Chemical compound
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 antimony 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 Bi0.4Sb0.6, as a thin film of thicknesses 150–1350 Å, superconducts (the critical temperature Tc) is approximately 2 K.[3] Single crystal Bi0.935Sb0.065 can superconduct at slightly higher temperatures, and at 4.2 K, its critical magnetic field Bc (the maximum magnetic field that the superconductor can expel) of 1.6 T at 4.2 K.[7]
Semiconductor
Electron mobility is one important parameter describing semiconductors because it describes the rate at which electrons can travel through the semiconductor. At 40 K, electron mobility ranged from 4.9×105 cm2/V·s at an antimony concentration of 0 to 2.4×105 cm2/V·s at an antimony concentration of 7.2%.[1] This is much greater than the electron mobility of other common semiconductors like silicon, which is 1400 cm2/V·s 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 2×10−3 me for x = 0.11 and 9×10−4 me at x = 0.06.[2] This is much less than the electron effective mass in many common semiconductors (1.09 in Si at 300 K, 0.55 in Ge, and 0.067 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 80 K, the figure of merit (zT) for Bi1−xSbx peaks at 6.5×10−3 K−1 when x = 0.15.[4] Also, the Seebeck coefficient (the ratio of the potential difference between ends of a material to the temperature difference between the sides) at 80 K of Bi0.9Sb0.1 is −140 μV/K, much lower than the Seebeck coefficient of pure bismuth, −50 μV/K.[9]
