A boride is a compound between
boron
Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...
and a less
electronegative
Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...
element, for example
silicon boride (SiB
3 and SiB
6). The borides are a very large group of compounds that are generally high melting and are covalent more than ionic in nature. Some borides exhibit very useful physical properties. The term boride is also loosely applied to compounds such as B
12As
2 (N.B. Arsenic has an electronegativity higher than boron) that is often referred to as
icosahedral boride.
Ranges of compounds
The borides can be classified loosely as boron rich or metal rich, for example the compound
YB66 at one extreme through to Nd
2Fe
14B 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,
lanthanide
The lanthanide () or lanthanoid () series of chemical elements comprises at least the 14 metallic chemical elements with atomic numbers 57–70, from lanthanum through ytterbium. In the periodic table, they fill the 4f orbitals. Lutetium (el ...
s and
actinide
The actinide () or actinoid () series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. Number 103, lawrencium, is also generally included despite being part ...
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 include examples of compounds that are semi conductors, superconductors,
diamagnetic
Diamagnetism is the property of materials that are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagn ...
,
paramagnetic
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...
,
ferromagnetic
Ferromagnetism is a property of certain materials (such as iron) that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagne ...
or
anti-ferromagnetic. They are mostly stable and refractory.
Some metallic dodecaborides contain boron
icosahedra, others (for example
yttrium
Yttrium is a chemical element; it has Symbol (chemistry), symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost a ...
,
zirconium
Zirconium is a chemical element; it has Symbol (chemistry), symbol Zr and atomic number 40. First identified in 1789, isolated in impure form in 1824, and manufactured at scale by 1925, pure zirconium is a lustrous transition metal with a greyis ...
and
uranium
Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
) have the boron atoms arranged in
cuboctahedra.
LaB6 is an inert
refractory
In materials science, a refractory (or refractory material) is a material that is resistant to decomposition by heat or chemical attack and that retains its strength and rigidity at high temperatures. They are inorganic, non-metallic compound ...
compound, used in
hot cathodes because of its low
work function
In solid-state physics, the work function (sometimes spelled workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" ...
which gives it a high rate of
thermionic emission
Thermionic emission is the liberation of charged particles from a hot electrode whose thermal energy gives some particles enough kinetic energy to escape the material's surface. The particles, sometimes called ''thermions'' in early literature, a ...
of electrons; YB
66 crystals, grown by an
indirect-heating floating zone method, are used as
monochromator
A monochromator is an optics, 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. The name is .
Uses
A device that can ...
s for low-energy
synchrotron
A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The strength of the magnetic field which bends the particle beam i ...
X-rays. VB
2 has shown some promise as potential material with higher energy capacity than lithium for batteries.
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
AlB2 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.
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, B
2 units, boron chains or boron sheets/layers.
Examples of the different types of borides are:
*isolated boron atoms, example Mn
4B
*B
2 units, example V
3B
*chains of boron atoms, example FeB
*sheets or layers of boron atoms CrB
2
*3-dimensional boron frameworks that include boron polyhedra, example NaB
15 with boron icosahedra
See also
*
Crystal structure of boron-rich metal borides
Metals, 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 ...
*
Iron tetraboride
*
Yttrium borides - a representative class of metal borides
*
Magnesium diboride - a superconductor
References
Books
*
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