Yttrium borides

Last updated
Yttrium borides
CaHexaboride.png
Identifiers
3D model (JSmol)
ChemSpider
PubChem CID
  • YB2:InChI=1S/B2.Y/c1-2;
    Key: YOSZWMPJVDQOCZ-UHFFFAOYSA-N
  • YB6:InChI=1S/6B.Y
    Key: LBQSRCUUAYSSDR-UHFFFAOYSA-N
  • YB2:[B][B].[Y]
  • YB6:[B].[B].[B].[B].[B].[B].[Y]
Properties
YB66/YB50/YB25/YB12/YB6/YB4
Molar mass 153.77
AppearanceGray-Black powder, Metallic
Density 2.52 g/cm3 --- YB66
2.72 g/cm3 --- YB50
3.02 g/cm3 --- YB25
3.44 g/cm3 --- YB12
3.67 g/cm3 --- YB6
4.32 g/cm3 --- YB4
Melting point 2,750–2,000 [1]  °C (4,980–3,630 °F; 3,020–2,270 K)
Insoluble
Structure
cubic, cP7
Pm3m, No. 221 [2]
a = 0.41132 nm [2]
Hazards
NFPA 704 (fire diamond)
NFPA 704.svgHealth 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g. sodium chlorideFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
0
0
Flash point Non-flammable
Safety data sheet (SDS) External MSDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

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. It exhibits superconductivity at relatively 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 (2.344 nm), high thermal and mechanical stability, and therefore is used as a diffraction grating for low-energy synchrotron radiation (1–2 keV).

Contents

YB2 (yttrium diboride)

Structure of YB2 Magnesium-diboride-3D-balls.png
Structure of YB2

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 (No 191), a = 0.33041 nm, c = 0.38465 nm and the calculated density is 5.05 g/cm3. [3] 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 completely oxidize at 800 °C. [4] YB2 melts at ~2100 °C. [5]

YB4 (yttrium tetraboride)

Structure of YB4 YB4structure.jpg
Structure of YB4

YB4 has tetragonal crystal structure with space group P4/mbm (No. 127), Pearson symbol tP20, a = 0.711 nm, c = 0.4019 nm, calculated density 4.32 g/cm3. [6] High-quality YB4 crystals of few centimeters in size can be grown by the multiple-pass floating zone technique. [7]

YB6 (yttrium hexaboride)

YB6 is a black odorless powder having density of 3.67 g/cm3; it has the same cubic crystalline structure as other hexaborides (CaB6, LaB6, etc., see infobox). [2] High-quality YB6 crystals of few centimeters in size can be grown by the multiple-pass floating zone technique. [7] [8] YB6 is a superconductor with the relatively high transition temperature (onset) of 8.4 K. [8] [9]

YB12 (yttrium dodecaboride)

YB12 crystals have a cubic structure with density of 3.44 g/cm3, Pearson symbol cF52, space group Fm3m (No. 225), a = 0.7468 nm. [10] Its structural unit is 12 cuboctahedron. The Debye temperature of YB12 is ~1040 K, and it is not superconducting at temperatures above 2.5 K. [11]

YB25

Crystal structure of YB25. Black and green spheres indicate Y and B atoms, respectively. YB25structure.png
Crystal structure of YB25. Black and green spheres indicate Y and B atoms, respectively.

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 ca. 60–70%, and the YB25 formula merely reflects the average atomic ratio [B]/[Y] = 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 (101) crystal plane (x-z plane in the figure). The bonding distance between the bridging boron and the equatorial boron atoms is 0.1755 nm, which is typical for the strong covalent B-B bond (bond length 0.17–0.18 nm); thus, the bridging boron atoms strengthen the individual network planes. On the other hand, the large distance between the boron atoms within the bridge (0.2041 nm) reveals a weaker interaction, and thus the bridging sites contribute little to the bonding between the network planes. [12] [13]

YB25 crystals can be grown by heating a compressed pellet of yttria (Y2O3) and boron powder to ~1700 °C. The YB25 phase is stable up to 1850 °C. Above this temperature it decomposes into YB12 and YB66 without melting. This makes it difficult to grow a single crystal of YB25 by the melt growth method. [12]

YB50

YB50 crystals have orthorhombic structure with space group P21212 (No. 18), a = 1.66251 nm, b = 1.76198 nm, c = 0.94797 nm. They can be grown by heating a compressed pellet of yttria (Y2O3) and boron powder to ~1700 0C. Above this temperature YB50 decomposes into YB12 and 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 also crystallize in the M50 form. [14]

YB66

Two single crystals of YB66 grown by floating zone technique using (100) oriented seeds. In the top crystal, the seed (left from the black line) has same diameter as the crystal. In the bottom crystal (sliced), the seed is much thinner and is on the right Yb66crystal.jpg
Two single crystals of YB66 grown by floating zone technique using (100) oriented seeds. In the top crystal, the seed (left from the black line) has same diameter as the crystal. In the bottom crystal (sliced), the seed is much thinner and is on the right
Borfig4a.png
Borfig4b.png
(a) Thirteen-icosahedron unit (B12)12B12 (supericosahedron), and (b) B80 cluster unit of the YB66 structure. The excessive bonding in panel (b) is because it assumes that all sites are occupied, whereas the total number of boron atoms is only 42. [15]
The boron framework of YB66 viewed along the z-axis. Borfig5a.png
The boron framework of YB66 viewed along the z-axis.
Borfig5b.png
Borfig6.png
Left: Schematically drawn boron framework of YB66. Light green spheres show the boron supericosahedra and their relative orientations are indicated by arrows. Dark green spheres correspond to the B80 clusters. Right: Pair of Y sites (pink spheres) in YB66. Light green spheres show the boron supericosahedron and dark green spheres correspond to the B80 clusters. [15]

YB66 was discovered in 1960 [17] and its structure was solved in 1969. [16] The structure is face-centered cubic, with space group Fm3c (No. 226), Pearson symbol cF1936 and lattice constant a = 2.3440(6) nm. There are 13 boron sites B1–B13 and one yttrium site. The B1 sites form one B12 icosahedron and the B2–B9 sites make up another icosahedron. These icosahedra arrange in a thirteen-icosahedron unit (B12)12B12 which is 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, which is rotated by 90°, is located at the center of the cell and at the cell edges. Thus, there are eight supericosahedra (1248 boron atoms) in the unit cell. [15]

Another structure unit of YB66 is B80 cluster of 80 boron sites formed by the B10 to B13 sites. [15] All those 80 sites are partially 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 (1/4, 1/4, 1/4); thus, there are eight such clusters (336 boron atoms) per unit cell. Two independent structure analyses [15] [16] 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, and the B80 clusters are depicted by light green and dark green spheres, respectively; at the top surface of the unit cell, the relative orientations of the supericosahedra are indicated by arrows. There are 48 yttrium sites ((0.0563, 1/4, 1/4) for YB62 [15] ) 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; this occupancy of 0.5 implies that the yttrium pair has always one Y atom with one empty site. [16]

YB66 has density 2.52 g/cm3, low thermal conductivity of 0.02 W/(cm·K), elastic constants c11 = 3.8×109 and c44 = 1.6×109 Newton/m2 and Debye temperature of 1300 K. [18] As all yttrium borides, YB66 is a hard material and exhibits Knoop hardness of 26 GPa. [19] High-quality YB66 crystals of few centimeters in size can be grown by the multiple-pass floating zone technique and be used as X-ray monochromators. [20]

The large unit cell of YB66 results in large lattice constant of 2.344 nm. [18] This property, together with high thermal and mechanical stability resulted in application of YB66 as dispersive elements of X-ray monochromators for low energy radiation (1–2 keV). [21] [22]

See also

Related Research Articles

<span class="mw-page-title-main">Boron</span> Chemical element, symbol B and atomic number 5

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 valence electrons for forming covalent bonds, resulting in many compounds such as boric acid, the mineral sodium borate, and the ultra-hard crystals of boron carbide and boron nitride.

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 is also sometimes considered a lanthanide, despite being a d-block element and a transition metal.

<span class="mw-page-title-main">Boron carbide</span> Extremely hard ceramic compound

Boron carbide (chemical formula approximately B4C) is an extremely hard boron–carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders, as well as numerous industrial applications. With a Vickers hardness of >30 GPa, it is one of the hardest known materials, behind cubic boron nitride and diamond.

<span class="mw-page-title-main">Superhard material</span> Material with Vickers hardness exceeding 40 gigapascals

A superhard material is a material with a hardness value exceeding 40 gigapascals (GPa) when measured by the Vickers hardness test. They are virtually incompressible solids with high electron density and high bond covalency. As a result of their unique properties, these materials are of great interest in many industrial areas including, but not limited to, abrasives, polishing and cutting tools, disc brakes, and wear-resistant and protective coatings.

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.

<span class="mw-page-title-main">Boron arsenide</span> Chemical compound

Boron arsenide is a chemical compound involving boron and arsenic, usually with a chemical formula BAs. Other boron arsenide compounds are known, such as the subarsenide B12As2. Chemical synthesis of cubic BAs is very challenging and its single crystal forms usually have defects.

Lutetium aluminum garnet (commonly abbreviated LuAG, molecular formula Lu3Al5O12) is an inorganic compound with a unique crystal structure primarily known for its use in high-efficiency laser devices. LuAG is also useful in the synthesis of transparent ceramics.

<span class="mw-page-title-main">Rhenium diboride</span> Chemical compound

Rhenium diboride (ReB2) is a synthetic high-hardness material that was first synthesized in 1962. The compound is formed from a mixture of rhenium, noted for its resistance to high pressure, and boron, which forms short, strong covalent bonds with rhenium. It has regained popularity in recent times in hopes of finding a material that possesses hardness comparable to that of diamond.

<span class="mw-page-title-main">Boron suboxide</span> Chemical compound

Boron suboxide (chemical formula B6O) is a solid compound with a structure built of eight icosahedra at the apexes of the rhombohedral unit cell. Each icosahedron is composed of twelve boron atoms. Two oxygen atoms are located in the interstices along the [111] rhombohedral direction. Due to its short interatomic bond lengths and strongly covalent character, B6O displays a range of outstanding physical and chemical properties such as great hardness (close to that of rhenium diboride and boron nitride), low mass density, high thermal conductivity, high chemical inertness, and excellent wear resistance.

Aluminium magnesium boride or Al3Mg3B56, colloquially known as BAM, is a chemical compound of aluminium, magnesium and boron. Whereas its nominal formula is AlMgB14, the chemical composition is closer to Al0.75Mg0.75B14. It is a ceramic alloy that is highly resistive to wear and has an extremely low coefficient of sliding friction, reaching a record value of 0.04 in unlubricated and 0.02 in lubricated AlMgB14−TiB2 composites. First reported in 1970, BAM has an orthorhombic structure with four icosahedral B12 units per unit cell. This ultrahard material has a coefficient of thermal expansion comparable to that of other widely used materials such as steel and concrete.

<span class="mw-page-title-main">Tungsten borides</span>

Tungsten borides are compounds of tungsten and boron. Their most remarkable property is high hardness. The Vickers hardness of WB or WB2 crystals is ~20 GPa and that of WB4 is ~30 GPa for loads exceeding 3 N.

<span class="mw-page-title-main">Allotropes of boron</span> Materials made only out of boron

Boron can be prepared in several crystalline and amorphous forms. Well known crystalline forms are α-rhombohedral (α-R), β-rhombohedral (β-R), and β-tetragonal (β-T). In special circumstances, boron can also be synthesized in the form of its α-tetragonal (α-T) and γ-orthorhombic (γ) allotropes. Two amorphous forms, one a finely divided powder and the other a glassy solid, are also known. Although at least 14 more allotropes have been reported, these other forms are based on tenuous evidence or have not been experimentally confirmed, or are thought to represent mixed allotropes, or boron frameworks stabilized by impurities. Whereas the β-rhombohedral phase is the most stable and the others are metastable, the transformation rate is negligible at room temperature, and thus all five phases can exist at ambient conditions. Amorphous powder boron and polycrystalline β-rhombohedral boron are the most common forms. The latter allotrope is a very hard grey material, about ten percent lighter than aluminium and with a melting point (2080 °C) several hundred degrees higher than that of steel.

This article contains crystal structure data used in the article crystal structure of boron-rich metal borides.

<span class="mw-page-title-main">Crystal structure of boron-rich metal borides</span> Boron chemical complexes

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 form B12 icosahedra which are linked into a three-dimensional boron framework, and the metal atoms reside in the voids of this framework. Those icosahedra are basic structural units of most allotropes of boron and boron-rich rare-earth borides. In such borides, metal atoms donate electrons to the boron polyhedra, and thus these compounds are regarded as electron-deficient solids.

Silicon borides (also known as boron silicides) 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 (n = 14, 15, 40, etc.). The n = 3 and n = 6 phases were reported as being co-produced together as a mixture for the first time by Henri Moissan and Alfred Stock in 1900 by briefly heating silicon and boron in a clay vessel. The tetraboride was first reported as being synthesized directly from the elements in 1960 by three independent groups: Carl Cline and Donald Sands; Ervin Colton; and Cyrill Brosset and Bengt Magnusson. It has been proposed that the triboride is a silicon-rich version of the tetraboride. Hence, the stoichiometry of either compound could be expressed as SiB4 - x where x = 0 or 1. All the silicon borides are black, crystalline materials of similar density: 2.52 and 2.47 g cm−3, respectively, for the n = 3(4) and 6 compounds. On the Mohs scale of mineral hardness, SiB4 - x and SiB6 are intermediate between diamond (10) and ruby (9). The silicon borides may be grown from boron-saturated silicon in either the solid or liquid state.

In chemistry, the Jemmis mno rules represent a unified rule for predicting and systematizing structures of compounds, usually clusters. The rules involve electron counting. They were formulated by E. D. Jemmis to explain the structures of condensed polyhedral boranes such as B20H16, which are obtained by condensing polyhedral boranes by sharing a triangular face, an edge, a single vertex, or four vertices. These rules are additions and extensions to Wade's rules and polyhedral skeletal electron pair theory. The Jemmis mno rule provides the relationship between polyhedral boranes, condensed polyhedral boranes, and β-rhombohedral boron. This is similar to the relationship between benzene, condensed benzenoid aromatics, and graphite, shown by Hückel's 4n + 2 rule, as well as the relationship between tetracoordinate tetrahedral carbon compounds and diamond. The Jemmis mno rules reduce to Hückel's rule when restricted to two dimensions and reduce to Wade's rules when restricted to one polyhedron.

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<span class="mw-page-title-main">Iron boride</span> Chemical compound

Iron boride refers to various inorganic compounds with the formula FexBy. Two main iron borides are FeB and Fe2B. Some iron borides possess useful properties such as magnetism, electrical conductivity, corrosion resistance and extreme hardness. Some iron borides have found use as hardening coatings for iron. Iron borides have properties of ceramics such as high hardness, and properties of metal properties, such as thermal conductivity and electrical conductivity. Boride coatings on iron are superior mechanical, frictional, and anti-corrosive. Iron monoboride (FeB) is a grey powder that is insoluble in water. FeB is harder than Fe2B, but is more brittle and more easily fractured upon impact.

<span class="mw-page-title-main">Niobium diboride</span> Chemical compound

Niobium diboride (NbB2) is a highly covalent refractory ceramic material with a hexagonal crystal structure.

An yttrium compound is a chemical compound containing yttrium. Among these compounds, yttrium generally has a +3 valence. The solubility properties of yttrium compounds are similar to those of the lanthanides. For example oxalates and carbonates are hardly soluble in water, but soluble in excess oxalate or carbonate solutions as complexes are formed. Sulfates and double sulfates are generally soluble. They resemble the "yttrium group" of heavy lanthanide elements.

References

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