A recent article published in Advanced Functional Materials investigated the influence of chemical pressure on the superconductivity in FeSe1−xSx crystals.
Selenium was replaced with sulfur (up to x = 0.52), and the crystals were intercalated with tetramethylammonium ions (TMA+).
Study: Effects of Chemical Pressure on Superconductivity in Electrochemically Intercalated (TMA)yFe2(Se1−xSx)2 (TMA = Tetramethylammonium). Image Credit: Casimiro PT/Shutterstock.com
Background
The β-modification of FeSe features an anti-PbO-type structure, consisting of stacked layers of edge-sharing FeSe₄ tetrahedra. Notably, it is a unique superconductor that requires neither doping nor applied pressure, exhibiting a critical temperature (Tc) of 8 K.
Despite its relatively low Tc, FeSe demonstrates remarkable tunability. Under pressure, Tc rises to 37 K, increases to 46 K with intercalation, and can reach as high as 99 K in thin films. This adaptability has been widely studied.
Additionally, Tc has been shown to respond to both compression and expansion of FeSe layers when ions or molecules are intercalated. Interestingly, for intercalated FeSe, Tc initially decreases under pressure before increasing at higher pressures. However, the effects of chemical pressure on FeSe intercalated with charged molecules remain unclear.
To explore this, FeSe₁₋ₓSₓ (x = 0–0.52; 1) crystals were synthesized in this study via chemical vapor transport (CVT) and subsequently intercalated electrochemically with TMA⁺.
Methods
The β-FeSe1−xSx precursors were synthesized via the CVT method, while the FeS precursor was hydrothermally synthesized. Subsequently, FeSe1−xSx host crystals (x = 0–0.52; 1) were electrochemically intercalated with TMA+ to form (TMA)yFe2(Se1−xSx)2.
During this process performed at three volts for three days, a tungsten rod was employed as the anode, an amalgamated copper spoon attached to a platinum wire was the cathode, and a saturated solution of tetramethylammonium iodide in N, N-dimethylformamide (DMF) was the electrolyte.
The formation of the tetragonal FeSe1−xSx precursors and the intercalated products was confirmed via powder X-ray diffraction (pXRD). Additionally, their compositions were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES), elemental carbon-hydrogen-nitrogen-sulfur analysis, and energy dispersive X-ray spectroscopy (EDX).
The superconducting properties of (TMA)yFe2(Se1−xSx)2 were analyzed by measuring their magnetic susceptibility with a vibrating sample magnetometer (VSM) incorporated into a physical property measurement system (PPMS).
Moreover, low- and high-temperature pXRD using a low-temperature device and a graphite furnace, respectively. Finally, morphological characterizations were performed via scanning electron microscopy (SEM).
Results and Discussion
All FeSe₁₋ₓSₓ host compounds crystallized in the tetragonal anti-PbO structure, with atomic positions nearly identical to those in FeSe (x = 0–0.52) and FeS (x = 1). These compounds exhibited superconductivity with Tc values ranging from 4 to 9 K. However, tetragonal FeS could not be synthesized via the CVT method and was instead prepared hydrothermally using sodium sulfide and iron.
The (TMA)ᵧFe₂(Se₁₋ₓSₓ)₂ superstructure was not detectable by pXRD due to the weak scattering of light atoms and the rotational disorder of TMA⁺ molecules. Therefore, the position of TMA⁺ ions was estimated based on pXRD data of FeSe₁₋ₓSₓ, while the TMA⁺ content was determined through chemical analysis. Notably, the proportion of TMA⁺ cations slightly decreased as sulfur incorporation increased.
SEM imaging revealed the layered morphology of the crystals, while EDX measurements confirmed that sulfur content in the FeSe₁₋ₓSₓ layers remained unchanged after intercalation. However, the structural transition observed in FeSe was rapidly suppressed with increasing sulfur content in FeSe₁₋ₓSₓ and was entirely absent in TMA⁺-intercalated compounds.
Thermal stability analysis showed that (TMA)₀.₅Fe₂Se₂ deintercalated to FeSe at 200 °C, whereas (TMA)₀.₃₆Fe₂S₂ decomposed at 150 °C, with FeS transitioning from a tetragonal to a hexagonal structure. This indicated that TMA⁺ intercalation into tetragonal FeS was irreversible.
Low-field susceptibility measurements of (TMA)ᵧFe₂(Se₁₋ₓSₓ)₂ confirmed superconductivity in samples with x = 0.06–0.28, with Tc decreasing from 41 to 35 K. The superconducting volume fractions ranged from 0.5 to 0.95 for compositions up to x = 0.28, indicating bulk superconductivity. Interestingly, this Tc variation in the intercalates differed from that observed in the precursor material.
Isothermal magnetization curves of (TMA)ᵧFe₂(Se₁₋ₓSₓ)₂ at 2 K displayed the characteristic ‘butterfly’ pattern typical of hard type-II superconductors with high upper critical fields. No flux jumps were detected in (TMA)₀.₅Fe₂Se₂. However, the magnetization behavior changed significantly for x = 0.28–0.52, where the critical fields became considerably smaller.
In contrast, no superconducting signal was observed for (TMA)₀.₃₆Fe₂S₂, suggesting that intercalation suppressed the low-temperature superconductivity of FeS. This sample exhibited only a weak isothermal magnetization signal, likely due to ferromagnetic impurities introduced during the hydrothermal synthesis of the FeS precursor.
Conclusion
Overall, the researchers comprehensively evaluated the impact of substituting selenium with sulfur in (TMA)yFe2(Se1−xSx)2.
This substitution applied a kind of chemical pressure, scaling the superconducting Tc at a rate nearly identical to that in the FeSe1−xSx host compounds. The considerably higher Tc of the intercalated specimens than the host compounds was primarily attributed to electron doping enabled by the TMA+ ions.
Notably, the electron doping, chemical pressure, and even minor compositional variations in the FeSe layer could significantly influence superconductivity in FeSe and its intercalates. Therefore, further research is important on these variations to facilitate the consistent formation of superconducting iron selenides.
Journal Reference
Lammer, N., Werhahn, D., Moritz, L., & Johrendt, D. (2025). Effects of Chemical Pressure on Superconductivity in Electrochemically Intercalated (TMA)yFe2(Se1−xSx)2 (TMA = Tetramethylammonium). Advanced Functional Materials. doi: 10.1002/adfm.202420840. https://advanced.onlinelibrary.wiley.com/doi/full/10.1002/adfm.202420840
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