Based on first-principles calculations, we discover a two-dimensional (2D) layered antiferromagnetic (AFM) electride ${\mathrm{Gd}}_2\mathrm{O}$, where anionic excess electrons exist in the interstitial spaces between positively charged cationic layers. It is revealed that each cationic layer composed of three-atom-thick Gd-O-Gd stacks has in-plane ferromagnetic and out-of-plane AFM superexchange interactions between the localized Gd $4f$ spins through O $2p$ orbitals. Interestingly, the interlayer superexchange mediated by the hybridized Gd $5d$ and interstitial-$s$-like states involves intimate couplings between the spin, lattice, and charge degrees of freedom, thereby inducing simultaneous magnetic, structural, and electronic phase transitions. The resulting ground state with the simple hexagonal lattice hosts massless Dirac fermions protected by nonsymmorphic magnetic symmetry, as well as massive Dirac fermions. We thus demonstrate that the anionic excess electrons in AFM ${\mathrm{Gd}}_2\mathrm{O}$ play an important role in the emergence of Dirac semimetal states, therefore offering an intriguing interplay between 2D magnetic electrides and topological physics.

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• Antiferromagnetism
• Exchange interaction
• Magnetic order
• Topological phase transition
• Density functional calculations