Phase stability, structures and properties of the (Bi2)m (Bi2Te3)n natural superlattices

The phase stability of the (Bi-2)(m) (Bi2Te3)(n) natural superlattices has been investigated through the low temperature solid state synthesis of a number of new binary BixTe1-x compositions. Powder X-ray diffraction revealed that an infinitely adaptive series forms for 0.44 <= x <= 0.70, while an unusual 2-phase region with continuously changing compositions is observed for 0.41 <= x <= 0.43. For x > 0.70, mixtures of elemental Bi and an almost constant composition (Bi-2)(m) (Bi2Te3)(n) phase are observed. Rietveld analysis of synchrotron X-ray powder diffraction data collected on Bi2Te (m=2, n=1) revealed substantial interchange of Bi and Te between Bi-2 and Bi2Te3 blocks, demonstrating that the block compositions are variable. All investigated phase pure compositions are degenerate semiconductors with low residual resistivity ratios and moderate positive magnetoresistances (R/R-0 = 1.05 in 9 T). The maximum Seebeck coefficient is +80 mu V K-1 for x=0.63, leading to an estimated thermoelectric figure of merit, zT=0.2 at 250 K. (C) 2012 Elsevier Inc. All rights reserved.