Monoclinic EuSn2As2: A Novel High-Pressure Network Structure

The layered crystal of EuSn2As2 has a Bi2Te3-type structure in rhombohedral (R-3m) symmetry and has been confirmed to be an intrinsic magnetic topological insulator at ambient conditions. Combining ab initio calculations and in-situ x-ray diffraction measurements, we identify a new monoclinic EuSn2As2 structure in C2/m symmetry above 15 GPa. It has a three-dimensional network made up of honeycomb-like Sn sheets and zigzag As chains, transformed from the layered EuSn2As2 via a two-stage reconstruction mechanism with the connecting of Sn-Sn and As-As atoms successively between the buckled SnAs layers. Its dynamic structural stability has been verified by phonon mode analysis. Electrical resistance measurements reveal an insulator-metal-superconductor transition at low temperature around 5 and 15 GPa, respectively, according to the structural conversion. These results establish a high-pressure EuSn2As2 phase with intriguing structural and electronic properties and expand our understandings about the layered magnetic topological insulators.