Intrinsically Low Lattice Thermal Conductivity in Natural Superlattice (Bi2)m(Bi2Te3)n Thermoelectric Materials

Understanding the origin of intrinsic lattice thermal conductivity in crystalline solids is critical to research fields ranging from thermoelectric materials to thermal barrier coatings. This work reports the high-temperature thermoelectric properties and reveals an intrinsically multiple-mechanism-driven low lattice thermal conductivity in the (Bi-2)(m)(Bi2Te3)(n) (m/n = 3:9, 2:7, and 1:5) natural superlattice series. Low-temperature heat capacity measurements provide compelling evidence for the existence of multiple low-lying Einstein oscillator modes in (Bi-2)(m)(Bi2Te3)(n) compounds, suggestive of the coupling between heat-carrying acoustic phonons and low-frequency optical phonons. This endows (Bi-2)(m)(Bi2Te3)(n) compounds with strong phonon resonance scattering and, thus, intrinsically low lattice thermal conductivity. Moreover, phonon velocity measurements demonstrate that the low lattice thermal conductivity originates from chemical bond softening and lattice anharmonicity. Additionally, a small volume of the Brillouin zone gives rise to low cutoff frequency of acoustic phonon modes in the (Bi-2)(m)(Bi2Te3)(n) natural superlattice series, which are also favorable for the realization of low lattice thermal conductivity. The tight integration of all these mechanisms into a single material not only makes (Bi-2)(m)(Bi2Te3)(n) compounds candidates for future thermoelectric applications but also enables a guide for designing materials with expected thermal transport properties.

Automated summary

Understanding the intrinsic lattice thermal conductivity in (Bi-2)(m)(Bi2Te3)(n) compounds involves the interaction between heat-carrying acoustic phonons and low-frequency optical phonons, chemical bond softening, lattice anharmonicity, and small Brillouin zone volume contributing to low cutoff frequency of acoustic phonon modes. These mechanisms together result in intrinsically low lattice thermal conductivity, making the compounds potential candidates for future thermoelectric applications and guiding the design of materials with desired thermal transport properties.