Understanding Doping, Vacancy, Lattice Stability, and Superconductivity in K x Fe2-y Se2

Two-Dimensional Pb Square Nets from Bulk (RO)nPb (R = Rare Earth Metals, n = 1,2)

All two-dimensional (2D) materials of group IV elements from Si to Pb are stabilized by carrier doping and interface bonding from substrates except graphene which can be free-standing. The involvement of strong hybrid of bonds, adsorption of exotic atomic species, and the high concentration of crystalline defects are often unavoidable, complicating the measurement of the intrinsic properties. In this work, we report the discovery of seven kinds of hitherto unreported bulk compounds (RO)nPb (R = rare earth metals, n = 1,2), which consist of quasi-2D Pb square nets that are spatially and electronically detached from the [RO]δ+ blocking layers. The band structures of these compounds near Fermi levels are relatively clean and dominantly contributed by Pb, resembling the electron-doped free-standing Pb monolayer. The R2O2Pb compounds are metallic at ambient pressure and become superconductors under high pressures with much enhanced critical fields. In particular, Gd2O2Pb (9.1 μB/Gd) exhibits an interesting bulk response of lattice distortion in conjunction with the emergence of superconductivity and magnetic anomalies at a critical pressure of 10 GPa. Our findings reveal the unexpected facets of 2D Pb sheets that are considerably different from their bulk counterparts and provide an alternative route for exploring 2D properties in bulk materials.

NaOH-Intercalated Iron Chalcogenides (Na1-xOH)Fe1-yX (X = Se, S): Ion-Exchange Synthesis and Physical Properties

The discovery of the (Li_1-xFe_xOH)FeSe superconductor has aroused significant interest in metal hydroxide-intercalated iron chalcogenides. However, all efforts made to intercalate NaOH between FeSe and FeS layers have failed so far. Here we report two NaOH-intercalated iron chalcogenides (Na_1-xOH)Fe_1-yX (X = Se, S) that were synthesized by a low-temperature hydrothermal ion-exchange method. Their crystal structures were solved through single-crystal X-ray diffraction and refined against powder X-ray and neutron diffraction data. Different from the (Li_1-xFe_xOH)FeX superconductors that crystallize in a tetragonal space group P4/nmm with Z = 2, (Na_1-xOH)Fe_1-yX belong to an orthorhombic space group Cmma with Z = 4. The structural solution also reveals that there are vacancies in both Na and Fe sites and there are not iron ions in the (Na_1-xOH) layer. This is probably why both Fe(II) and Fe(III) species exist in the title compounds, as detected by X-ray photoelectron spectroscopy. Based on magnetization and electrical resistivity measurements, the two compounds were found to be paramagnetic semiconductors. The absence of superconductivity should be closely related to the iron vacancies in the Fe_1-yX layer. Theoretical calculations suggest that inducing superconductivity in (Na_1-xOH)Fe_1-ySe is promising due to the similarity of the electronic structures between stoichiometric (NaOH)FeSe and the (Li_1-xFe_xOH)FeSe superconductor.

Highly-Tunable Crystal Structure and Physical Properties in FeSe-Based Superconductors

Here, crystal structure, electronic structure, chemical substitution, pressure-dependent superconductivity, and thickness-dependent properties in FeSe-based superconductors are systemically reviewed. First, the superconductivity versus chemical substitution is reviewed, where the doping at Fe or Se sites induces different effects on the superconducting critical temperature (Tc). Meanwhile, the application of high pressure is extremely effective in enhancing Tc and simultaneously induces magnetism. Second, the intercalated-FeSe superconductors exhibit higher Tc from 30 to 46 K. Such an enhancement is mainly caused by the charge transfer from the intercalated organic and inorganic layer. Finally, the highest Tc emerging in single-unit-cell FeSe on the SrTiO3 substrate is discussed, where electron-phonon coupling between FeSe and the substrate could enhance Tc to as high as 65 K or 100 K. The step-wise increment of Tc indicates that the synergic effect of carrier doping and electron-phonon coupling plays a critical role in tuning the electronic structure and superconductivity in FeSe-based superconductors.

Effects of Rb Intercalation on NbSe2: Phase Formation, Structure, and Physical Properties

Here, we report the crystal structures and properties of Rb _xNbSe₂, with 0 ≤ x ≤ 0.5. With Rb intercalation, Rb _xNbSe₂ evolves from 2H (Phase I) for 0 ≤ x ≤ 0.025, to 6R (Phase II) for x ∼ 0.2 with space group R3 m (no. 166), and finally to 2H (Phase IV) for 0.375 ≤ x ≤ 0.5 with space group P6₃/ mmc (no. 194). In addition, Phase II is found to transform to a rare 6H structure (Phase III) with space group P6₃/ mmc (no. 194) by annealing at a relatively low temperature. We show the first 6H phase in the intercalated transition-metal dichalcogenides (TMDs) family obtained through a solid-state reaction. Moreover, both 6R and 6H phases are new polymorphs in the NbSe₂ system. For the range 0.2 ≤ x ≤ 0.5 in Rb _xNbSe₂, we show metallic electronic transport behavior and a paramagnetic feature. The lack of superconductivity (SC) down to 2 K is most probably due to the decrease of hole carrier density with increasing Rb content. Through careful analysis of the structural data, we were able to assemble a phase diagram covering the range of 0 ≤ x ≤ 0.5 in Rb _xNbSe₂.

Quantum conductance-temperature phase diagram of granular superconductor K x Fe2-ySe2

It is now well established that the microstructure of Fe-based chalcogenide K _x Fe_2-ySe₂ consists of, at least, a minor (~15 percent), nano-sized, superconducting K _s Fe₂Se₂ phase and a major (~85 percent) insulating antiferromagnetic K₂Fe₄Se₅ matrix. Other intercalated A_1-xFe_2-ySe₂ (A = Li, Na, Ba, Sr, Ca, Yb, Eu, ammonia, amide, pyridine, ethylenediamine etc.) manifest a similar microstructure. On subjecting each of these systems to a varying control parameter (e.g. heat treatment, concentration x,y, or pressure p), one obtains an exotic normal-state and superconducting phase diagram. With the objective of rationalizing the properties of such a diagram, we envisage a system consisting of nanosized superconducting granules which are embedded within an insulating continuum. Then, based on the standard granular superconductor model, an induced variation in size, distribution, separation and Fe-content of the superconducting granules can be expressed in terms of model parameters (e.g. tunneling conductance, g, Coulomb charging energy, E _c , superconducting gap of single granule, Δ, and Josephson energy J = πΔg/2). We show, with illustration from experiments, that this granular scenario explains satisfactorily the evolution of normal-state and superconducting properties (best visualized on a [Formula: see text] phase diagram) of A _x Fe_2-ySe₂ when any of x, y, p, or heat treatment is varied.

Cs0.9Ni3.1Se3: A Ni-Based Quasi-One-Dimensional Conductor with Spin-Glass Behavior

In this work, we report the discovery of a new Ni-based quasi-one-dimensional selenide: Cs_0.9Ni_3.1Se₃. This compound adopts the TlFe₃Te₃-type structure with space group P6₃/ m, which consists of infinite [Ni₃Se₃] chains with face-sharing Ni₆ octahedra along the c direction. The lattice parameters are calculated as a = 9.26301(4) Å and c = 4.34272(2) Å, with the Ni-Ni distance in the ab plane as 2.582(3) Å, suggesting the formation of a Ni-Ni metallic bond in this compound. Interestingly, it has been found that Cs_0.9Ni_3.1Se₃ is nonstoichiometric, which is different from the other TlFe₃Te₃-type phases reported so far. Structure refinement shows that the extra Ni atom in the structure may occupy the 2c site, together with Cs atoms. Cs_0.9Ni_3.1Se₃ shows metallic behavior with monotonously decreased resistivity with temperatures from 300 to 0.5 K. Measurements on the magnetic susceptibility display a spin-glass state below 7 K. The specific heat curve gives a Sommerfeld coefficient of 14.6 mJ·K^-2·mol^-1 and a Debye temperature of 143.6 K. The discovery of this new compound enriches the diversity of low-dimensional materials in a transition-metal-based family and also sheds light on the structure-property relationship of this system.

Structure Evolution and Spin-Glass Transition of Layered Compounds ALiFeSe2 (A = Na, K, Rb)

Three new layered compounds, namely NaLiFeSe₂, KLiFeSe₂, and RbLiFeSe₂, have been discovered. NaLiFeSe₂ adopts a trigonal CaAl₂Si₂-type structure with space group P3̅m1, while the other two possess a tetragonal ThCr₂Si₂-type structure with space group I4/mmm. Structural refinements reveal that Li and Fe atoms randomly occupy the same sites in all these compounds without ordering. It is found that the radius of the alkali metals plays a vital role in determining the symmetry of this series of compounds. The substitution of Li at the Fe site shortens the layer spacing and elongates the A-Se bond length in the ThCr₂Si₂-type structure. The elongated Na-Se bond length would destabilize the ThCr₂Si₂-type structure in NaLiFeSe₂, suggesting that Na_xFe_2-ySe₂ lies at the border of ThCr₂Si₂-type and CaAl₂Si₂-type structures. Magnetic and resistivity measurements demonstrate that these compounds exhibit anisotropic spin-glass and narrow-band-gap semiconducting characteristics. First-principles calculations indicate that the introduction of Li enhances strong localization and weakens the correlation of the 3d electrons of Fe, which are responsible for the observed spin-glass transition and semiconducting conductions.

Kx(C2H8N2)yFe2−zS2: synthesis, phase structure and correlation between K+ intercalation and Fe depletion

Co-intercalated FeS K_x(C₂H₈N₂)_yFe_2−zS₂ has been synthesized by a sonochemical method. It has been found that the charged K⁺ intercalation depleted the Fe from [FeS] layers, resulting in disordered Fe vacancies and weak ferrimagnetic semiconducting behavior.

High-Tc superconducting phases in organic molecular intercalated iron selenides: synthesis and crystal structures

Phase pure hybrid iron-based superconductors with various structural types can be obtained by sonochemical insertion of organic molecules into FeSe-layers.