Unique Arrangement of Atoms Leads to Low Thermal Conductivity: A Comparative Study of Monolayer MgC.

Two-dimensional MgC, one of the typical representative MXene materials, is attracting lots of attention due to its outstanding properties. In this study, we find the thermal conductivity of monolayer MgC is more than 2 orders of magnitude lower than graphene and is even lower than MoS despite the relatively lighter atoms of Mg and C. Based on the comparative analysis with graphene, silicene, and MoS, the underlying mechanism is found lying in the unique arrangement of atoms (lighter atoms in the middle plane) and large electronegativity difference in MgC. The phonon anharmonicity is strong due to the resonant bonding. In addition, dual band gaps emerge in the phonon dispersion of MgC, which limit the phonon-phonon scattering and reduce the phonon relaxation time. This study reveals a new mechanism responsible for low thermal conductivity, which would be helpful for designing thermal functional materials and pave the way for applications in thermoelectrics.