Image courtesy of Don Morelli
Upper: In the thermoelectric alloycopper antimony selenide (Cu3SbSe4, left), Sb atoms are bonded to four neighbors and are in the pentavalent (+5) charge state; in a second alloy, Cu3SbSe3 (right), Sb atoms have three neighbors and retain an electron lone-pair. Lower: Calculations (lines) and experiments (filled circles) show that the copper antimony selenide compound with an electron lone-pair (Cu3SbSe3, red) has significantly lower lattice thermal conductivity.
Scientists demonstrated that compounds containing antimony (Sb) atoms with a “lone pair” of electrons—that is, electrons that are not involved in coordination of the Sb atom to another atom through the formation of a chemical bond—exhibit minimal lattice thermal conductivity. This intrinsic material behavior was showed computationally to be due to strong anharmonic lattice vibrations in the antimony-containing compound.
Structural defects and impurities that are often added to thermoelectric materials to reduce the material’s thermal conductivity and improve the heat-to-electricity conversion efficiency are usually not thermally stable. These results enable the design of a new class of thermoelectric materials with an intrinsically low thermal conductivity (k) “built into” a thermally stable crystallographic arrangement.
The primary challenge in designing thermoelectric materials with efficient heat-to-electricity conversion is maintaining good transmission of electrons through an atomic structure designed to have low thermal conductivity. Adding nanosized defects and impurities to the material can block the lattice vibrations responsible for heat transport, but this approach can also degrade electrical transport and may reduce thermally stability. In the search for new thermoelectric materials which have intrinsically low thermal conductivity, researchers in the Revolutionary Materials for Solid State Energy Conversion EFRC discovered a direct link between the structure and bonding of compounds containing antimony atoms and the lattice thermal conductivity. Antimony (Sb) ions can assume either a pentavalent (+5) state with coordination to four nearest neighbor atoms via bonding or a trivalent (+3) state with coordination to only three nearest neighbor atoms and the retention of two “lone-pair” electrons in one of the four Sb coordination sites. Theoretical lattice dynamics computations determined that strong anharmonicity is induced in the lattice vibrational spectrum of Sb compounds with “lone-pair” electrons. Because lattice anharmonicity drives thermal resistance in solids, these compounds and others like them that contain loan pairs should exhibit intrinsically low thermal conductivity that is “built into” the crystal structure. This was confirmed experimentally in copper antimony selenide semiconductors.
Director of the Center for Revolutionary Materials for Solid State Energy Conversion EFRC
DOE Office of Science, Office of Basic Energy Sciences, Energy Frontier Research Centers (EFRC) Program (theory and material synthesis); National Science Foundation (experiment and material synthesis)
Zhang, Yongsheng; Skoug, Eric J; Cain, Jeffrey D; Ozolins, Vidvuds; Morelli, Donald T; and Wolverton, Christopher “First-principles description of anomalously low lattice thermal conductivity in thermoelectric Cu-Sb-Se ternary semiconductors” Physical Review B, 85, 054306 (2012). [DOI: 10.1103/PhysRevB.85.054306]
Skoug, EricJ; and Morelli, Donald T “The role of lone-pair electrons in producing minimum thermal conductivity in Group VA chalcogenide compounds” Physical Review Letters, 107, 235901 (2011). [DOI: 10.1103/PhysRevLett.107.235901]
Center for Revolutionary Materials for Solid State Energy Conversion EFRC
Collaborations, Non-DOE Interagency Collaboration