The A15 phases (also known as β-W or Cr₃Si structure types) are series of

intermetallic compounds An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic eleme ...

with the chemical formula ''A''₃''B'' (where A is a

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that ca ...

and B can be any element) and a specific structure. The A15 phase is also one of the members in the

Frank–Kasper phases Topologically close pack (TCP) phases, also known as Frank-Kasper (FK) phases, are one of the largest groups of intermetallic compounds, known for their complex crystallographic structure and physical properties. Owing to their combination of ...

family. Many of these compounds have superconductivity at around , which is comparatively high, and remain superconductive in magnetic fields of tens of teslas (hundreds of

kilogauss The gauss, symbol (sometimes Gs), is a unit of measurement of magnetic induction, also known as ''magnetic flux density''. The unit is part of the Gaussian system of units, which inherited it from the older CGS-EMU system. It was named after th ...

). This kind of superconductivity ( Type-II superconductivity) is an important area of study as it has several practical applications.

History

The first time that A15 structure was observed was in 1931 when an electrolytically deposited layer of

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

was examined. Discussion of whether the β-tungsten structure is an allotrope of tungsten or the structure of a tungsten suboxide was long-standing, but since the 1950s there has been many publications showing that the material is a true allotrope of tungsten. The first

intermetallic An intermetallic (also called an intermetallic compound, intermetallic alloy, ordered intermetallic alloy, and a long-range-ordered alloy) is a type of metallic alloy that forms an ordered solid-state compound between two or more metallic eleme ...

compound discovered with typical A₃B composition was chromium silicide Cr₃Si, discovered in 1933. Several other compounds with A15 structure were discovered in following years. No large interest existed in research on those compounds. This changed with the discovery that vanadium silicide V₃Si showed superconductivity at around 17 K in 1953. In following years, several other A₃B superconductors were found. Niobium-germanium held the record for the highest temperature of 23.2 K from 1973 until the discovery of the

cuprate superconductor Cuprate superconductors are a family of high-temperature superconducting materials made of layers of copper oxides (CuO2) alternating with layers of other metal oxides, which act as charge reservoirs. At ambient pressure, cuprate superconductor ...

s in 1986. It took time for the method to produce wires from the very brittle A15 phase materials to be established. This method is still complicated. Though some A15 phase materials can withstand higher

magnetic field intensity A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

and have higher critical temperatures than the NbZr and NbTi alloys, NbTi is still used for most applications due to easier manufacturing. Nb₃Sn is used for some high field applications, for example high-end MRI scanners and NMR spectrometers. A relaxed form of the

Voronoi diagram In mathematics, a Voronoi diagram is a partition of a plane into regions close to each of a given set of objects. In the simplest case, these objects are just finitely many points in the plane (called seeds, sites, or generators). For each seed ...

of the A15 phase seems to have the least surface area among all the possible partitions of three-dimensional Euclidean space in regions of equal volume. This partition, also known as the

Weaire–Phelan structure In geometry, the Weaire–Phelan structure is a three-dimensional structure representing an idealised foam of equal-sized bubbles, with two different shapes. In 1993, Denis Weaire and Robert Phelan found that this structure was a better solution ...

, is often present in clathrate hydrates.

Examples

* Vanadium-silicon * Vanadium-gallium * Niobium-germanium * Niobium-tin * Titanium-gold *

Tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

(β-phase)

History

Examples

References

Further reading