Crystalline anharmonicity and ultralow thermal conductivity in layered Bi2GeTe4 for thermoelectric applications

Anharmonicity related to crystal structures strongly affects the phonon propagation and reduces lattice thermal conductivity (kappa(l)). Associated with a higher order of anharmonicity, thermoelectric (TE) materials with a large Gruneisen parameter (gamma) possess ultralow thermal conductivity (kappa(total)), which plays a decisive role in engineering of their TE performances. We report on the structural and TE transport properties of layered Bi2GeTe4, which belongs to the Bi2Te3 family of TE materials albeit with a larger unit cell. Bi2GeTe4 shows the electron dominated metallic nature throughout the temperature range with a carrier density of n=8.23x10(19)cm(-3) at 300K. Bi2GeTe4 exhibits an ultralow kappa(total)similar to 0.52W m(-1) K-1 at 350K with a high degree of anharmonicity as estimated by a large value of gamma similar to 4.24. Here, the calculated average phonon speed (nu(avg)=1.54km s(-1)) and larger bond lengths support the observations of ultralow kappa(total). The sufficiently lower value of kappa(total) makes Bi2GeTe4 a good candidate for high performance TE materials through appropriate electronic transport modulation.