Giant thermoelectric effect governed by unique two-dimensional electronic structure and strong anharmonicity in layered nitrides

Abstract

Layered complex nitrides have emerged as a novel class of thermoelectric materials due to their unique geometrical and electronic structures. In this work, we employ density functional theory calculations combined with Boltzmann transport equation and thermal transport unified theory to explore the electrical and thermal transport properties and thermoelectric performance of layered nitrides ${AM\mathrm{N}}_{2}$ $(A=\mathrm{Sr}, \mathrm{Ba}; M=\mathrm{Ti}, \mathrm{Zr}, \mathrm{Hf})$. It is found that the ${AM\mathrm{N}}_{2}$ family exhibits a large Seebeck coefficient and high power factor, ascribed to its multiband degeneracy and the presence of unique two-dimensional (2D) electronic structure. As the atomic mass of M increases, the lattice thermal conductivity ${\ensuremath{\kappa}}_{L}$ decreases significantly. In particular, ${\mathrm{BaHfN}}_{2}$ demonstrates ultralow lattice thermal conductivity of 0.18 W/mK at room temperature. Even after accounting for both the particlelike propagation and wavelike phonon tunneling transport, ultralow ${\ensuremath{\kappa}}_{L}$ is found across a wide temperature range. The ultralow ${\ensuremath{\kappa}}_{L}$ is mainly attributed to the strong anharmonicity in ${\mathrm{BaHfN}}_{2}$ as reflected by the high phonon-phonon scattering rate and short phonon lifetime. Importantly, we find that $p$-type ${\mathrm{BaHfN}}_{2}$ exhibits a high figure of merit (ZT), achieving values of 2.19 at 300 K and 7.29 at 900 K, respectively. Additionally, through strain engineering, applying a moderate tensile stress of 0.5% remarkably increases the ZT of $p$-type increases to 2.58 at 300 K and reaches a maximum of 9.02 at 900 K, respectively. The present work unveils the importance of unique 2D electronic structures and strong anharmonicity induced by the weak-bonded heavy element on the thermoelectric performance, suggesting that the layered ${AM\mathrm{N}}_{2}$ family is a promising candidate for the development of high-performance thermoelectric devices.

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