Monoclinic EuSn2As2: A Novel High-Pressure Network Structure

The layered crystal of EuSn2As2 has a Bi2Te3-type structure in rhombohedral ( R ¯ 3 m ) 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 ∼14 GPa . It has a three-dimensional network made up of honeycomblike 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, and the superconductivity with a TC value of ∼4K is observed up to 30.8 GPa. These results establish a high-pressure EuSn2As2 phase with intriguing structural and electronic properties and expand our understandings about the layered magnetic topological insulators.