The magnetic topological insulator ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ is known to crystallize in a ${\mathrm{Bi}}_2{\mathrm{Te}}_3$-type layered structure in rhombohedral symmetry at ambient conditions and undergo a structural phase transition toward a monoclinic $\beta$ phase under increasing pressure. Here, we identify by ab initio calculations a new high-pressure rhombohedral structure (termed the $\gamma$ phase) in $R\overline{3}m$ symmetry comprising Sn-As-As-Sn linear chains along the $z$ axis via the formation of As-As bonds in the AsSn/SnAs bilayers with strong anisotropic lattice distortion along the [001] orientation. This pressure-densified new rhombohedral phase has a metallic A-type antiferromagnetic ground state and is energetically more favorable than the original rhombohedral structure and the monoclinic $\beta$ phase above 11 GPa. Simulated x-ray diffraction patterns provide an excellent match to the previously unexplained distinct diffraction peak around $14.7^{\circ }$ found in experiments for ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ above 13 GPa. These findings offer key insights into a new type of high-pressure phase of layered magnetic topological insulators, laying a foundation for further exploration of this intriguing class of materials.

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• First-principles calculations
• Phase transitions
• Pressure effects
• Structural phase transition
• Crystal structures
• Topological insulators
• Density functional calculations