Abstract
The properties of bismuth triselenide (Bi2Se3) are already known to a certain extent through the work of several authors, while it was still an open question whether there exists an individual solid phase of BiSe. Further information on this subject could be obtained by the successful growth and investigation of single crystals of both Bi2Se3 and Bi2Se2. X-ray analysis by means of goniometry, Weisenberg, Laue, and Debye-Scherrer diagrams confirmed the known crystal structure of Bi2Se3 (ditrigonal scalenohedral;D 3 5 −R—m; with the hexagonal axes:a=4·15 A andc=28·55 A, and 3 molecules per unit cell). As to Bi2Se2 it can be shown that it belongs to the same class but to a different space group (D 3 1 −P— 1m orD 3 3 −P—m 1; hexagonal axes:a=4·15 A,c=22·84 A, unit cell: 3 molecules, if the formula Bi2Se2 is adopted). Common to both is a subcell with the dimensions:a′=a=4·15 A andc′=5·71 A. The temperature dependence of electrical conductivity and Hall coefficient was measured on several specimens having different crystal orientations. The most striking difference is the high anisotropy of Bi2Se3 (σ a σ c =10) as compared with Bi2Se2 (σ a /σ c <2). All specimens turned out to ben-type. The room temperature carrier concentration observed was:n (Bi2Se3)=8·1018 cm−3 andn (Bi2Se2)=4·1020 cm−3, the carrier mobility:μ(Bi2Se3)=2·103 cm2/V·s andμ(Bi2Se3)=20 cm2/V·s.
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