Phase Stability, Structures and Properties of the (Bi2)m(Bi2Te3)n Natural Superlattices

TL;DR

This study investigates the phase stability, structures, and thermoelectric properties of (Bi2)m(Bi2Te3)n natural superlattices, revealing adaptive series, phase regions, and variable block compositions with potential thermoelectric applications.

Contribution

It provides new insights into the phase behavior, structural variability, and thermoelectric performance of (Bi2)m(Bi2Te3)n superlattices through synthesis and detailed analysis.

Findings

Adaptive series form for 0.44 < x < 0.70

Variable Bi/Te interchange in blocks

Maximum Seebeck coefficient of +80 μV/K at x=0.63

Abstract

The phase stability of the (Bi2)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 (Bi2)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 the Bi2 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/R0 = 1.05 in 9 T). The maximum Seebeck coefficient is +80 muV K-1 for x = 0.63, leading to an estimated thermoelectric figure of merit, zT = 0.2 at 250 K.