Given the unexpected , initial increase in resistance , we conclude that the transport properties as a function of annealing are , in the n - type regime , more strongly governed by the diminishing carrier concentration than by the carrier mobility , which generally increases upon structural ordering . The latter process , in combination with multi - band effects , explains the transition to the p - type regime characterized by higher carrier concentrations and lower resistivity . For Pb 2 Sb 2 Te 5 , the influence of two bands with different carrier types even leaves a fingerprint in the Seebeck coefficient changing its sign as a function of the measurement temperature . For PbSb 2 Te 4 , on the other hand , the changing relevance of conduction - band and valence - band tails over the course of annealing can best be tracked in tunneling measurements of the DOS . Altogether , the experiments imply that the Fermi level must shift downward with respect to the band extrema upon structural ordering . The differences between PbSb 2 Te 4 and Pb 2 Sb 2 Te 5 in the Seebeck data suggest that the two processes involvedreduction of band tails and Fermi - energy shift - concur in different ways in the two materials , with Pb 2 Sb 2 Te 5 starting already with a comparatively low Fermi energy but yet noticeable band tails .