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Bismuth antimonides, Bismuth-antimonys, or Bismuth-antimony alloys, (Bi1−xSbx) are binary alloys of bismuth and antimony in various ratios.

Some, in particular Bi0.9Sb0.1, were the first experimentally-observed three-dimensional topological insulators, materials that have conducting surface states but have an insulating interior.[2]

Various BiSb alloys also superconduct at low temperatures,[3] are semiconductors,[1] and are used in thermoelectric devices.[4]

Bismuth antimonide itself (see box to right) is sometimes described as Bi2Sb2.[5]

Synthesis

Crystals of bismuth antimonides are synthesized by melting bismuth and antimony together under inert gas or vacuum. Zone melting is used to decrease the concentration of impurities.[4] When synthesizing single crystals of bismuth antimonides, it is important that impurities are removed from the samples, as oxidation occurring at the impurities leads to polycrystalline growth.[1]

Properties

Topological Insulator

Pure bismuth is a semimetal, containing a small band gap, which leads to it having a relatively high conductivity (7.7*105 S/m at 20 °C). When the bismuth is doped with antimony, the conduction band decreases in energy and the valence band increases in energy. At an Sb concentration of 4%, the two bands intersect, forming a Dirac point[2] (which is defined as a point where the conduction and valence bands intersect). Further increases in the concentration of antimony result in a band inversion, in which the energy of the valence band becomes greater than that of the conduction band at specific momenta. Between Sb concentrations of 7 and 22%, the bands no longer intersect, and the Bi1−xSbx becomes an inverted-band insulator.[6] It is at these higher concentrations of Sb that the band gap in the surface states vanishes, and the material thus conducts at its surface.[2]

Superconductor

The highest temperatures at which Bi.4Sb.6 thin film of thicknesses 150-1350A superconduct, the critical temperature Tc, is approximately 2K.[3] Single crystal Bi.935Sb.065 can superconduct at slightly higher temperatures, and at 4.2K, its critical magnetic field Bc (the maximum magnetic field that the superconductor can expel) of 1.6T at 4.2K.[7]

Semiconductor

Electron mobility is one important parameter describing semiconductors because it describes the rate at which electrons can travel through the semiconductor. At 40K, electron mobility ranged from 0.49*106 cm2/Vs at an Sb concentration of 0 to .24*106 cm2/Vs at a Sb concentration of 7.2%.[1] This is much greater than the electron mobility of other common semiconductors like Si, which is 1400 cm2/Vs at room temperature.[8]

Another important parameter of Bi1−xSbx is the effective electron mass (EEM), a measure of the ratio of the acceleration of an electron to the force applied to an electron. The effective electron mass is .002me for x=.11 and .0009me at x=.06.[2] This is much less than the electron effective mass in many common semiconductors (1.09 in Si at 300K, .55 in Ge, and .067 in GaAs). A low EEM is good for Thermophotovoltaic applications.

Thermoelectric

Bismuth antimonides are used as the n-type legs in many thermoelectric devices below room temperature. The thermoelectric efficiency, given by its figure of merit zT = σS2T/λ, where S is the Seebeck coefficient, λ is the thermal conductivity, and σ is the electrical conductivity, describes the ratio of the energy provided by the thermoelectric to the heat absorbed by the device. At 80K, the figure of merit (zT) for Bi1−xSbx peaks at 6.5*10−3/K when x = 15%.[4] Also, The Seebeck coefficient (the ratio of the potential difference between ends of a material to the temperature difference between the sides) at 80K of Bi.9Sb.1 is -140μV/K, much smaller than the Seebeck coefficient of pure bismuth, -50μV/K.[9]

References

  1. ^ a b c d Jain, A. L. "Temperature Dependence of the Electrical Properties of Bismuth-Antimony Alloys". Physical Review. 114 (6): 1518–1528. doi:10.1103/physrev.114.1518. 
  2. ^ a b c d Hsieh, D.; Qian, D.; Wray, L.; Xia, Y.; Hor, Y. S.; Cava, R. J.; Hasan, M. Z. (2008-04-24). "A topological Dirac insulator in a quantum spin Hall phase". Nature. 452 (7190): 970–974. doi:10.1038/nature06843. ISSN 0028-0836. PMID 18432240. 
  3. ^ a b Zally, G. D.; Mochel, J. M. "Fluctuation Heat Capacity in Superconducting Thin Films of Amorphous BiSb". Physical Review Letters. 27 (25): 1710–1712. doi:10.1103/physrevlett.27.1710. 
  4. ^ a b c Smith, G. E.; Wolfe, R. (1962-03-01). "Thermoelectric Properties of Bismuth‐Antimony Alloys". Journal of Applied Physics. 33 (3): 841–846. doi:10.1063/1.1777178. ISSN 0021-8979. 
  5. ^ Bismuth Antimonide
  6. ^ "Phase transition between the quantum spin Hall and insulator phases in 3D: emergence of a topological gapless phase". New Journal of Physics. 9: 356. doi:10.1088/1367-2630/9/9/356. 
  7. ^ Kasumov, A. Yu.; Kononenko, O. V.; Matveev, V. N.; Borsenko, T. B.; Tulin, V. A.; Vdovin, E. E.; Khodos, I. I. "Anomalous Proximity Effect in the Nb-BiSb-Nb Junctions". Physical Review Letters. 77 (14): 3029–3032. doi:10.1103/physrevlett.77.3029. PMID 10062113. 
  8. ^ "Electrical properties of Silicon (Si)". www.ioffe.rssi.ru. Retrieved 2015-12-11. 
  9. ^ Goldsmid, H. J. (1970-01-16). "Bismuth–antimony alloys". physica status solidi (a). 1 (1): 7–28. doi:10.1002/pssa.19700010102. ISSN 1521-396X.