Thermoelectric CoGeTe with an Orthorhombic Crystal Symmetry and Balance of the Electrical and Thermal Properties.

Applying crystal symmetry to discover and optimize the performance of thermoelectric (TE) materials has attracted much attention. Here, we report CoGeTe with a middle-class crystalline system as a novel n-type TE material. Density functional theory indicates that orthorhombic CoGeTe shows multiband dispersion near the bottom of the conduction band, which is mainly occupied by the Co 3d states. Through Ni doping, these multiple bands can be activated, leading to a maximum power factor of 1.14 mW/m K@786 K for CoNiGeTe. In addition, phonon-dispersion calculations reveal that CoGeTe possesses relatively strong harmonic properties, including sound velocity and Debye temperature. Furthermore, the local distorted CoGeTe octahedron in the matrix is beneficial for anharmonic phonon scattering. In particular, the Grüneisen parameter of Te in the crystal structure is clearly larger than those of Co and Ge. The observed thermal conductivity of CoNiGeTe is between 6.50 and 5.38 W/m K in the temperature range 300-860 K. Owing to the combination of the enhanced power factor and reduced thermal conductivity, the maximum zT value reaches 0.18 at 860 K. This study suggests that TE materials with orthorhombic structures provide an ideal platform to balance the power factor and thermal conductivity in search of high-performance thermoelectrics.