Pressure-densified new rhombohedral phase of EuSn2As2

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 $\ensuremath{\beta}$ phase under increasing pressure. Here, we identify by ab initio calculations a new high-pressure rhombohedral structure (termed the $\ensuremath{\gamma}$ 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 $\ensuremath{\beta}$ phase above 11 GPa. Simulated x-ray diffraction patterns provide an excellent match to the previously unexplained distinct diffraction peak around $14.{7}^{\ensuremath{\circ}}$ 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.