A15 phases

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A series of intermetallic compounds which have a particular crystal structure and the chemical formula A₃B, where A represents a transition element and B can be either a transition or a nontransition element. Many A15 compounds exhibit the phenomenon of superconductivity at relatively high temperatures in the neighborhood of 20 K (−424°F) and in high magnetic fields on the order of several tens of teslas (several hundred kilogauss). High-temperature–high-field superconductivity has a number of important technological applications and is a challenging fundamental research area in condensed-matter physics. See also Superconductivity.

The A15 compounds crystallize in a structure in which the unit cell, the repeating unit of the crystal structure, has the overall shape of a cube. The B atoms are located at the corners and in the center of the cube, while the A atoms are arranged in pairs on the cube faces (see illustration). A special characteristic of the A15 crystal structure is that the A atoms form mutually orthogonal linear chains that run throughout the crystal lattice, as shown in the illustration. The extraordinary superconducting properties of the A15 compounds are believed to be primarily associated with these linear chains of transition-element A atoms. See also Crystal.

A15 crystal structure of the A₃B intermetallic compounds. The light spheres represent the A atoms; the dark spheres represent the B atoms. The linear chains of A atoms are emphasized.

Processes have been developed for preparing multifilamentary superconducting wires that consist of numerous filaments of a superconducting A15 compound, such as Nb₃Sn, embedded in a nonsuperconducting copper matrix. Superconducting wires can be used in electric power transmission lines and to wind electrically lossless coils (solenoids) for superconducting electrical machinery (motors and generators) and magnets. Superconducting magnets are employed to produce intense magnetic fields for laboratory research, confinement of high-temperature plasmas in nuclear fusion research, bending beams of charged particles in accelerators, levitation of highspeed trains, mineral separation, and energy storage. See also Superconducting devices.

Unit cell of the A15 phases of Nb₃Sn

The A15 phases (also known as β-W or Cr₃Si structure types) are series of intermetallic compounds with the chemical formula A₃B (where A is a transition metal and B can be any element) and a specific structure. Many of these compounds have superconductivity at around 20 K (−424 °F), which is comparatively high, and remain superconductive in magnetic fields of tens of teslas (hundreds of kilogauss). This kind of superconductivity (Type-II superconductivity) is an important area of study as it has several practical applications.

Contents 1 History 2 References 3 Further reading 4 Examples

History

The first time the A15 structure was observed in 1931 when an electrolytically deposited layer of tungsten was examined.^[1] The discussion if the β–tungsten structure is an allotrope of tungsten or a the structure of a tungsten suboxide was long-standing and in 1998 still articles about the discussion were published. In the end it seems most likely that the material is a true allotrope of tungsten.^[2]

The first inter-metallic compound with the typical A₃B composition was the chromium silicide Cr₃Si, discovered in 1933.^[3] Several other compounds of the A15 structure were discovered in the following years. No large interest existed in the research on those compounds. This changed with the discovery that Vanadium silicide V₃Si showed superconductivity at around 17 K in 1953.^[4] In the following years several other A₃B superconductors were found.^[5] Niobium-germanium held the record for the highest temperature of 23.2 K from 1971 till the discovery of the cuprate superconductors in 1986. It took time before the method to produce wires from the very brittle A15 phase materials was established. This method is still complicated. Though some A15 phase materials can withstand higher magnetic field intensity and have higher critical temperatures than the NbZr and NbTi alloys, NbTi is still used for most of the applications due to the easier manufacturing.^[6] Nb₃Sn is used for some high field applications, for example high-end MRI scanners and NMR spectrometers.

The Voronoi diagram of the A15 phase is known to have the least surface area among point sets in three-dimensional Euclidean space. This partition, also known as the Weaire-Phelan structure, is often present in clathrate hydrates.

References

1. ^ Hartmann, Hellmuth; Ebert, Fritz; Bretschneider, Otto (1931). "Elektrolysen in Phosphatschmelzen. I. Die elektrolytische Gewinnung von α- und ?β-Wolfram". Zeitschrift für anorganische und allgemeine Chemie 198: 116. doi:10.1002/zaac.19311980111. 
2. ^ Kiss, A. B. (1998). Journal of Thermal Analysis and Calorimetry 54 (3): 815. doi:10.1023/A:1010143904328. 
3. ^ Boren, B. (1933). "X-Ray Investigation of Alloys of Silicon with Chromium, Manganese, Cobalt and Nickel". Ark. Kern., Min. Geol 11A (10): 2–10. 
4. ^ Hardy, George; Hulm, John (1953). "Superconducting Silicides and Germanides". Physical Review 89 (4): 884. Bibcode 1953PhRv...89Q.884H. doi:10.1103/PhysRev.89.884. 
5. ^ Izyumov, Yurii A; Kurmaev, Z Z (1974). "Physical properties and electronic structure of superconducting compounds with theβ-tungsten structure". Soviet Physics Uspekhi 17 (3): 356. Bibcode 1974SvPhU..17..356I. doi:10.1070/PU1974v017n03ABEH004136. 
6. ^ Sheahen, Thomas P (1994). Introduction to high-temperature superconductivity. p. 32. ISBN 978-0-306-44793-8. http://books.google.de/books?id=dRhQZBqjT14C&pg=PA32. 

Further reading

• Graef, Marc De; McHenry, Michael E (2007). Structure of materials: an introduction to crystallography, diffraction and symmetry. pp. 518–521. ISBN 978-0-521-65151-6. http://books.google.de/books?id=nJHSqEseuIUC&pg=PA518. 
• Sauthoff, G (1995). Intermetallics. pp. 95–97. ISBN 978-3-527-29320-9. http://books.google.de/books?id=ORV48cYEPMYC&pg=PA95. 

Examples

• Vanadium-silicon
• Vanadium-gallium
• Niobium-germanium
• Niobium-tin

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