Boride
Encyclopedia
In chemistry a boride is a chemical compound between boron
and a less electronegative
element, for example silicon boride
(SiB3 and SiB6). The borides are a very large group of compounds that are generally high melting and are not ionic in nature. Some borides exhibit very useful physical properties. The term boride is also loosely applied to compounds such as B12As2 (N.B. Arsenic has an electronegativity higher than boron) that is often referred to as icosahedral boride
.
at one extreme through to Nd2Fe14B at the other. The generally accepted definition is that if the ratio of boron atoms to metal atoms is 4:1 or more the compound is boron rich, if it is less, then it is metal rich.
s and actinide
s form a wide variety of boron-rich borides, with metal:boron ratios up to YB66
.
The properties of this group vary from one compound to the next, and includes examples of compounds that are semi conductors, superconductors, diamagnetic
, paramagnetic
, ferromagnetic
or anti-ferromagnetic
. They are mostly stable and refractory.
Some metallic dodecaborides contain boron icosahedra, others (for example yttrium
, zirconium
and uranium
) have the boron atoms arranged in cuboctahedra
.
LaB6
is an inert refractory
compound, used in hot cathodes because of its low work function for emission of electrodes; YB66 crystals, grown by an indirect-heating floating zone method, are used as monochromator
s for low-energy synchrotron
X-rays.
and TiB2
. Recent investigations into this class of borides have revealed a wealth of interesting properties such as super conductivity at 39 K in MgB2
and the ultra-incompressibility of OsB2 and ReB2
.
The metal rich borides contain single boron atoms, B2 units, boron chains or boron sheets/layers.
Examples of the different types of borides are:
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
and a less electronegative
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
element, for example silicon boride
Silicon boride
Silicon borides are lightweight ceramic compounds formed between silicon and boron. Several stoichiometric silicon boride compounds, SiBn, have been reported: silicon triboride, SiB3, silicon tetraboride, SiB4, silicon hexaboride, SiB6, as well as SiBn...
(SiB3 and SiB6). The borides are a very large group of compounds that are generally high melting and are not ionic in nature. Some borides exhibit very useful physical properties. The term boride is also loosely applied to compounds such as B12As2 (N.B. Arsenic has an electronegativity higher than boron) that is often referred to as icosahedral boride
Boron arsenide
Boron arsenide is a chemical compound of boron and arsenic. It is a cubic semiconductor with a lattice constant of 0.4777 nm and an indirect bandgap of roughly 1.5 eV...
.
Ranges of compounds
The borides can be classified loosely as boron rich or metal rich, for example the compound YB66Yttrium 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 gray-colored, hard solids having high melting temperatures. The most common form is the yttrium hexaboride YB6...
at one extreme through to Nd2Fe14B at the other. The generally accepted definition is that if the ratio of boron atoms to metal atoms is 4:1 or more the compound is boron rich, if it is less, then it is metal rich.
Boron rich borides (B:M 4:1 or more)
The main group metals, lanthanideLanthanide
The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
s and actinide
Actinide
The actinide or actinoid series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium.The actinide series derives its name from the group 3 element actinium...
s form a wide variety of boron-rich borides, with metal:boron ratios up to YB66
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 gray-colored, hard solids having high melting temperatures. The most common form is the yttrium hexaboride YB6...
.
The properties of this group vary from one compound to the next, and includes examples of compounds that are semi conductors, superconductors, diamagnetic
Diamagnetism
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the...
, paramagnetic
Paramagnetism
Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...
, ferromagnetic
Ferromagnetism
Ferromagnetism is the basic mechanism by which certain materials form permanent magnets, or are attracted to magnets. In physics, several different types of magnetism are distinguished...
or anti-ferromagnetic
Antiferromagnetism
In materials that exhibit antiferromagnetism, the magnetic moments of atoms or molecules, usuallyrelated to the spins of electrons, align in a regular pattern with neighboring spins pointing in opposite directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered magnetism...
. They are mostly stable and refractory.
Some metallic dodecaborides contain boron icosahedra, others (for example yttrium
Yttrium
Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...
, zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
and uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
) have the boron atoms arranged in cuboctahedra
Cuboctahedron
In geometry, a cuboctahedron is a polyhedron with eight triangular faces and six square faces. A cuboctahedron has 12 identical vertices, with two triangles and two squares meeting at each, and 24 identical edges, each separating a triangle from a square. As such it is a quasiregular polyhedron,...
.
LaB6
Lanthanum hexaboride
]]Lanthanum hexaboride is an inorganic chemical, a boride of lanthanum. It is a refractory ceramic material that has a melting point of 2210 °C, and is insoluble in water and hydrochloric acid. It has a low work function and one of the highest electron emissivities known, and is stable in...
is an inert refractory
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
compound, used in hot cathodes because of its low work function for emission of electrodes; YB66 crystals, grown by an indirect-heating floating zone method, are used as monochromator
Monochromator
A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input...
s for low-energy synchrotron
Synchrotron
A synchrotron is a particular type of cyclic particle accelerator in which the magnetic field and the electric field are carefully synchronised with the travelling particle beam. The proton synchrotron was originally conceived by Sir Marcus Oliphant...
X-rays.
Metal rich borides (B:M less than 4:1)
The transition metals tend to form metal rich borides. Metal-rich borides, as a group, are inert and have high melting temperature. Some are easily formed and this explains their use in making turbine blades, rocket nozzles, etc. Some examples include AlB2Aluminium diboride
Aluminium diboride is a chemical compound made from the metal aluminium and the non-metal boron. It is one of two compounds of aluminium and boron, the other being AlB12 that are both commonly referred to as aluminium boride....
and TiB2
Titanium boride
Titanium diboride is an extremely hard ceramic compound composed of titanium and boron which has excellent resistance to mechanical erosion...
. Recent investigations into this class of borides have revealed a wealth of interesting properties such as super conductivity at 39 K in MgB2
Magnesium diboride
Magnesium diboride is a simple ionic binary compound that has proven to be an inexpensive and useful superconducting material.Its superconductivity was announced in the journal Nature in March 2001. Its critical temperature of is the highest amongst conventional superconductors...
and the ultra-incompressibility of OsB2 and ReB2
Rhenium diboride
Rhenium diboride is a synthetic superhard material. It was first synthesized in 1962 and re-emerged recently due to hopes of achieving high hardness comparable to that of diamond...
.
Boride structures
The boron rich borides contain 3-dimensional frameworks of boron atoms that can include boron polyhedra.The metal rich borides contain single boron atoms, B2 units, boron chains or boron sheets/layers.
Examples of the different types of borides are:
- isolated boron atoms, example Mn4B
- B2 units, example V3B
- chains of boron atoms, example FeB
- sheets or layers of boron atoms CrB2
- 3-dimensional boron frameworks that include boron polyhedra, example NaB15 with boron icosahedra
See also
- Crystal structure of boron-rich metal boridesCrystal structure of boron-rich metal boridesMetals, and specifically rare-earth elements , form numerous chemical complexes with boron. Their crystal structure and chemical bonding depend strongly on the metal element M and on its atomic ratio to boron. When B/M ratio exceeds 12, boron atoms form B12 icosahedra which are linked into a...
- Yttrium boridesYttrium boridesYttrium 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 gray-colored, hard solids having high melting temperatures. The most common form is the yttrium hexaboride YB6...
- a representative class of metal borides