Structure inhomogeneities, shallow defects, and charge transport in the series of thermoelectric materials K₂Bi_8-xSb _xSe₁₃

The charge transport properties of the low-dimensional thermoelectric materials K2 Bi8-x Sbx Se13 (0<x≤8) were studied as a function of temperature and composition. The Seebeck coefficient shows an evolution from n - to p -type character with increasing incorporation of Sb, and at the same time the temperature dependence of the electrical conductivity changes from that of a degenerate semiconductor to that of an intrinsic or compensated semiconductor. These changes, however, are not monotonic with composition due to the nonuniform substitution of Sb atoms at the Bi sites of the structure. Three separate composition regions can be assigned depending on x each with different charge transport characters. Electronic transport in K2 Bi8-x Sbx Se13 was analyzed on the basis of the classical semiconductor theory and discussed in the context of recent band calculations. The results suggest that the K2 Bi8-x Sbx Se13 materials possess coexisting domains with semimetallic and semiconducting characters whose ratio is influenced by the value of x and by local defects. The extent and relative distribution of these domains control the charge transport properties. Electron diffraction experiments performed on samples of K2 Bi8-x Sbx Se13 with x=1.6 show evidence for such domains by indicating regions with long range ordering of K+ Bi3+ atoms and regions with increased disorder. The semiconducting behavior is enhanced with increasing x (i.e., SbBi ratio) in the composition through a decrease of the semimetallic fraction.