Yttrium boride refers to a crystalline material composed of different proportions of
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 com ...
and
boron
Boron is a chemical element with the 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 th ...
, such as YB
2, YB
4, YB
6, YB
12, YB
25, YB
50 and YB
66. They are all gray-colored, hard solids having high melting temperatures. The most common form is the yttrium hexaboride YB
6. It exhibits
superconductivity
Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...
at relatively high temperature of 8.4 K and, similar to LaB
6, is an
electron cathode. Another remarkable yttrium boride is YB
66. It has a large lattice constant (2.344 nm), high thermal and mechanical stability, and therefore is used as a
diffraction grating
In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structura ...
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 ...
radiation (1–2 keV).
YB2 (yttrium diboride)
![Magnesium-diboride-3D-balls](https://upload.wikimedia.org/wikipedia/commons/d/d2/Magnesium-diboride-3D-balls.png)
Yttrium diboride has the same hexagonal crystal structure as
aluminium diboride
Aluminium diboride (AlB2) is a chemical compound made from the metal aluminium and the metalloid boron. It is one of two compounds of aluminium and boron, the other being AlB12, which are both commonly referred to as aluminium boride.
Structu ...
and
magnesium diboride
Magnesium diboride is the inorganic compound with the formula MgB2. It is a dark gray, water-insoluble solid. The compound has attracted attention because it becomes superconductor, superconducting at 39 K (−234 °C). In terms of its ...
– an important superconducting material. Its
Pearson symbol
The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W. B. Pearson. The symbol is made up of two letters followed by a number. For example:
* Diamond structure ...
is ''hP3'',
space group
In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...
P6/mmm (No 191), ''a'' = 0.33041 nm, ''c'' = 0.38465 nm and the calculated density is 5.05 g/cm
3. In this structure, the boron atoms form graphite like sheets with yttrium atoms between them. YB
2 crystals are unstable to moderate heating in air – they start oxidizing at 400 °C and completely oxidize at 800 °C. YB
2 melts at ~2100 °C.
YB4 (yttrium tetraboride)
![YB4structure](https://upload.wikimedia.org/wikipedia/commons/7/7e/YB4structure.jpg)
YB
4 has
tetragonal
In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square ...
crystal structure with
space group
In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...
''P4/mbm'' (No. 127), Pearson symbol ''tP20'', ''a'' = 0.711 nm, ''c'' = 0.4019 nm, calculated density 4.32 g/cm
3. High-quality YB
4 crystals of few centimeters in size can be grown by the multiple-pass
floating zone technique.
YB6 (yttrium hexaboride)
YB
6 is a black odorless powder having density of 3.67 g/cm
3; it has the same cubic crystalline structure as other hexaborides (
CaB6,
LaB6, etc., see infobox).
[ High-quality YB6 crystals of few centimeters in size can be grown by the multiple-pass floating zone technique.][ YB6 is a superconductor with the relatively high transition temperature (onset) of 8.4 K.]
YB12 (yttrium dodecaboride)
YB12 crystals have a cubic structure with density of 3.44 g/cm3, Pearson symbol
The Pearson symbol, or Pearson notation, is used in crystallography as a means of describing a crystal structure, and was originated by W. B. Pearson. The symbol is made up of two letters followed by a number. For example:
* Diamond structure ...
''cF52'', space group
In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...
''Fmm'' (No. 225), ''a'' = 0.7468 nm. Its structural unit is 12 cuboctahedron
A cuboctahedron is a polyhedron with 8 triangular faces and 6 square faces. A cuboctahedron has 12 identical vertices, with 2 triangles and 2 squares meeting at each, and 24 identical edges, each separating a triangle from a square. As such, it ...
. The Debye temperature
In thermodynamics and solid-state physics, the Debye model is a method developed by Peter Debye in 1912 for estimating the phonon contribution to the specific heat (Heat capacity) in a solid. It treats the vibrations of the atomic lattice (hea ...
of YB12 is ~1040 K, and it is not superconducting at temperatures above 2.5 K.
YB25
The structure of yttrium borides with B/Y ratio of 25 and above consists of a network of B12 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 symmetrica ...
. 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 = 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.
YB25 crystals can be grown by heating a compressed pellet of yttria
Yttrium oxide, also known as yttria, is Y2 O3. It is an air-stable, white solid substance.
The thermal conductivity of yttrium oxide is 27 W/(m·K).
Uses Phosphors
Yttria is widely used to make Eu:YVO4 and Eu:Y2O3 phosphors that give the red ...
(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.
YB50
YB50 crystals have orthorhombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...
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
Yttrium oxide, also known as yttria, is Y2 O3. It is an air-stable, white solid substance.
The thermal conductivity of yttrium oxide is 27 W/(m·K).
Uses Phosphors
Yttria is widely used to make Eu:YVO4 and Eu:Y2O3 phosphors that give the red ...
(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.
YB66
YB66 was discovered in 1960 and its structure was solved in 1969. The structure is face-centered cubic
In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals.
There are three main varieties of ...
, with space group ''Fmc'' (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
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 symmetrica ...
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.[
Another structure unit of YB66 is B80 cluster of 80 boron sites formed by the B10 to B13 sites.][ 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 8''a'' position (1/4, 1/4, 1/4); thus, there are eight such clusters (336 boron atoms) per unit cell. Two independent structure analyses ][ 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][) 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.][
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
In thermodynamics and solid-state physics, the Debye model is a method developed by Peter Debye in 1912 for estimating the phonon contribution to the specific heat (Heat capacity) in a solid. It treats the vibrations of the atomic lattice (hea ...
of 1300 K. As all yttrium borides, YB66 is a hard material and exhibits Knoop hardness
The Knoop hardness test is a microhardness test – a test for mechanical hardness used particularly for very brittle materials or thin sheets, where only a small indentation may be made for testing purposes. A pyramidal diamond point is pressed i ...
of 26 GPa.
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.
The large unit cell of YB66 results in large lattice constant of 2.344 nm.[ 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).]
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 atom ...
References
{{Yttrium compounds
Borides
Yttrium compounds