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 literature that, the lone-pair electrons (LPE) are generally thought to lead to low lattice thermal conductivity (κ_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 κ_L (660.71 Wm^-1K^-1) than the LPE free counterpart of penta-graphene (252.95 Wm^-1K^-1), which is unexpected and contradictory to the traditional theory of LPE leading to low κ_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. penta-graphene), the underlying mechanism is found lying in the bonds homogenization in penta-CN₂ due to the wide spatial extension of the non-symmetrically distributed LPE, which compensates the lattice anharmonicity due to LPE and is responsible for the opposite tendency of LPE affected κ_L in the four 2D systems.