Khang Hoang
Materials Department, University of California, Santa Barbara,
California 93106, USA
S. D. Mahanti
Department of Physics and Astronomy, Michigan State University, East
Lansing, Michigan 48824, USA
Mercouri G. Kanatzidis
Department of Chemistry, Northwestern University, Evanston, Illinois
60208, USA
Materials Science Division, Argonne National Laboratory, Argonne,
Illinois 60439, USA
Complex multicomponent systems based on PbTe, SnTe, and GeTe are of
great interest for infrared devices and high-temperature thermoelectric
applications. A deeper understanding of the atomic and electronic
structure of these materials is crucial for explaining, predicting,
and optimizing their properties, and to suggest new materials for better
performance. In this work, we present our first-principles studies of
the energy bands associated with various monovalent (Na, K, Ag) and
trivalent (Sb, Bi) impurities and impurity clusters in PbTe, SnTe, and
GeTe using supercell models. We find that monovalent and trivalent
impurity atoms tend to come close to one another and form impurity-rich
clusters, and the electronic structure of the host materials is strongly
perturbed by the impurities. There are impurity-induced bands associated
with the trivalent impurities that split off from the conduction-band
bottom with large shifts towards the valence-band top. This is due to
strong hybridization between the p states of the trivalent impurity
cations (Sb, Bi) and the divalent anion (Te) which tends to drive the
systems towards metallicity. The introduction of the monovalent
impurities (in the presence of the trivalent impurities) to PbTe,
SnTe, and GeTe significantly reduces (in PbTe and GeTe) or slightly
enhances (in SnTe) the effect of the trivalent impurities. One,
therefore, can tailor the band gap and band structure near the band
gap (hence transport properties) by choosing the type of impurity and
its concentration or tuning the monovalent/trivalent ratio. Based on the
calculated band structures, we are able to explain qualitatively the
measured transport properties of the whole class of PbTe-, SnTe-, and
GeTe-based bulk thermoelectrics.