Novel High-Pressure Network Structure of β-EuSn2As2 and its Two-Stage Reconstruction Mechanism
Topological insulators have attracted much research interest due to their novel band structure and quantum phenomenon. In the past few years, a good deal of attention has been focused on the intrinsic magnetic topological insulators because the interaction between magnetism and topological surface states can produce many exotic topological quantum effects, such as the quantum anomalous Hall effect, Majorana bound states and axion insulator state. In 2019, Prof. Tian Qian, Hongming Weng, Hong Ding, and Wentao Zhang, et al. [Phys. Rev. X 9, 041039 (2019)] reported that EuSn2As2 is an intrinsic magnetic topological insulator by combining angle resolved photoemission spectroscopy (ARPES) measurements and first-principles calculation.
Pressure, as a basic thermodynamic parameter, plays an important role in the research of topological materials, and can effectively tune the crystal and electronic structure of material to form a new state of matter. The pressure-induced structural transitions and superconductivity have been successfully observed in the typical topological insulators such as Bi2Se3, Bi2Te3 and Sb2Te3, which crystallize in a rhombohedral structure in R-3m symmetry at ambient pressure. Meanwhile, the layered crystal of EuSn2As2 has a Bi2Te3-type structure in rhombohedral (R-3m) symmetry under ambient pressure (termed α-EuSn2As2 hereafter), however, a systematic high-pressure study on EuSn2As2 is lacking.
Recently, Prof. Jian-Tao Wang, Xiaohui Yu, Fang Hong, Jinguang Cheng, etc from Institute of Physics of the Chinese Academy of Sciences collaborated with Dr. Lin Zhao and Prof. Yonghao Han from Jilin University reported a new three-dimensional network structure of β-EuSn2As2 [see Figure 1(c)] by combining ab initio calculations and in situ x-ray diffraction measurements. The β-EuSn2As2 has a monoclinic (C2/m) symmetry comprising of honeycomb-like Sn sheets and zigzag As chains and can be transformed from the layered α-EuSn2As2 via a two-stage phase transition process under pressure [see Fig. 1(a-c)]. At the first stage, the two buckled SnAs layers connect to each other via the nearest neighbor Sn-Sn atoms [see Fig. 1(b)]; at the second stage, the buckled Sn-Sn bonds become planar and form honeycomb-like Sn sheets [see Fig. 1(d)], meanwhile the SnAs layers further connect to each other via the As-As bonds across the Eu layers to form zigzag As chains between the Sn sheets. As a result, a three-dimensional monoclinic network structure comprising honeycomblike Sn sheets and zigzag As chains is achieved via a two-stage reconstruction mechanism. The enthalpy calculations show that β-EuSn2As2 is more stable than α-EuSn2As2 at above 14.3 GPa [see Fig. 1(e)]. The phonon mode analysis also verifies the dynamic structural stability of β-EuSn2As2 structure [see Fig. 2]. Experimentally, the in situ high-pressure XRD experiments identify that the EuSn2As2 sample undergoes a structure transition from the layered α-EuSn2As2 to the three-dimensional monoclinic β-EuSn2As2 at 12.6 GPa [see Fig. 3]. Electrical resistance measurements reveal an insulator-metal-superconductor transition at low temperature around 5 and 15 GPa, respectively [see Fig. 4], according to the two-stage structural conversion process, and the superconductivity with a TC value of ～4 K is observed up to 30.8 GPa. These discoveries spread over the research fields of superconductivity, topological insulators, and quantum magnetism, and thus will provide a new venue for studying various topics of current condensed matter physics.
The EuSn2As2 samples were provided by Dr. Changjiang Yi and Prof. Youguo Shi from the Institute of Physics of Chinese Academy of Sciences. The in situ XRD measurements were performed at 4W2 High Pressure Station, Beijing Synchrotron Radiation Facility (BSRF). The electrical resistance measurements are performed at the Huairou Synergic Extreme Condition User Facility.
This study entitled “Monoclinic EuSn2As2: A Novel High-Pressure Network Structure” was published on Phys. Rev. Lett. 126, 155701 (2021).
This work was supported by the National Key R&D Program of China (Grants No. 2016YFA0401503, No. 2016YFA0300604, No. 2018YFA0305700, No. 2018YFA0305900, and No. 2020YFA0711502), the National Natural Science Foundation of China (Grants No. 11974387, No. 12004416, No. 11674328, No. U2032204, No. 11774126, No. 12004014, No. U1832123, and No. U1930401), the Strategic Priority Research Program and Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (Grants No. XDB33000000, No. XDB25000000, and No. QYZDBSSW-SLH013), the K. C. Wong Education Foundation (No. GJTD-2018-01), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (No. 2016006).
 Monoclinic EuSn2As2: A Novel High-Pressure Network Structure, Lin Zhao, Changjiang Yi, Chang-Tian Wang, Zhenhua Chi, Yunyu Yin, Xiaoli Ma, Jianhong Dai, Pengtao Yang, Binbin Yue, Jinguang Cheng, Fang Hong, Jian-Tao Wang,* Yonghao Han,* Youguo Shi,* and Xiaohui Yu,* Phys. Rev. Lett. 126, 155701 (2021)
 Supplemental Materials for “Monoclinic EuSn2As2: A Novel High-Pressure Network Structure”, http://link.aps.org/supplemental/10.1103/PhysRevLett.126.155701
FIG. 1. Phase transition and structural stability under pressure. (a) α-EuSn2As2 in rhombohedral R-3m (No. 166) symmetry at 0 GPa with four buckled As-Sn layers and two Eu layers. (b) α-EuSn2As2 in rhombohedral R-3m symmetry at 10 GPa with the connecting of Sn-Sn atoms between the nearest neighbor SnAs layers. (c) β-EuSn2As2 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 α- and β-EuSn2As2 phases as a function of pressure.
FIG. 2. Phonon band structures and partial density of states (PDOS) for β-EuSn2As2 at 20 GPa in a monoclinic (P2/m) magnetic primitive cell. The high frequency models around 200 and 140 cm-1 are mainly contributed by As atoms, while the low frequency modes around 55 cm-1 are mainly contributed by Sn atoms. Throughout the entire Brillouin zone, no imaginary frequencies are observed, confirming the dynamic stability of this new β-EuSn2As2 phase.
FIG. 3. (a) The in situ XRD patterns of EuSn2As2 under 0.16–30.7 GPa. The synchrotron x-ray wavelength λ=0.6199 Å. (b)–(d) The refined XRD patterns at 0.16, 15.6, and 27.8 GPa. (e) Simulated XRD patterns of the β phase at 20 GPa. The two main peaks correspond to the new diffraction peaks in (a) above 12.6 GPa.
FIG. 4. (a)–(c) Temperature dependent resistance of EuSn2As2 under various pressures up to 30.8 GPa. The insulator-metal-superconductor transitions at low temperature are shown around 5 and 15 GPa, respectively. The arrows indicate the Neel temperature TN. (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.
Institute of Physics
High pressure; Structural phase transition; Magnetic topological insulators; X-ray diffraction; Electrical transport properties
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 ～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 ～4 K 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.