of the total quicklime production is converted to calcium hydroxide before use. Both quick- and hydrated lime are used in the treatment of drinking water. Petroleum industry: Water detection pastes contain a mix of calcium oxide and phenolphthalein. Should this paste come into contact with water in a fuel storage tank, the CaO reacts with the water to form calcium hydroxide. Calcium hydroxide has a high enough pH to turn the phenolphthalein a vivid purplish-pink color, thus indicating the presence of water. Paper: Calcium oxide is used to regenerate sodium hydroxide from sodium carbonate in the chemical recovery at Kraft pulp mills. Plaster: There is archeological evidence that Pre-Pottery Neolithic B humans used limestone-based plaster for flooring and other uses; such Lime-ash floor remained in use until the late nineteenth century. Chemical or power production: Solid sprays or slurries of calcium oxide can be used to remove sulfur dioxide from exhaust streams in a process called flue-gas desulfurization.
Mining: Compressed lime cartridges exploit the exothermic properties of quicklime to break rock. A shot hole is drilled into the rock in the usual way and a sealed cartridge of quicklime is placed within and tamped. A quantity of water is injected into the cartridge and the resulting release of steam, together with the greater volume of the residual hydrated solid, breaks the rock apart; the method does not work if the rock is hard. In 80 BC, the Roman general Sertorius deployed choking clouds of caustic lime powder to defeat the Characitani of Hispania, who had taken refuge in inaccessible caves. A similar dust was used in China to quell an armed peasant revolt in 178 AD, when lime chariots equipped with bellows blew limestone powder into the crowds. Quicklime is thought to have been a component of Greek fire. Upon contact with water, quicklime would ignite the fuel. David Hume, in his History of England, recounts that early in the reign of Henry III, the English Navy destroyed an invading French fleet by blindin
Thermionic emission
Thermionic emission is the thermally induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the work function of the material; the charge carriers can be electrons or ions, in older literature are sometimes referred to as thermions. After emission, a charge, equal in magnitude and opposite in sign to the total charge emitted is left behind in the emitting region, but if the emitter is connected to a battery, the charge left behind is neutralized by charge supplied by the battery as the emitted charge carriers move away from the emitter, the emitter will be in the same state as it was before emission. The classical example of thermionic emission is that of electrons from a hot cathode into a vacuum in a vacuum tube; the hot cathode can be a metal filament, a coated metal filament, or a separate structure of metal or carbides or borides of transition metals. Vacuum emission from metals tends to become significant only for temperatures over 1,000 K.
The term "thermionic emission" is now used to refer to any thermally-excited charge emission process when the charge is emitted from one solid-state region into another. This process is crucially important in the operation of a variety of electronic devices and can be used for electricity generation or cooling; the magnitude of the charge flow increases with increasing temperature. Because the electron was not identified as a separate physical particle until the work of J. J. Thomson in 1897, the word "electron" was not used when discussing experiments that took place before this date; the phenomenon was reported in 1853 by Edmond Becquerel. It was rediscovered in 1873 by Frederick Guthrie in Britain. While doing work on charged objects, Guthrie discovered that a red-hot iron sphere with a negative charge would lose its charge, he found that this did not happen if the sphere had a positive charge. Other early contributors included Johann Wilhelm Hittorf, Eugen Goldstein, Julius Elster and Hans Friedrich Geitel.
The effect was rediscovered again by Thomas Edison on February 13, 1880, while he was trying to discover the reason for breakage of lamp filaments and uneven blackening of the bulbs in his incandescent lamps. Edison built several experimental lamp bulbs with an extra wire, metal plate, or foil inside the bulb, separate from the filament and thus could serve as an electrode, he connected a galvanometer, a device used to measure current, to the output of the extra metal electrode. If the foil was put at a negative potential relative to the filament, there was no measurable current between the filament and the foil; when the foil was raised to a positive potential relative to the filament, there could be a significant current between the filament through the vacuum to the foil if the filament was heated sufficiently. We now know that the filament was emitting electrons, which were attracted to a positively charged foil, but not a negatively charged one; this one-way current was called the Edison effect.
He found that the current emitted by the hot filament increased with increasing voltage, filed a patent application for a voltage-regulating device using the effect on November 15, 1883. He found; this was exhibited at the International Electrical Exposition in Philadelphia in September 1884. William Preece, a British scientist, took back with him several of the Edison effect bulbs, he presented a paper on them in 1885, where he referred to thermionic emission as the "Edison Effect." The British physicist John Ambrose Fleming, working for the British "Wireless Telegraphy" Company, discovered that the Edison Effect could be used to detect radio waves. Fleming went on to develop the two-element vacuum tube known as the diode, which he patented on November 16, 1904; the thermionic diode can be configured as a device that converts a heat difference to electric power directly without moving parts. Following J. J. Thomson's identification of the electron in 1897, the British physicist Owen Willans Richardson began work on the topic that he called "thermionic emission".
He received a Nobel Prize in Physics in 1928 "for his work on the thermionic phenomenon and for the discovery of the law named after him". From band theory, there are one or two electrons per atom in a solid that are free to move from atom to atom; this is sometimes collectively referred to as a "sea of electrons". Their velocities follow a statistical distribution, rather than being uniform, an electron will have enough velocity to exit the metal without being pulled back in; the minimum amount of energy needed for an electron to leave a surface is called the work function. The work function is characteristic of the material and for most metals is on the order of several electronvolts. Thermionic currents can be increased by decreasing the work function; this often-desired goal can be achieved by applying various oxide coatings to the wire. In 1901 Richardson published the results of his experiments: the current from a heated wire seemed to depend exponentially on the temperature of the wire with a mathematical form similar to the Arrhenius equation.
He propose
Kelvin
The Kelvin scale is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. The kelvin is the base unit of temperature in the International System of Units; until 2018, the kelvin was defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. In other words, it was defined such that the triple point of water is 273.16 K. On 16 November 2018, a new definition was adopted, in terms of a fixed value of the Boltzmann constant. For legal metrology purposes, the new definition will come into force on 20 May 2019; the Kelvin scale is named after the Belfast-born, Glasgow University engineer and physicist William Thomson, 1st Baron Kelvin, who wrote of the need for an "absolute thermometric scale". Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or written as a degree; the kelvin is the primary unit of temperature measurement in the physical sciences, but is used in conjunction with the degree Celsius, which has the same magnitude.
The definition implies that absolute zero is equivalent to −273.15 °C. In 1848, William Thomson, made Lord Kelvin, wrote in his paper, On an Absolute Thermometric Scale, of the need for a scale whereby "infinite cold" was the scale's null point, which used the degree Celsius for its unit increment. Kelvin calculated; this absolute scale is known today as the Kelvin thermodynamic temperature scale. Kelvin's value of "−273" was the negative reciprocal of 0.00366—the accepted expansion coefficient of gas per degree Celsius relative to the ice point, giving a remarkable consistency to the accepted value. In 1954, Resolution 3 of the 10th General Conference on Weights and Measures gave the Kelvin scale its modern definition by designating the triple point of water as its second defining point and assigned its temperature to 273.16 kelvins. In 1967/1968, Resolution 3 of the 13th CGPM renamed the unit increment of thermodynamic temperature "kelvin", symbol K, replacing "degree Kelvin", symbol °K. Furthermore, feeling it useful to more explicitly define the magnitude of the unit increment, the 13th CGPM held in Resolution 4 that "The kelvin, unit of thermodynamic temperature, is equal to the fraction 1/273.16 of the thermodynamic temperature of the triple point of water."In 2005, the Comité International des Poids et Mesures, a committee of the CGPM, affirmed that for the purposes of delineating the temperature of the triple point of water, the definition of the Kelvin thermodynamic temperature scale would refer to water having an isotopic composition specified as Vienna Standard Mean Ocean Water.
In 2018, Resolution A of the 26th CGPM adopted a significant redefinition of SI base units which included redefining the Kelvin in terms of a fixed value for the Boltzmann constant of 1.380649×10−23 J/K. When spelled out or spoken, the unit is pluralised using the same grammatical rules as for other SI units such as the volt or ohm; when reference is made to the "Kelvin scale", the word "kelvin"—which is a noun—functions adjectivally to modify the noun "scale" and is capitalized. As with most other SI unit symbols there is a space between the kelvin symbol. Before the 13th CGPM in 1967–1968, the unit kelvin was called a "degree", the same as with the other temperature scales at the time, it was distinguished from the other scales with either the adjective suffix "Kelvin" or with "absolute" and its symbol was °K. The latter term, the unit's official name from 1948 until 1954, was ambiguous since it could be interpreted as referring to the Rankine scale. Before the 13th CGPM, the plural form was "degrees absolute".
The 13th CGPM changed the unit name to "kelvin". The omission of "degree" indicates that it is not relative to an arbitrary reference point like the Celsius and Fahrenheit scales, but rather an absolute unit of measure which can be manipulated algebraically. In science and engineering, degrees Celsius and kelvins are used in the same article, where absolute temperatures are given in degrees Celsius, but temperature intervals are given in kelvins. E.g. "its measured value was 0.01028 °C with an uncertainty of 60 µK." This practice is permissible because the degree Celsius is a special name for the kelvin for use in expressing relative temperatures, the magnitude of the degree Celsius is equal to that of the kelvin. Notwithstanding that the official endorsement provided by Resolution 3 of the 13th CGPM states "a temperature interval may be expressed in degrees Celsius", the practice of using both °C and K is widespread throughout the scientific world; the use of SI prefixed forms of the degree Celsius to express a temperature interval has not been adopted.
In 2005 the CIPM embarked on a programme to redefine the kelvin using a more experimentally rigorous methodology. In particular, the committee proposed redefining the kelvin such that Boltzmann's constant takes the exact value 1.3806505×10−23 J/K. The committee had hoped tha
Yttrium borides
Yttrium boride refers to a crystalline material composed of different proportions of yttrium and boron, such as YB2, YB4, YB6, YB12, YB25, YB50 and YB66. They are all hard solids having high melting temperatures; the most common form is the yttrium hexaboride YB6. It exhibits superconductivity at high temperature of 8.4 K and, similar to LaB6, is an electron cathode. Another remarkable yttrium boride is YB66, it has a large lattice constant, high thermal and mechanical stability, therefore is used as a diffraction grating for low-energy synchrotron radiation. Yttrium diboride has the same hexagonal crystal structure as aluminium diboride and magnesium diboride – an important superconducting material, its Pearson symbol is hP3, space group P6/mmm, a = 0.33041 nm, c = 0.38465 nm and the calculated density is 5.05 g/cm3. In this structure, the boron atoms form graphite like sheets with yttrium atoms between them. YB2 crystals are unstable to moderate heating in air – they start oxidizing at 400 °C and oxidize at 800 °C.
YB2 melts at ~2100 °C. YB4 has tetragonal crystal structure with space group P4/mbm, Pearson symbol tP20, a = 0.711 nm, c = 0.4019 nm, calculated density 4.32 g/cm3. High-quality YB4 crystals of few centimeters in size can be grown by the multiple-pass floating zone technique. YB6 is a black odorless powder having density of 3.67 g/cm3. High-quality YB6 crystals of few centimeters in size can be grown by the multiple-pass floating zone technique. YB6 is a superconductor with the high transition temperature of 8.4 K. YB12 crystals have a cubic structure with density of 3.44 g/cm3, Pearson symbol cF52, space group Fm3m, a = 0.7468 nm. Its structural unit is 12 cuboctahedron; the Debye temperature of YB12 is ~1040 K, it is not superconducting at temperatures above 2.5 K. The structure of yttrium borides with B/Y ratio of 25 and above consists of a network of B12 icosahedra; the boron framework of YB25 is one of the simplest among icosahedron-based borides – it consists of only one kind of icosahedra and one bridging boron site.
The bridging boron site is tetrahedrally coordinated by four boron atoms. Those atoms are another boron atom in the counter bridge site and three equatorial boron atoms of one of three B12 icosahedra; the yttrium sites have partial occupancies of. 60–70%, the YB25 formula reflects the average atomic ratio / = 25. Both the Y atoms and B12 icosahedra form zigzags along the x-axis; the bridging boron atoms connect three equatorial boron atoms of three icosahedra and those icosahedra make up a network parallel to the crystal plane. The bonding distance between the bridging boron and the equatorial boron atoms is 0.1755 nm, typical for the strong covalent B-B bond. On the other hand, the large distance between the boron atoms within the bridge reveals a weaker interaction, thus the bridging sites contribute little to the bonding between the network planes. YB25 crystals can be grown by heating a compressed pellet of yttria and boron powder to ~1700 °C; the YB25 phase is stable up to 1850 °C. Above this temperature it decomposes into YB66 without melting.
This makes it difficult to grow a single crystal of YB25 by the melt growth method. YB50 crystals have orthorhombic structure with space group P21212, a = 1.66251 nm, b = 1.76198 nm, c = 0.94797 nm. They can be grown by heating a compressed pellet of yttria and boron powder to ~1700 0C. Above this temperature YB50 decomposes into YB66 without melting; this makes it difficult to grow a single crystal of YB50 by the melt growth method. Rare earth elements from Tb to Lu can crystallize in the M50 form. YB66 was discovered in 1960 and its structure was solved in 1969; the structure is face-centered cubic, with space group Fm3c, Pearson symbol cF1936 and lattice constant a = 2.3440 nm. There are 13 boron sites B1 -- one yttrium site; the B1 sites form the B2 -- B9 sites make up another icosahedron. These icosahedra arrange in a thirteen-icosahedron unit 12B12, called supericosahedron; the icosahedron formed by the B1 site atoms is located at the center of the supericosahedron. The supericosahedron is one of the basic units of the boron framework of YB66.
There are two types of supericosahedra: one occupies the cubic face centers and another, rotated by 90°, is located at the center of the cell and at the cell edges. Thus, there are eight supericosahedra in the unit cell. Another structure unit of YB66 is B80 cluster of 80 boron sites formed by the B10 to B13 sites. All those 80 sites are occupied and in total contain only ca. 42 boron atoms. The B80 cluster is located at the body center of the octant of the unit cell, i.e. at the 8a position. Two independent structure analyses came to the same conclusion that the total number of boron atoms in the unit cell is 1584; the boron framework structure of YB66 is shown in the figure to the right. A schematic drawing under it indicates relative orientations of the supericosahedra, the B80 clusters are depicted by light green and dark green spheres, respectively. There are 48 yttrium sites in the unit cell. Fixing the occupancy of the Y site to 0.5 results in 24 Y atoms in the unit cell and the chemical composition of YB66.
