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Density functional calculations have been performed to investigate the role of Ag defects in PbTe thermoelectric materials. Ag-defects can be either donor, acceptor, or isovalent neutral defect. When Ag is heavily doped in PbTe, the neutral (Ag-Ag) dimer defect at Pb-site is formed and the environment changes to the Pb-rich/Te-poor condition. Under Pb-rich condition, the ionized Ag-interstitial defect (AgI+) becomes the major donor. The formation energy of AgI+ is smaller than other native and Ag-related defects. Also it is found that AgI+ is an effective dopant. There is no additional impurity state near the band gap and the conduction band minimum. The charge state of AgI+ defect is maintained even when the Fermi level is located above the conduction band minimum. The diffusion constant of AgI+ is calculated based on the temperature dependent Fermi level, formation energy, and migration energy. When T > 550 K, the diffusion length of Ag within a few minutes is comparable to the grain size of the polycrystalline PbTe, implying that Ag is dissolved into PbTe and this donor defect is distributed over the whole lattice in Ag-excess doped polycrystalline PbTe. The predicted solubility of AgI+ well explains the increased electron carrier concentration and electrical conductivity reported in Ag-excess doped polycrystalline PbTe at T= 450-750 K [Pei et ah, Adv. Energy Mater. 1, 291 (2011)]. In addition, we suggest that this abnormal doping behavior is also found for Au-doped PbTe. [ABSTRACT FROM AUTHOR]

We have grown Mn-doped V2VI3 single crystals using the temperature gradient solidification method. We report on the structural and magnetic properties of Mn-doped Bi2Te3, Sb2Te3, Bi2Se3, and Sb2Se3 compound semiconductors. The lattice constants of several percent Mn-doped V2VI3 were slightly smaller than those of the undoped V2VI3 due to the smaller Mn atomic radius (1.40 Å) than those of Bi (1.60 Å) and Sb (1.45 Å). Mn-doped Bi2Te3 and Sb2Te3 had ferromagnetic ordering at TC=10 and 17 K, respectively. However, Mn-doped Bi2Se3 and Sb2Se3 showed spin glass and paramagnetic properties, respectively. [ABSTRACT FROM AUTHOR]