A boride is a 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 covalent more than 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.

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 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,

lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and y ...

s and

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 first element in the series, actinium. The inf ...

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 Diamagnetic materials 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 ferromagnetic materials are attracted ...

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) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...

or anti-ferromagnetic. They are mostly stable and refractory. Some metallic dodecaborides contain boron

icosahedra In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons". There are infinitely many non- similar shapes of icosahedra, some of them being more symmetri ...

, others (for example

yttrium Yttrium is a chemical element with the 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 always found in c ...

zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...

and

uranium Uranium is a chemical element with the 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. Uranium is weakly ...

) have the boron atoms arranged in

cuboctahedra A cuboctahedron is a polyhedron with 8 triangular faces and 6 square faces. A cuboctahedron has 12 identical vertex (geometry), vertices, with 2 triangles and 2 squares meeting at each, and 24 identical edge (geometry), edges, each separating a t ...

. LaB₆ is an inert

refractory In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase ...

compound, used in

hot cathode In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating elem ...

s because of its low

work function In solid-state physics, the work function (sometimes spelt 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 electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat). This occurs because the thermal energy given to the charge carrier overcomes the work function of the mate ...

of electrons; YB₆₆ crystals, grown by an indirect-heating floating zone method, are used as

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. The name is from the Greek roots ''mono-'', "si ...

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 magnetic field which bends the particle beam into its closed p ...

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 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, 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 Unit Cell of RuB2

References

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