Soft Phonon Modes Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance in AgCuTe

• Published in: Angewandte Chemie International Edition Vol. 57; no. 15; pp. 4043 - 4047
• Main Authors: Roychowdhury, Subhajit, Jana, Manoj K., Pan, Jaysree, Guin, Satya N., Sanyal, Dirtha, Waghmare, Umesh V., Biswas, Kanishka
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 03.04.2018
• Edition: International ed. in English
• Subjects: AgCuTe; Cations; Crystal structure; Crystallinity; Heat conductivity; Heat transfer; Hexagonal phase; High temperature; Lattice vibration; Power factor; soft phonons; Temperature; Temperature effects; Thermal conductivity; thermoelectric figure of merit; Thermoelectric materials; thermoelectrics; ultralow thermal conductivity; Vibrations; ultralow thermal conductivity; soft phonons; thermoelectric figure of merit; thermoelectrics; AgCuTe
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.201801491

Crystalline solids with intrinsically low lattice thermal conductivity (κL) are crucial to realizing high‐performance thermoelectric (TE) materials. Herein, we show an ultralow κL of 0.35 Wm−1 K−1 in AgCuTe, which has a remarkable TE figure‐of‐merit, zT of 1.6 at 670 K when alloyed with 10 mol % Se. First‐principles DFT calculation reveals several soft phonon modes in its room‐temperature hexagonal phase, which are also evident from low‐temperature heat‐capacity measurement. These phonon modes, dominated by Ag vibrations, soften further with temperature giving a dynamic cation disorder and driving the superionic transition. Intrinsic factors cause an ultralow κL in the room‐temperature hexagonal phase, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high‐temperature rocksalt phase. Despite the cation disorder at elevated temperatures, the crystalline conduits of the rigid anion sublattice give a high power factor. Low thermal conduction: Soft phonon modes and optical‐acoustic phonon coupling cause an ultralow lattice thermal conductivity in the room‐temperature hexagonal phase of AgCuTe, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high‐temperature rocksalt phase. A high thermoelectric figure of merit (zT) of 1.6 is achieved in the p‐type AgCuTe at around 670 K.