  We theoretically propose possible magnetism-induced negative thermal
expansion in honeycomb-lattice antiferromagnets with edge-sharing networks of
$MX_6$ octahedra where $M$ and $X$ are transition-metal and ligand ions,
respectively. In this crystal structure, the nearest-neighbor exchange
interaction is composed of two competing contributions, i.e., the
antiferromagnetic contribution from a direct 180$^\circ$ $M$-$M$ bond and the
ferromagnetic contribution from 90$^\circ$ $M$-$X$-$M$ bonds, amplitudes of
which have different bond-length dependence. Numerical analysis of the
spin-lattice model of the honeycomb-lattice antiferromagnets demonstrates that
the negative thermal expansion can occur when the system enters the
antiferromagnetic phase with lowering temperature so as to maximize the energy
gain associated with the bond-length dependent antiferromagnetic exchange
interaction. The present work provides a guiding principle for searching new
materials and eventually contributes to diversify the family of materials that
host the negative thermal expansion originating from the spin-lattice coupling
on the honeycomb lattices or related crystal structures.
