Monoclinic ${\mathrm{EuSn}}_2{\mathrm{As}}_2$: A Novel High-Pressure Network Structure

The layered crystal of ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ has a ${\mathrm{Bi}}_2{\mathrm{Te}}_3$-type structure in rhombohedral ($R\overline{3}m$) 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 ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ structure in $C2/m$ symmetry above $\sim 14\mathrm{GPa}$. It has a three-dimensional network made up of honeycomblike Sn sheets and zigzag As chains, transformed from the layered ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ 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 $T_C$ value of $\sim 4\mathrm{K}$ is observed up to 30.8 GPa. These results establish a high-pressure ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ phase with intriguing structural and electronic properties and expand our understandings about the layered magnetic topological insulators.

Figure 1

(a) $\alpha \text{−}{\mathrm{EuSn}}_2{\mathrm{As}}_2$ in rhombohedral $R\overline{3}m$ (No. 166) symmetry at 0 GPa with four buckled As-Sn layers and two Eu layers. (b) $\alpha \text{−}{\mathrm{EuSn}}_2{\mathrm{As}}_2$ in rhombohedral $R\overline{3}m$ symmetry at 10 GPa with the connecting of Sn-Sn atoms between the nearest neighbor SnAs layers. (c) $\beta \text{−}{\mathrm{EuSn}}_2{\mathrm{As}}_2$ in monoclinic $C2/m$ (No. 12) symmetry at 20 GPa with the connecting of As-As atoms between the SnAs layers across the Eu layers. It has a network structure comprising honeycomblike Sn sheets and zigzag As chains. The magnetic unit cell in $P2/m$ (No. 10) symmetry is marked with dashed lines. (d) Top view of the honeycomblike Sn sheet in (c). (e) Relative enthalpy between the $\alpha$- and $\beta \text{−}{\mathrm{EuSn}}_2{\mathrm{As}}_2$ phases as a function of pressure.

Figure 2

Phonon band structures and PDOS for $\beta \text{−}{\mathrm{EuSn}}_2{\mathrm{As}}_2$ at 20 GPa in a monoclinic ($P2/m$) magnetic primitive cell.

Figure 3

(a) The in situ XRD patterns of ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ under 0.16–30.7 GPa. The synchrotron x-ray wavelength $\lambda =0.6199\AA$. (b)–(d) The refined XRD patterns at 0.16, 15.6, and 27.8 GPa. The blue, green, and pink vertical tick markers are corresponding to the Bragg reflections of $\alpha$ phase, $\beta$ phase, and Sn, respectively. (e) Simulated XRD patterns of the $\beta$ phase at 20 GPa. The two main peaks correspond to the new diffraction peaks in (a) above 12.6 GPa.

Figure 4

(a)–(c) Temperature dependent resistance of ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ under various pressures up to 30.8 GPa. The insulator-metal-superconductor transition at low temperature are shown around 5 and 15 GPa, respectively. The arrows indicate the Néel temperature $T_{\mathrm{N}}$. (d) The resistivity-pressure curve at 150 K. The two discontinuous change points near 5 and 12.6 GPa are in corresponding to the first-stage and second-stage structural conversion, respectively.