Superconducting Material Revived by Pressure Could Unlock Lossless Power Transmission

Researchers have long sought to optimise iron selenide (FeSe) as a high-temperature superconductor, but interstitial iron impurities commonly suppress its superconducting properties. Mingzhang Yang from the Institute of Physics, Chinese Academy of Sciences, alongside Yuxin Ma and Qi Li, and their colleagues, present a new study detailing the synthesis of Fe1.11Se single crystals via a hydrothermal ion-exchange and de-intercalation route. This method yields a material exhibiting superconductivity with an onset temperature of 30.4 K, despite containing a small excess of interstitial iron, exceeding previously established limits. Significantly, the team demonstrate that applying physical pressure not only modulates the superconducting temperature, creating a distinctive “V”-shaped response, but also induces a possible magnetic order, offering valuable new understanding of how interstitial iron governs the superconducting and transport characteristics of this complex material.

Enhanced superconductivity via controlled non-stoichiometry in iron selenide

Scientists have synthesized a new form of iron selenide, Fe1.11Se, exhibiting superconductivity at 30.4 K, a significant advancement in materials science. This non-stoichiometric single crystal was created using a hydrothermal ion-exchange and de-intercalation route, overcoming a long-standing challenge in achieving superconductivity in iron selenide due to the detrimental effects of interstitial iron.
The research demonstrates that incorporating 0.11% interstitial iron, exceeding the previously established equilibrium phase diagram limit, can, surprisingly, enhance superconducting properties. This breakthrough challenges conventional understanding of impurity effects in this material system and opens new avenues for tailoring its electronic structure.

Intriguingly, the superconducting onset temperature of Fe1.11Se displays a unique “V”-shaped evolution under applied physical pressure. Initially decreasing to a minimum between 2 and 2.6 GPa, the superconducting behaviour then rebounds, forming a second superconducting region. This behaviour mirrors observations in iron selenide intercalates, suggesting a shared underlying mechanism.

Furthermore, the application of pressure induces a possible magnetic order within the material, a phenomenon previously observed only in pressurized stoichiometric iron selenide. These observations provide crucial insights into the complex interplay between interstitial iron, pressure, and superconductivity.

Hydrothermal Synthesis and Structural Characterisation of Iron-Rich Fe1.11Se Single Crystals

A two-step hydrothermal ion-exchange and de-intercalation route was developed to synthesize new Fe1.11Se bulk single crystals containing a high concentration of interstitial iron ions. This process began with the hydrothermal intercalation of precursor materials, followed by selective de-intercalation to achieve the desired non-stoichiometry.

Single-crystal X-ray diffraction performed on a 152 × 66 × 11 μm3 specimen at 299 K confirmed the LiFeAs-type structure with the P4/nmm space group. Refinement of the crystal structure revealed that Fe and Se fully occupy the 2b [1/4, 3/4, 0] and 2c [1/4, 1/4, 0.266(1)] Wyckoff sites respectively. Differential Fourier maps identified residual electron density between layers, indicating the presence of 11% interstitial Fe2+ ions randomly occupying the 2c site [1/4, 1/4, 0.866(1)].

Powder X-ray diffraction was then employed on a 0.5 × 0.3 cm2 specimen to verify the single crystal purity, revealing only sharp (00l) reflections and no detectable peaks from precursor compounds. The lowest-angle Bragg reflection exhibited a d-spacing of 5.588(6) Å, representing a 1.2% increase compared to stoichiometric FeSe, suggesting the incorporation of additional species between the FeSe layers.

Elemental compositions were further determined using inductively coupled plasma-atomic emission spectroscopy to confirm complete de-intercalation of alkali metals. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to examine the morphology and elemental distribution within the crystals, providing further validation of the successful synthesis0.57Fe Mössbauer spectroscopy at 23 K was also performed, providing insights into the local electronic environment of iron atoms within the Fe1.11Se structure and confirming the presence of interstitial iron. These combined techniques enabled the characterization of a novel material with a significantly altered stoichiometry compared to conventionally synthesized FeSe.

Enhanced superconductivity in iron-rich Fe1.11Se single crystals via iron intercalation

Single crystals of Fe1.11Se were successfully synthesized exhibiting a superconducting onset temperature (Tconset) of 30.4 K. This achievement surpasses the typical 8.5 K observed in stoichiometric FeSe and approaches the 46 K seen in potassium-intercalated FeSe compounds. The synthesis involved a hydrothermal ion-exchange and de-intercalation route, resulting in a non-stoichiometric composition with 11% interstitial Fe ions, exceeding the equilibrium phase diagram limit.

Single-crystal X-ray diffraction confirmed the LiFeAs-type structure with space group P4/nmm, revealing that interstitial Fe randomly occupies sites within the crystal lattice. Powder X-ray diffraction analysis showed a d-spacing of 5.588 Å, representing a 1.2% increase compared to stoichiometric FeSe, indicating the incorporation of additional species between the FeSe layers.

Inductively coupled plasma-atomic emission spectroscopy yielded a Fe:Se ratio of 1.18:1, while energy-dispersive X-ray spectroscopy confirmed a ratio of 1.11:1, aligning with the structural analysis0.57Fe Mössbauer spectroscopy at 23 K further characterized the electronic environment within the Fe1.11Se crystals. Under physical pres.

Interstitial iron induces unconventional superconductivity and magnetic behaviour

Researchers have successfully grown a non-stoichiometric iron selenide (Fe1.11Se) single crystal exhibiting superconductivity with an onset temperature of 30.4 K. This achievement utilized a hydrothermal ion-exchange and de-intercalation process, resulting in a material with 11% interstitial iron ions exceeding the typical equilibrium composition limit for this compound.

Comprehensive structural and spectroscopic analyses confirmed the incorporation of these interstitial iron ions into the crystal lattice. The superconducting behaviour of this new material under applied pressure demonstrates a distinctive “V”-shaped evolution, initially decreasing before increasing again, similar to observations in iron selenide intercalates.

Furthermore, evidence of pressure-induced magnetic order, previously noted in standard iron selenide, was also detected. These findings suggest that controlling non-equilibrium synthesis conditions can induce emergent physical properties and potentially lead to higher-temperature superconductivity in layered materials.

The authors acknowledge that the precise mechanism behind the observed pressure-induced magnetic order requires further investigation. They also note that the metastable nature of the synthesized material presents challenges for large-scale production. Future research will likely focus on refining the synthesis process to improve crystal quality and exploring the relationship between interstitial iron content and superconducting properties in greater detail, potentially extending these strategies to other layered superconductors with weak interlayer forces. This work establishes a viable route for discovering new high-temperature superconductors and tailoring material properties through non-equilibrium approaches.