Lone-Pair Electrons Do Not Necessarily Lead to Low Lattice Thermal Conductivity: An Exception of Two-Dimensional Penta-CN2

It has long been documented in the literature that lone-pair electrons (LPE) are generally thought to lead to low lattice thermal conductivity (kappa(L)) of bulk materials by inducing strong phonon anharmonicity. Herein, we show an exceptional case of two-dimensional (2D) penta-CN2 that possesses LPE but exhibits more than doubled kappa(L), (660.71 W m(-1) K-1) than the LPE free counterpart of penta-graphene (252.95 W m(-1) K-1), which is unexpected and contradictory to the traditional theory of LPE leading to low kappa(L). Based on the comparative study of four 2D systems possessing LPE and their respective LPE free counterparts (planar C3N vs graphene and penta-CN2 vs pentagraphene), the underlying mechanism is found lying in the bonds homogenization in penta-CN2 due to the wide spatial extension of the nonsymmetrically distributed LPE, which compensates the lattice anharmonicity due to LPE and is responsible for the opposite tendency of LPE-affected kappa(L), in the four 2D systems.