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Research Article  |  Open Access  |  26 Feb 2024

Pressure-induced superconductivity in SnSb2Te4

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Microstructures 2024;4:2024012.
10.20517/microstructures.2023.60 |  © The Author(s) 2024.
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Abstract

Owing to their unique compositional and structural characteristics, layered van der Waals solids in binary and ternary chalcogenide families provide a fertile testbed for exploring novel exotic structures and states, e.g., topological insulators and superconductors. Herein, a comprehensive study on the structural variations and correlated electrical transport behavior of SnSb2Te4, a ternary member, has been carried out considering elevated pressures. Under 45.6 GPa, three distinct structural phase transitions have been observed, with strong evidence from the variations of high-pressure X-ray diffraction patterns. The onsets of phase II (monoclinic, C2/m) at 6.3 GPa, phase III (monoclinic, C2/c) at 15.5 GPa, and phase IV (body-centered cubic with substitutional disorder, Im-3m) at 17.2 GPa have been observed owing to the emergence of new diffractions. Based on electrical measurements at low temperature and high pressure conditions, two pressure-induced superconducting states have been distinguished in SnSb2Te4. The first state occurs in the range of 12.3-17.1 GPa. The positive pressure dependence on Tc indicates that the aforementioned state is related to the monoclinic C2/m phase. At > 17.1 GPa, the second superconducting state emerges, with the negative pressure dependence on Tc. It relates to the body-centered cubic solid solution phase, which is characteristic of a substitutional disordered crystal structure. The discovery that the pressure-induced superconductivity in SnSb2Te4 is affected by structural phase transitions under pressure may help understand the universal relationship between the ambient condition topological insulating state and derived superconductivity. Ab initio theoretical calculations reveal that an electronic topological transition takes place at approximately 2.0 GPa, which is featured by the obvious changes in the distribution of electronic density of states near the Fermi level.

Keywords

Superconductivity, structural phase transitions, topological insulators, high pressure, diamond anvil cell

INTRODUCTION

The pursuit of new superconductors and the comprehension of the superconductivity mechanism constitute one of the most active forefront subjects in condensed matter physics and materials science[1-4]. The recent discovery of topological superconductors (TSs) has introduced new concepts in the family of superconductors and elucidated the superconducting mechanism. TSs are bulk superconductors; however, they exhibit spin-polarized metallic topological surface states. Therefore, the transport properties of materials with topological features, including but not restricted to topological insulators and TSs, have attracted considerable research interest. Based on both the theoretical simulations[5,6] and the experimental measurements[7,8], typical layered ternary chalcogenides, AB2X4 (A = Ge, Sn, and Pb; B = Sb and Bi; X = Se and Te), have been recognized as topological insulators under ambient conditions. GeBi2Te4, SnSb2Te4, and SnBi2Te4 (and their parent binary counterparts, e.g., Sb2Te3[9], Bi2Te3[10], and Bi2Se3[11]) have been considered as promising TS candidates.

These binary or ternary chalcogenide compounds have been intensively studied. They comprise a flexible and versatile system to develop new applications and explore the relationship of various ordered states. Moreover, the compounds exhibit extraordinary thermoelectric[12], galvanomagnetic, and thermomagnetic properties[13,14] and novel and exotic phenomena, such as electronic topological transition (ETT), metal-insulator transition, topological quantum phase transition, structural phase transition, and superconductivity[15-18]. While crystalizing in a similar tetradymite-like layered structure, ternary topological compounds, i.e., AB2X4, possess the advantage (over their binary counterparts) of tuning their structures and properties based on the appropriate selection and substitution of atoms in the A site. Specifically, SnSb2Te4 acts as an intrinsic p-doped 3D TI with a narrow indirect Eg band gap of approximately 0.1 eV. Based on the angle-resolved two-photon photoemission measurement combined with the density functional theory (DFT) calculations about the electronic energy band structure, the Dirac cone is observed at 0.32 eV above the Fermi level[19]. High-pressure techniques have played an important role in tuning the superconducting states and the electronic properties of materials[20-23]. For SnSb2Te4, the pressure-induced superconducting transition occurs at 8.1 GPa with an onset Tc value of approximately 2 K[22]. With a further increase in pressure, Tc increases monotonously, indicating the pressure-induced enhancement of superconductivity. However, a subsequent study indicates that SnSb2Te4 tends to decompose into its constituent binary compounds (α-Sb2Te3 and SnTe) at medium high pressures (> 7 GPa)[20]. Thus, the observed superconductivity at higher pressures needs to be investigated to exclude the interference of the high pressure phases of the binary constituents. Moreover, other layered ternary chalcogenides, such as SnBi2Se4, GeSb2Te4, and PbBi2Te4[15,22,24-26], have been reported to undergo complicated but sequential superconducting transitions under compression. The characteristic superconducting transition parameters, such as Tc and Pc, vary considerably. However, in these homologous materials, pressure-induced decomposition into corresponding binary constituents has not been reported to date. From this perspective, the stability and phase transition of SnSb2Te4 and related layered ternary chalcogenides need to be investigated.

To address the aforementioned problem and clarify the evolution of the structures and electronic transport features of SnSb2Te4 under compression, we have conducted systematic experimental studies combined with theoretical simulations on SnSb2Te4 at elevated pressures. The observation of pressure-induced superconductivity is confirmed at pressures above approximately 12.3 GPa with variable critical temperatures. Superconductivity is enhanced at the initial pressure range, and the maximum superconductivity is achieved at 8.2 K (Tc) and 17.1 GPa. After that, a kink appears in the Tc-P plot. The sharp reduction and negative pressure dependence of Tc at higher pressures suggest the presence of a second superconducting state. Moreover, it is revealed by the high pressure synchrotron X-ray diffraction (XRD) studies that the initial R-3m structure transforms to a C2/m phase (phase II) at 6.3 GPa, a nominal phase III at 15.5 GPa, and a substitutional alloy (phase IV, Im-3m) at 17.2 GPa. The consistency in the critical pressures of the structural and superconducting transitions indicates that the two high pressure superconducting states are closely correlated to the pressure-induced structural transitions in SnSb2Te4.

EXPERIMENTAL AND CALCULATION DETAILS

Sample preparation and characterization

The coarse-grained single crystals of SnSb2Te4 were prepared by a self-flux synthetic route. Powders of the elements Sn, Sb, and Te with a purity better than 99.99% were used as purchased (Aladdin Company, China). They were mixed together according to the expected atomic ratio (Sn:Sb:Te = 1:2:4). The initial reactants were ground vigorously in an agate mortar to turn them into a homogenous mixture. The mixture was then transferred into a quartz ampule. The ample was evacuated and sealed to ensure airtightness. The temperature of the ampule was raised to 950 °C at a heating rate of 40 °C per hour. The high temperature transformed the reactants into a melting flux and induced reactions among them. Afterward, the temperature of the ampule was lowered to 500 °C at a rate of 20 °C per hour. The products were annealed at this temperature for four days. At last, the power supply was turned off, and the temperature of the ampule was lowered to room temperature in a natural cooling mode. In this way, single crystals of SnSb2Te4, with a black color, were successfully prepared and finely powdered in an agate mortar for convenience in high pressure studies. The morphologies of the obtained powders were characterized using SEM images taken on a Hitachi S4800 microscope. The elemental compositions were characterized using an energy dispersive X-ray spectrometer (EDX).

ADXRD studies at high pressures

In situ high pressure angle dispersive XRD (ADXRD) experiments were carried out using a symmetric diamond anvil cell (DAC). The culets of the two diamond anvils were about 300 µm in diameter. The gasket was made of T301 stainless steel. A hole about 50 µm in diameter was drilled at the center of the gasket, serving as the sample chamber. The prepared powders of SnSb2Te4 were loaded into the sample chamber together with a small chip of ruby, which was used as the pressure indicator[27]. The mixture of methanol and ethanol in a volume ratio of 4:1 was used as the pressure transmitting medium. In situ high pressure ADXRD experiments were performed at the high pressure station on the 4W2 beam line of Beijing Synchrotron Radiation Facility (BSRF). The wavelength of the incident monochromatic synchrotron X-ray was 0.6199 Å. The XRD data were recorded using a MAR image plate. The Dioptas package was employed to process the raw XRD data[28]. The full profile indexing and refinements of the diffraction patterns were carried out using the GSAS+EXPGUI package, employing the Le Bail method[29].

Electrical measurements at high pressures and low temperatures

The electrical transport characteristics of SnSb2Te4 at high pressures and low temperatures (HPLT) were investigated using a symmetric DAC with two 300 µm culet diamond anvils. In order to prevent short circuits between the electrode leads and the gasket, fine powders of Al2O3 were employed to cover the T301 stainless steel gasket on one side to form a layered composite gasket. The gasket was initially 250 µm in thickness and pre-indented to about 30 µm. A hole with a diameter of 260 µm was drilled at the center of the indented region. Then, fine powders of Al2O3 were filled and compressed in the hole. Finally, a hole with a diameter of 150 µm was drilled at the center of compact Al2O3 to serve as the sample chamber, and the sample powders of SnSb2Te4 were loaded into it. To ensure reliable contact between the electrodes and the sample powders, pressure transmitting medium was avoided in electrical measurements. The pressure was calibrated using either the ruby fluorescence R1 line[27,30,31] or the high frequency edge of the Raman scattering peak of diamond[32] (at low temperatures). Pt wires with a diameter of 4 µm were employed as the electrodes. The electrical resistance of the sample was measured using the traditional four-probe method. The electric voltages and currents involved were measured using a Keithley 2182A nanovoltmeter and 6221 AC and DC sources, respectively. The low temperature conditions were generated and maintained using an integrated low temperature system (Janis PTSHI-950-LT). The sample was compressed under 41 GPa at increments of approximately 2 GPa and cooled down to 5.2 K from the ambient temperature. After each increment, a temperature cycle was performed. The rate of temperature slowly decreased to ensure temperature equilibrium.

Theoretical simulation

Ab initio energetic and electronic calculations of SnSb2Te4 were performed based on the framework of DFT as implemented in the Vienna ab initio simulation package (VASP)[33]. The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional[34] was adopted to describe the exchange-correlation energies. The electron-ion interaction was described using the projector augmented wave (PAW) method[35], considering 5s25p2, 5s25p3, and 5s25p4 as valence electrons for Sn, Sb, and Te atoms. The cutoff energy of the plane waves was set at 900 eV. The fine Monkhorst-Pack[36]k meshes with a spacing of 2π × 0.18 Å-1 were adopted in order that the calculated enthalpy were well converged to values with an accuracy higher than 1 meV/atom. The spin-orbit coupling effect was considered in the calculations of the electronic band structures.

RESULTS AND DISCUSSION

Physical characterization under ambient pressure

The crystal structure of SnSb2Te4 is shown in Figure 1A. Under ambient conditions, the crystal structure of SnSb2Te4 belongs to the trigonal system with the R-3m space group (phase I). In the structure, rocksalt-type building blocks comprising septuple layers (SLs) of alternating cations and anions are stacked along the crystallographic c axis. The SLs, with the ideal sequence of Te1-Sb-Te2-Sn-Te2-Sb-Te1, are held together through the van der Waals gaps between the hexagonal Te layers terminating the blocks. The hexagonal unit cell includes three SLs. Based on the Le Bail refinement of the XRD patterns of SnSb2Te4 powders [Figure 1B] using GSAS, the unit cell parameters are determined: a = 4.301 Å and c = 41.598 Å. The results are consistent with those reported previously[37]. The typical scanning electron microscope (SEM) image of the as-prepared SnSb2Te4 sample is shown in Figure 1C. It shows that the prepared sample consists mainly of flakes with irregular shapes due to vigorous grinding treatment. However, all the flakes show clearly flat basal planes since they were actually broken single crystals. The size of the flakes ranges from 1 to 10 μm. The chemical composition of the sample was measured by energy-dispersive X-ray spectroscopy (EDS). Figure 1D demonstrates that the sample comprises mainly Sn, Sb, and Te elements. In a wide spectrum range (0-10 keV), no signal from any other element is detectable. Through quantitative analysis based on the EDX spectra, the atomic ratio of Sn:Sb:Te is estimated to be 1:2:4, in good agreement with the proposed chemical formula.

Pressure-induced superconductivity in SnSb<sub>2</sub>Te<sub>4</sub>

Figure 1. (A) The crystal structure of SnSb2Te4. The square frame shows a single SL building block. Blue, green, and magenta balls stand for Sn, Sb, and Te atoms. (B) The typical XRD patterns of SnSb2Te4 powders at ambient pressure. (C) The typical SEM image of the as-prepared SnSb2Te4 powders. (D) The typical EDX spectra of the as-prepared SnSb2Te4 powders.

Pressure-induced structural transformations

In the ADXRD studies, it was observed that SnSb2Te4 experienced three sequential structural phase transitions under compression at 0-45.6 GPa [Figure 2A]. As pressure increased to approximately 6.3 GPa, the emergence of new diffractions (marked with solid circles) indicated that the initial trigonal structure (phase I) lost stability, and a new phase (denoted as phase II) appeared. Phase II was fully established at > 12.9 GPa. It was noteworthy that phase I persisted and coexisted with phase II up to approximately 8.9 GPa. Previous studies on SnSb2Te4[20] under high pressures demonstrated the pressure-induced decomposition at > 7 GPa. However, our experimental studies did not observe any signs of decomposition [Supplementary Figures 1-3]. To determine the structure of phase II, the pressure-induced phase transition sequences of the binary VA-VIA compound counterparts of SnSb2Te4, such as Bi2Te3[38,39], Bi2Se3[40], Sb2Te3[41], Sb2Se3[42], and Sb2S3[17,43], and the related ternary ones, such as Bi2Te2Se[44] and SnBi2Te4[15], were considered. Therefore, phase II was assigned to a C2/m space group. The assignment could be validated based on the Le Bail refinement of the ADXRD patterns [Figure 3A]. The reflections from phase II can be indexed to the C2/m structure, yielding the lattice parameters summarized in Table 1. As the pressure further increased to 15.5 GPa, additional diffraction peaks appeared in the ADXRD patterns [marked with solid diamonds in Figure 2A, indicating the emergence of another new phase (phase III). Phase III remained stable at 15.5-40.2 GPa. Under the stable pressure range, phase III coexisted with phase II and/or phase IV. Owing to the complicacy of the ADXRD profiles, it becomes difficult to determine the exact crystal structure and lattice parameters of phase III. However, similar to the assignment of phase II, phase III might be tentatively assigned to a C2/c structure [Figure 3B]. With further increase of the pressure to 17.2 GPa, new diffraction peaks started to emerge, which were marked with black arrows in Figure 2A. They indicated the onset of the third phase transformation into phase IV. Phase IV at high pressures can be indexed to the body-centered cubic structure (space group No.229, Im-3m) with the unit cell parameter of a = 3.459 Å. Phase IV remained stable up to 45.6 GPa, the highest pressure achieved herein. Le Bail fitting for the XRD patterns at various pressures was shown in Figure 3A-D.

Pressure-induced superconductivity in SnSb<sub>2</sub>Te<sub>4</sub>

Figure 2. The structural evolution of SnSb2Te4 at high pressures. (A) The typical ADXRD patterns of SnSb2Te4 acquired at various high pressures and room temperature. The solid circles, diamonds, and arrows are used to indicate the emerging diffractions of phases II, III, and IV. (B) The pressure-dependent evolution of the d-spacings of SnSb2Te4. The pressure ranges in which the structural phases exist are labeled with vertical dash lines. Across the phase transition pressures, the slopes of the variation curves of the d-spacings change discontinuously.

Pressure-induced superconductivity in SnSb<sub>2</sub>Te<sub>4</sub>

Figure 3. The full profile diffraction patterns of SnSb2Te4 at (A) 8.9, (B) 15.5, (C) 3.1 and (D) 44.7 GPa. The solid lines in red represent the fitting curves of the diffraction patterns of the involved structures via Le Bail refinements, and open circles represent the experimental data points. The solid lines in black at the bottom represent the residual deviations between the fitted and the observed intensities. The vertical bars indicate the theoretical peak positions of the involved structural phases. The diffraction patterns in (A) comprise a mixture of phases I and II. Those in (B) comprise a mixture of phases II and III. (E) The pressure dependence of the lattice parameters for phase I. The lattice parameters are indicated by squares, solid circles, and hollow circles, corresponding to the data reported by Sans et al.[20] and those obtained experimentally and theoretically in the present study. The inset shows the pressure dependence of the axial ratio c/a.

Table 1

The lattice parameters of SnSb2Te4 phases at 3.2, 8.9, 15.5, and 44.7 GPa, derived from Le Bail refinements, as shown in Figure 3

PhaseSpace groupPressureLattice parameters (Å)Volume (Å3)
phase IR-3m3.1 GPaa = b = 4.224 (3)
c = 39.742 (7)
614.18 (3)
phase IIC2/m8.9 GPaa = 14.253 (2)
b = 4.128 (6)
c = 17.120 (2)
β = 149.059 (3)
519.04 (7)
phase IIIC2/c15.5 GPaa = 10.137 (9)
b = 7.105 (3)
c = 10.660 (7)
β = 136.320 (5)
530.30 (5)
phase IVIm-3m44.7 GPaa = b = c = 3.459 (6)41.10 (6)

In the bcc (Im-3m) structure, it was demanded by symmetry that only one inequivalent atomic position at the 2a Wyckoff site was allowed. As a result, in phase IV of SnSb2Te4, the distribution of Sn, Sb, and Te atoms was disordered, and they had to share the bcc lattice sites in a random manner. Hence, an Sn-Sb-Te substitutional solid solution is formed. The similar phenomena have also been observed in the high pressure-induced substitutional solid solution phases in Bi2Te3, Sb2Te3, and Sb2Se3. The formation of the high pressure substitutional solid solutions can be roughly explained in terms of atomic size and electronegativity. As pressure was gradually increased, these parameters for different atoms tended to become almost homogenous. Thus, it turned out to be possible for them to form substitutional solid solutions at high pressures. Moreover, the pressure-dependent variation trends of the lattice d-spacings [Figure 2B] and the volume [Supplementary Figure 4] indicated additional signs of the phase transitions. Thus, based on the aforementioned results and discussion, for SnSb2Te4, the pressure-induced structural phase transition sequence might be R-3mC2/mC2/c → disordered Im-3m.

Several factors may contribute to the broadening of the diffraction peaks in high pressure XRD patterns. First, the non-hydrostatic stress in the samples may lead to systematic broadening and weakening of the diffraction peaks with increasing pressure. Second, amorphization, if it occurs, may lead to diffuse diffractions with large width and low intensity. In the third place, the overlapping of the diffraction peaks of coexisting phases may exhibit broad convolutional profiles when they have similar d-spacings. In this work, coexistence of phases II, III, and IV may be the major origin of the broadening of the diffraction peaks around 15-17 GPa. The occurrence of amorphization was hardly possible since the diffraction peaks in the low-angle range remained sharp and strong.

Pressure-induced electron topological transition

The pressure dependences of the experimental and theoretical lattice parameters and the axial ratio c/a of the initial phase are illustrated in Figure 3E. A minimum can be observed in the variation curve of c/a at approximately 3 GPa. Generally, it suggests the occurrence of the electron topological transition (ETT) near this pressure. These experimental results agree well with our theoretical calculations. The abnormal variation in the c/a ratio with pressure is considered as a good indicator of ETT. It is caused by the anomalous compression behavior of the a axis under high pressures, which results in a profound change in compressibility during ETT[41]. The topological transition of electrons related to electrical transportation in SnSb2Te4 can be visualized based on the first-principles calculations of the electron energy bands [Figure 4]. Under ambient conditions, the minimum conduction band (MCB) and maximum valence band (MVB) appear at the high symmetric point (A) of the Brillouin zone (BZ). The calculation results of the density of states (DOS) indicate that Te mainly contributes to the MVB, and Sn contributes to the MCB. In agreement with previous reports[22], SnSb2Te4 is a direct band gap semiconductor. However, the narrow band gap of 0.02 eV indicates that SnSb2Te4 may behave as a metal at room temperature owing to self-doping effects.

Pressure-induced superconductivity in SnSb<sub>2</sub>Te<sub>4</sub>

Figure 4. Electronic band structures of SnSb2Te4 at (A) 0.1, (B) 2.0, (C) 4.0, and (D) 6.0 GPa in the presence of spin–orbit coupling.

Under compression, the repulsion between the electron states of Te and Sn considerably increases with pressure. As a result, the positions of the MCB and MVB change substantially. SnSb2Te4 becomes an indirect band gap semiconductor. At 2.0 GPa, the MVB moves to a high symmetric point along the Γ-M direction in the BZ, with the complex contributions of Sn, Sb, and Te. Along the Γ-M direction near the Γ point, at least three dispersive branches compete for the MCB. The indirect band gap decreases with an increase in pressure. ETT occurs when an extreme of the electronic band structure, which is associated with the Van Hove singularity in the DOS, exceeds the Fermi energy (EF) and leads to a strong redistribution of the EDOS near the EF[43]. In one of the previous studies that reported the analysis of SnBi2Te4 under high pressures, a discontinuous change in the c/a ratio under pressure was found to be related to the inflection point of electrical resistance at 2 GPa, which implies the occurrence of ETT in phase I[15]. Moreover, ETT existed based on the phonon spectrum measurements observed in the parent binary compound, i.e., Sb2Te3[45], and the compounds of the A2B3 chalcogenide series, e.g., Bi2Se3, Sb2S3, and Bi2Te3[40,43,46].

Pressure-induced superconductivity

The temperature dependence of the electrical resistance of SnSb2Te4 was obtained at 12.3-41 GPa[Figure 5A]. At < 12.3 GPa, the conducting behavior is characteristic of a metal without the marked signature of superconductivity down to the lowest temperature measured. When the pressure increased to 12.3 GPa, superconducting transition with a Tc value of approximately 5.2 K was observed [Figure 5A]. When the pressure increased to 13.8 GPa, the decrease in electrical resistance became more evident, and the zero-resistance state was fully realized. Moreover, Tc increased rapidly with pressure. Herein, the zero-resistance critical temperature (Tczero) and the onset temperature of superconducting transition (Tconset) were determined. At > 25.1 GPa, there was a little difference between Tczero and Tconset, hovering near a width of about 1 K. Herein, the maximum value of Tc was calculated to be approximately 8.2 K at 17.1 GPa, very close to those reported for SnBi2Te4 (8.9 K)[15] and GeSb2Te4 (8 K)[25] under pressure. A comparison of Tc and critical pressures for other TS candidates is shown in Supplementary Table 1.

Pressure-induced superconductivity in SnSb<sub>2</sub>Te<sub>4</sub>

Figure 5. (A) The temperature dependence of the electrical resistance of SnSb2Te4 at various high pressures. (B) The pressure dependence of the critical temperatures Tconset and Tczero of SnSb2Te4. The solid and hollow circles stand for Tconset and Tczero, respectively. The inset depicts the differential curve dR/dT of the electrical resistance near Tc. The symbols Tconset, Tcmid, and Tczero are defined accordingly. Tconset refers to the temperature at which the drop in electrical resistance begins; Tczero refers to the temperature at which electrical resistance decreases to zero; Tcmid refers to the temperature at which the maximum of the dR/dT curve occurs.

Figure 5B shows the superconducting phase diagram of the SnSb2Te4 powders. Two regions can be distinguished based on the changes in Tc with elevated pressure. At 12.3-17.1 GPa, in which phases II and III coexist, Tconset increases monotonically with pressure. The steep slope indicates the strong pressure-induced enhancement of superconductivity. The pressure dependence of Tc at the aforementioned pressure range is similar to that reported in the previous study[22]. It can be concluded that the positive pressure dependence on Tc observed under pressures of up to 17.1 GPa is correlated to monoclinic C2/m phase II. For Sb2Te3[9] or the binary VA-VIA compound counterparts of SnSb2Te4, the evolution of TC as a function of pressure indicates a positive trend as well. At > 17.1 GPa, the sharp decrease in and the negative pressure dependence of Tc indicate the presence of a new superconducting phase [Figure 5B]. TC monotonically decreases to 7.2 K at 41.0 GPa. The new superconducting phase corresponds to phase IV (the substitutional solid solution with the bcc structure), which exists at > 17.1 GPa and room temperature. Similar trends in the pressure effect on TC for the substitutional solid solution phases of Bi2Te3[47], Bi4Te3[48], and SbBi2Te4[15] are also observed. For these materials, superconductivity is depressed by high pressure. As a whole, pressure-driven superconductivity in SnSb2Te4 relates closely to the structural phase transitions. The obtained phase diagram reveals a relationship between the structural transition and the appearance of superconductivity.

To ensure the superconducting transition illustrated in Figure 5, we performed electrical resistance measurements near the critical temperature at a varying external magnetic field. The results were shown in Figure 6A with the pressure fixed at 22.3 GPa. It is clear that Tc decreased with increasing magnetic field, confirming that the transition is really superconductive. It is also shown that, with and without a magnetic field, the drop in the electrical resistance is very sharp, indicating bulk and homogeneous occurrence of superconductivity. Figure 6B shows the plot of the temperature dependence of the reduced upper critical field μ0H(T) (red circles). The data points are fitted to the Werthdamer-Helfand-Hohenberg (WHH) formula[49]

$$ \begin{equation} \begin{aligned} \mu_{0} H_{c 2}(T)=\frac{\mu_{0} H_{C 2}(0)}{0.693}\left(\left(1-\frac{T}{T_{C}}\right)-0.153\left(1-\frac{T}{T_{C}}\right)^{2}-0.152\left(1-\frac{T}{T_{C}}\right)^{4}\right), \end{aligned} \end{equation} $$

Pressure-induced superconductivity in SnSb<sub>2</sub>Te<sub>4</sub>

Figure 6. (A) The temperature dependence of the electrical resistance under various external magnetic fields with the pressure held constant at 22.3 GPa. (B) The temperature dependence of the reduced upper critical field μ0H(T) (red circles) and the fitting curve according to the WHH formula (green line). The inset shows the dependence of Tc on the external magnetic field. The solid and hollow circles stand for Tconset and Tczero, respectively.

and the obtained fitting curve (green line) was also shown in Figure 6B. Through extrapolation of the fitting curve, the upper critical field Hc2(0) was estimated to be 3.3 Tesla for H||c. In the inset of Figure 6B, the variations of the critical temperatures TConset and TCzero with magnetic field H were demonstrated.

CONCLUSIONS

In summary, as a promising candidate material for topological quantum states, SnSb2Te4 has attracted considerable research interest owing to its structural and electric transport behaviors, which are related to not only fundamental sciences but also potential applications in spintronics and quantum technologies. Herein, powdered samples of SnSb2Te4 were subjected to severe compression using DAC techniques. Similar to its binary end-member, i.e., Sb2Te3, SnSb2Te4 experienced structural phase transformations under high pressures in the following sequence: R-3mC2/mC2/c → disordered Im-3m. The focus was on the disordered site in high-pressure phases; i.e., in all the cases, the lattice sites were randomly occupied by substituent atoms, i.e., Sn, Sb, and Te. Such structural features, which are consistent with its binary and ternary chalcogenide counterparts, might enable the pressure-driven superconductivity of materials in this family. In this case study on SnSb2Te4, two distinct superconducting states, bulk and homogeneous, were observed at high pressures; they were closely related to the high-pressure monoclinic C2/m and cubic Im-3m phases, respectively. This study offers the opportunity for a deeper comprehension of the correlations between the metastable structural phases and the pressure-induced superconducting states of SnSb2Te4, which can clarify the general physical phenomenon giving rise to superconducting states in potential candidates for topological quantum materials.

DECLARATIONS

Authors’ contributions

Conceptualization, investigation, writing - review & editing: Ma Y

Investigation: Wang H

Investigation, writing - original draft: Li R

Methodology: Liu H

Conceptualization, writing-review & editing: Zhang J

Synthesis: Wang X

Synthesis, formal analysis: Jing Q

Investigation: Wang X, Dong W, Chen J, Wu B

Validation, investigation: Han Y

Investigation, writing-review & editing: Zhou D

Validation, conceptualization: Gao C

Availability of data and materials

The authors declare that the main data supporting the findings of this study are contained within the paper and its associated Supplementary Materials. All other relevant data are available from the corresponding author upon reasonable request.

Financial support and sponsorship

This work was supported by the National Natural Science Foundation of China (Grant No. 11974130, No. 12004015, No. 11874028, and No. 11704044). High pressure XRD experiments were performed at the 4W2 beamline of Beijing Synchrotron Radiation Facility and BL15U1 beamline of Shanghai Synchrotron Radiation Facility. Additionally, we acknowledge support from the User Experiment Assist System of Shanghai Synchrotron Radiation Facility (SSRF).

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Copyright

© The Author(s) 2024.

Supplementary Materials

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Cite This Article

OAE Style

Ma Y, Wang H, Li R, Liu H, Zhang J, Wang X, Jing Q, Wang X, Dong W, Chen J, Wu B, Han Y, Zhou D, Gao C. Pressure-induced superconductivity in SnSb2Te4. Microstructures 2024;4:2024012. http://dx.doi.org/10.20517/microstructures.2023.60

AMA Style

Ma Y, Wang H, Li R, Liu H, Zhang J, Wang X, Jing Q, Wang X, Dong W, Chen J, Wu B, Han Y, Zhou D, Gao C. Pressure-induced superconductivity in SnSb2Te4. Microstructures. 2024; 4(1): 2024012. http://dx.doi.org/10.20517/microstructures.2023.60

Chicago/Turabian Style

Ma, Yanmei, Hongbo Wang, Ruihong Li, Han Liu, Jian Zhang, Xianyu Wang, Qiang Jing, Xu Wang, Wenping Dong, Jinman Chen, Bingze Wu, Yonghao Han, Dan Zhou, Chunxiao Gao. 2024. "Pressure-induced superconductivity in SnSb2Te4" Microstructures. 4, no.1: 2024012. http://dx.doi.org/10.20517/microstructures.2023.60

ACS Style

Ma, Y.; Wang H.; Li R.; Liu H.; Zhang J.; Wang X.; Jing Q.; Wang X.; Dong W.; Chen J.; Wu B.; Han Y.; Zhou D.; Gao C. Pressure-induced superconductivity in SnSb2Te4. Microstructures. 2024, 4, 2024012. http://dx.doi.org/10.20517/microstructures.2023.60

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