The magnetic ground state (MGS) and physical properties of monolayer (ML) $1T\text{−}{\mathrm{CrTe}}_2$ remain unresolved. Using first-principles calculations, we elucidate the mechanism underlying its antiferromagnetic (AFM) MGS, accompanied by structural dimerization, and reveal a unique lattice-driven topological phase transition. The MGS exhibits pronounced sensitivity to the lattice constant, governed by the competition between AFM direct exchange and ferromagnetic (FM) superexchange interactions. Below a critical lattice constant of 7.56 Å, AFM coupling dominates, inducing magnetic frustration and subsequent structural dimerization. Within the dimerized phase, the magnetic easy axis transitions from in-plane to out-of-plane orientation as the lattice constant increases. Furthermore, we identify a different AFM topological phase transition driven by lattice variations unique to the dimerized system. These findings resolve the long-standing controversy regarding the MGS and uncover a lattice-driven $\mathcal{PT}$-symmetric AFM topological insulator state.

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• Phase transitions
• Structural properties
• Topological materials
• 2-dimensional systems
• Magnetic systems
• Transition metal dichalcogenides
• Density functional theory