Boron Allotropes and Forms

Dec 6, 2021 | RESEARCH, Chemistry

Boron Allotropes

Boron Allotropes take several forms and account for boron’s presence in multiple natural and man-made products. Allotropes are different structural forms of an element: the atoms are bonded together in a different ways. for example, carbon has diamond and graphene forms. Each allotrope has a distinct arrangement of atoms and their binding electrons and as a result very different physical and chemical properties. There are many Boron Allotropes and and these can be seen in the wide range of minerals, forms and uses of Boron.

icosahedral atoms models

icosahedral atoms models

Boron is found everywhere: glass, ceramics, plastics, dyes, lubricants, pesticides, fertilizers, explosives, drugs, insecticides, fungicides, flame retardant materials, food preservatives, cosmetics, toothpaste, detergents, paints, adhesives, rubber products, textiles, paper, fibers, insulation, semiconductors, batteries, catalysts, pharmaceuticals. The list goes on.

Boron can be prepared in several crystalline or amorphous forms. Three crystalline forms are 𝜶-rhombohedral, 𝜷-rhombohedral, and 𝜶-tetragonal. When exceptional circumstances arise, boron can form tetragonal and orthomorphic allotropes: two amorphous forms, one as a finely divided powder and a glassy solid. At least 14 more allotropes have been reported. Still, these are based on tenuous evidence or have not been experimentally proven or are thought to represent mixed allotropes of boron frameworks stabilized by impurities. 

Types of Boron

There are three main types of boron: elemental, metallic, and non-metallic. The elemental boron consists of just two protons and six neutrons. In comparison, metallic boron contains both positive and negative charges. Non-metallic boron does not have any charge.

Elemental Boron

Elemental Boron is one of the allotropes of Boron and is extensively used in every industry. Its chemical symbol is B. There are four isotopes of boron: 10B,11B,12B,13B. Isotope refers to an atom with the same number of protons but differing numbers of neutrons.

  • 10B is stable and radioactive. It decays slowly over time through alpha decay emitting helium nuclei. When it reaches equilibrium, there will be no further production of 10B. However, 10B is produced continuously in nature. Most often, 10B comes from cosmic rays hitting nitrogen molecules in Earth’s atmosphere.
  • 11B is unstable and radioactive. It undergoes beta decay releasing energy and changing into 12C. Beta particles emitted from 11B travel faster than light, so they cannot escape our planet. Therefore, 11B is never seen directly except in nuclear reactors.
  • 12B is stable and radioactive, decaying via electron capture. Electron capture means that an outer shell electron changes place with another inner shell electron. As a result, the nucleus becomes positively charged and emits a gamma-ray photon. Gamma radiation travels at the speed of light.
  • 13B is stable and radioactive, undergoing proton emission. Proton emission means that a neutron loses a single proton leaving behind a negatively charged particle. 13B is formed when a 14N nucleus captures a free neutron.

Metallic Boron

The high oxygen environment on Earth doesn’t allow for elemental Boron, a metal found in stardust and meteorites. It is hard to get out of its compounds. Reducing Boric oxide with metals such as magnesium or aluminum was the earliest method, but the product is almost always contaminated with metal. Reducing volatile borons with hydrogen halides can be done at high temperatures.

The most common form of metallic boron is hexaborides such as aluminum boride AlB2. Other examples include magnesium diboride MgBO2; calcium pentaborate Ca, sodium tetraborate Na4, potassium octahydrate K8, and lithium heptoxide Li7

Boron has many allotropic forms, including white solid; black powder; yellowish-brown glassy material; red crystalline form; green amorphous form; blue transparent liquid; violet colorless solution; gray-colored liquid; pale orange to dark purple viscous fluid; transparent or opaque yellowish amber oil; light pink to deep red semi-solid mass; bright bluish-green vapor; and reddish-brown gas.

Nonmetallic Boron

Non-metallic boron includes boric acid H3BO3 and its salts. These materials do not conduct electricity like metals. Instead, they act much like glass. Glasses are solid solids where individual molecules bond together tightly. This makes them very strong. Some glasses are more robust than steel!

The structure of boric acid is similar to sugar. Sugar crystals consist of many small units known as monomers. Each molecule of boric acid also consists of many tiny building blocks called dimers. A dimer looks like a cross between a triangle and a square. Two dimers stick together, forming a larger unit called a trimer. Trimers then link together, making a cluster of 4 dimers. Clusters of 8 dimers become large clusters of 64 dimers. Finally, these clusters join together to create crystalline structures.

Boric acid has many applications. For example, it can be used for cleaning metal surfaces. It dissolves away rust on iron and copper pipes. It removes stains from wood furniture. And it helps remove grease from cooking pans.

There are other uses for boron besides those listed above. Some of these uses include:

  • Borax is commonly found in laundry detergent. It acts as a mild abrasive agent. It cleans clothes by removing dirt and oil without damaging fabric fibers.
  • Boracite is a mineral that occurs naturally in volcanic areas. It is sometimes mined for use in jewelry manufacturing.
  • Calcium bromide, which is made up of two atoms of bromine and three atoms of calcium, is used in photography. It reacts with silver halides to produce images. Silver chloride is added to make photographs permanent.
  • Ceramic glazes contain varying percentages of borosilicate compounds. Glaze manufacturers add boric oxide to their products because this compound improves the durability of ceramics.
  • Chlorobenzene is a solvent used in organic chemistry. It contains chlorine and benzene rings. It is toxic if inhaled.
  • Diammonium phosphate is a fertilizer containing phosphorus and ammonia. It is widely used in agriculture.

Kramer Borate deposit, Boron, Kern Co, California

Kramer Borate deposit, Boron, Kern Co, California

Different forms of Boron

Boron can be prepared in many different forms. In general, crystalline boron occurs primarily in tetragonal and rhombohedral forms. There are three well-known forms in our knowledge:

1. 𝛂-Rhombohedral (α-R)) -α-rhombohedral boron has twelve boron atoms in a unit cell. B12 icosahedra consist of five nearest neighbors for each boron atom. A conventional covalent bond would have involved each boron donating five electrons. It is believed that the B12 icosahedra are formed by so-called 3-center electron-deficient bonds in which the electron charge accumulates in the center of a triangle formed by three adjacent atoms.

The isolated B12 icosahedra are not stable; thus, boron is not a molecular solid, but strong covalent bonds connect the icosahedra in it.

2. 𝜷- Rhombohedral(β-R) – β-rhombohedral boron comprises 105–108 atoms per unit cell. Atoms form discrete icosahedra of B12, partially interpenetrated; there are also two deltahedra of B10 and a single central B atom. The 𝜷 phase has been thought to be the most thermodynamically stable allotrope for a long time, but gradually it has been determined that the 𝛂 or 𝜷 phase is the most durable. 

3. 𝜶- Tetragonal (𝜶-T) – Tetragonal boron (T-B 50) in elementary boron phases is said to be mechanically unstable due to the electron deficiency in the icosahedra. Using transmission electron microscopy (TEM) and quantum mechanics simulations, we discovered a unique distorted structure within T-B 50 that stabilizes this phase. A QM simulation confirmed that the vacancy and shear deformation is essential for forming the distorted structure observed experimentally. 

More interestingly, QM simulations suggest that the distorted structure has lower energy than the vacancy structure, while it is similar in energy to the perfect tetragonal structure. Stabilizing distorted structures is achieved by electron counting for B12 icosahedral clusters.

A pure α-tetragonal can only be synthesized as thin layers deposited on an underlying substrate of isotropic boron carbide (B50C2) or nitride (B50N2). Most examples of α-tetragonal boron are, in fact, boron-rich carbide or nitrides.

4. 𝜸-phase clusters can be characterized as NaCl-type clusters of two types, B12 icosahedra, and B2 pairs. Boron carbide can be formed by compressing other boron phases to 12–20 GPa and heating them to 1500–1800 °C while remaining stable at ambient temperatures. The lattice dynamics of the structure suggest that there are substantial long-range electrostatic interactions between B2 pairs and the B12 icosahedra.

When Wentorf reported this phase in 1965, there was no information on its structure or chemical composition. The structure was confirmed by single-crystal X-ray diffraction.

There are exceptional circumstances in which boron can be synthesized with its -tetragonal (-T) and -orthorhombic allotropes. At least 14 allotropes are reported, but they are based on tenuous evidence or are thought to be mixed allotrope. Polycrystalline-rhombohedral boron is one of the most common forms.

Borophene

Experimentally various atomically-thin, crystalline and metallic borophenes were synthesized on clean metal surfaces under ultrahigh-vacuum conditions. The atomic structure consists of mixed triangular and hexagonal motifs. The atomic structure is a consequence of an interplay between two-center and multi-center in-plane bonding, typical for electron-deficient elements like boron.

Borophenes exhibit in-plane elasticity and ideal strength. They can be stronger than graphene and more flexible in some configurations. For example, boron nanotubes have a higher 2D Young’s modulus than any other known carbon and non-carbon nanostructures. Borophenes undergo novel structural phase transitions under in-plane tensile loading due to the fluxional nature of their multi-center in-plane bonding. Borophene has potential as an anode material for batteries due to its high theoretical specific capacities, electronic conductivity, and ion transport properties. Hydrogen quickly adsorbs to borophene, offering the potential for hydrogen storage – over 15% of its weight. Borophene can catalyze the breakdown of molecular hydrogen into hydrogen ions and reduce water.

Amorphous Boron

Amorphous boron has a different structure than crystalline boron. It can be considered like glass, but it’s not that hard and brittle like glass. Instead, it flows easily when you heat or cool it. This makes it helpful in making things out of thin sheets and even jewelry with the right kind of metal clasps.

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