Layered Iron Arsenide Research Articles

Synthesis and physical properties of Ca1−xRExFeAs2 with RE = La–Gd

The synthesis of a series of layered iron arsenides Ca1−xRExFeAs2(112) was attempted by heating at 1000 °C under a high pressure of 2 GPa. The 112 phase successfully forms with rare earth (RE) = La, Ce, Nd, Sm, Eu, and Gd, while RE-free samples do not contain the 112 phase. While a Ce-doped sample does not show superconductivity, La-, Pr-, Nd-, Sm-, Eu-, and Gd-doped samples show superconducting transition temperatures of 24.5, 13.2, 11.9, 11.6, 9.3, and 12.6 K, respectively, in magnetization measurements. Their Tc and Fe–Fe interlayer distances decreased with decreasing ionic radii of RE, but there are exceptions.

Enhancement of the superconducting transition temperature of FeSe by intercalation of a molecular spacer layer

The discovery of high-temperature superconductivity in a layered iron arsenide has led to an intensive search to optimize the superconducting properties of iron-based superconductors by changing the chemical composition of the spacer layer between adjacent anionic iron arsenide layers. Superconductivity has been found in iron arsenides with cationic spacer layers consisting of metal ions (for example, Li(+), Na(+), K(+), Ba(2+)) or PbO- or perovskite-type oxide layers, and also in Fe(1.01)Se (ref. 8) with neutral layers similar in structure to those found in the iron arsenides and no spacer layer. Here we demonstrate the synthesis of Li(x)(NH(2))(y)(NH(3))(1-y)Fe(2)Se(2) (x~0.6; y~0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer between FeSe layers, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far. We have determined the crystal structure using neutron powder diffraction and used magnetometry and muon-spin rotation data to determine the superconducting properties. This new synthetic route opens up the possibility of further exploitation of related molecular intercalations in this and other systems to greatly optimize the superconducting properties in this family.

Relationship Between Crystal Structures and Physical Properties in Iron Arsenides with Perovskite-type Layers

Effect of two-dimensionality of crystal structure on superconducting transition temperature(Tc) in iron arisenides with thick perovskite-type layer was investigated. Solid solutions of (Fe2As2)(Ca4(Sc,Ti)3Oy) and (Fe2As2)(Ca4(Mg,Ti)3Oy), as well as (Fe2As2)(Ca5(Sc,Ti)4Oy) and (Fe2As2)(Ca5(Mg,Ti)4Oy) were successfully fabricated. Lattice constants were systematically controlled by mixing ratio of Mg, Sc and Ti. All samples show bulk superconductivity at 30 – 40 K. Maximum Tc was achieved at a = 3.88 Å in both (Fe2As2)(Ca4M3Oy) and (Fe2As2)(Ca5M4Oy) compounds. Tc slightly decreases with increasing thickness of the perovskite-type layer. It indicates that the interlayer distance does not have positive effect on Tc in layered iron arsenides with thick blocking layers.

Irreversibility lines of layered Fe-based superconductors with thick blocking layers

Irreversibility lines of the layered iron-arsenide superconductors with thick perovskite-type oxide blocking layers have been systematically studied. Five compounds with different interlayer distance between iron layers from 1.5 to 3.0 nm were selected for the magnetization measurements. These compounds showed very large magnetically reversible region. The irreversibility lines of these compounds were found to locate at very low fields compared to the other iron-based superconductors and shift towards lower fields with an increase in the thickness of the blocking layer. In addition, the analysis on the irreversibility line suggested that the change of dimensionality of the vortex system occurs at T/Tc∼0.6. These results indicate that the electromagnetic anisotropy originated from the layered crystal structure plays a crucial role to determine the irreversibility line and magnetic phase diagram of layered iron arsenides as is the case in the layered cuprates.

Hydrogen in layered iron arsenides: Indirect electron doping to induce superconductivity

Utilizing the high stability of calcium and rare-earth hydrides, CaFeAsF${}_{1\ensuremath{-}x}$H${}_{x}$ ($x$ $=$ 0.0--1.0) and SmFeAsO${}_{1\ensuremath{-}x}$H${}_{x}$ ($x$ $=$ 0.0--0.47) have been synthesized using high pressure to form hydrogen-substituted 1111-type iron-arsenide superconductors. Neutron diffraction and density functional calculations have demonstrated that the hydrogens are incorporated as H${}^{\ensuremath{-}}$ ions occupying F${}^{\ensuremath{-}}$ sites in the blocking layer of CaFeAsF. The resulting CaFeAsF${}_{1\ensuremath{-}x}$H${}_{x}$ is nonsuperconducting, whereas, SmFeAsO${}_{1\ensuremath{-}x}$H${}_{x}$ is a superconductor, with an optimal ${T}_{\mathrm{c}}$ $=$ 55 K at $x$ \ensuremath{\sim} 0.2. It was found that up to 40% of the O${}^{2\ensuremath{-}}$ ions can be replaced by H${}^{\ensuremath{-}}$ ions, with electrons being supplied into the FeAs layer to maintain neutrality (O${}^{2\ensuremath{-}}$ $=$ H${}^{\ensuremath{-}}$ $+$ ${e}^{\ensuremath{-}}$). When $x$ exceeded 0.2, ${T}_{\mathrm{c}}$ was reduced corresponding to an electron overdoped region.

A new iron pnictide oxide (Fe2As2)(Ca5(Mg, Ti)4Oy) and a new phase in the Fe–As–Ca–Mg–Ti–O system

A new layered iron arsenide oxide(Fe2As2)(Ca5(Mg, Ti)4Oy) and its structural derivative were found in the Fe–As–Ca–Mg–Ti–O system. The crystal structure of(Fe2As2)(Ca5(Mg, Ti)4Oy) is identicalto that of (Fe2As2)(Ca5(Sc, Ti)4Oy), which was reported in our previous study. The lattice constants of this compound area = 3.86(4) Å andc = 41.05(2) Å.In addition, another phase with a thicker blocking layer was found. The structure of the compoundwas tentatively assigned through scanning transmission electron microscope observation as(Fe2As2)(Ca8(Mg, Ti)6Oy) withsextuple perovskite-type sheets divided by a rock salt layer. The interlayer Fe–Fe distance of this compoundis ∼ 30 Å. These compounds exhibited bulk superconductivity, as found from magnetization andresistivity measurements.

Structure, Magnetism, and Superconductivity of the Layered Iron Arsenides Sr1-xNaxFe2As2

The synthesis and evolution of the structure, magnetic ordering, and superconductivity of the layered iron arsenides Sr1-xNaxFe2As2 is reported. In the Sr1-xNaxFe2As2 solid solution, the limiting Na-rich composition in samples made using conventional solid state synthesis at elevated temperatures occurs at an unusually small value (x = 0.4) compared with other alkali-metal-doped alkaline earth iron arsenides. Above this limiting value of the sodium content, competing phases are formed: for x = 0.42, an elemental iron impurity is evident, and additional impurities appear for x > 0.42. Superconductivity is detected in the compositions approaching the phase limit (Tc = 26 K for x = 0.4) in line with analogous isoelectronic materials. However, the magnetically ordered state which competes with the superconducting state appears not to be completely suppressed even at the limiting composition. The Na doping of SrFe2As2 is contrasted with the K-doping of SrFe2As2 and Na-doping of BaFe2As2 and other “122” iron ar...

Indifference of Superconductivity and Magnetism to Size-Mismatched Cations in the Layered Iron Arsenides Ba1−xNaxFe2As2

The evolution of the structure, magnetic ordering, and superconductivity in the series Ba1−xNaxFe2As2 is reported up to the limiting Na-rich composition with x = 0.6; the more Na-rich compositions are unstable at high temperatures with respect to competing phases. The magnetic and superconducting behaviors of the Ba1−xNaxFe2As2 members are similar to those of the better-investigated Ba1−xKxFe2As2 analogues. This is evidently a consequence of the quantitatively similar evolution of the structure of the FeAs layers in the two series. In Ba1−xNaxFe2As2 antiferromagnetic order and an associated structural distortion are evident for x ≤ 0.35 and superconductivity is evident when x exceeds 0.2. For 0.4 ≤ x ≤ 0.6 bulk superconductivity is evident, and the long-range antiferromagnetically ordered state is completely suppressed. The maximum Tc in the Ba1−xNaxFe2As2 series, as judged by the onset of diamagnetism, is 34 K in Ba0.6Na0.4Fe2As2. Despite the large mis-match in sizes between the two electropositive cations which separate the FeAs layers, there is no evidence for ordering of these cations on the length scale probed by electron diffraction.

Low-temperature Crystal Structure and 57Fe Mössbauer Spectroscopy of Sr3Sc2O5Fe2As2

The crystal structure of the layered iron arsenide Sr3Sc2O5Fe2As2 was determined between 300 and 10 K. The lattice parameters of the tetragonal cell decrease anisotropically according to Δc/c : Δa/a ≈ 4.2, which results in a slight flattening of the As-Fe-As bond angle within the FeAs layers. No indication of a structural instability could be detected. 57Fe Mössbauer spectroscopic data show only a single signal at 4.2, 77, and 298 K, subjected to quadrupole splitting. The isomer shift increases from 0.36(1) mms−1 at 298 K to 0.49(1) mms−1 at 4.2 K. No indication for magnetic ordering was found.

Response of Superconductivity and Crystal Structure of LiFeAs to Hydrostatic Pressure

On the application of hydrostatic pressures of up to 1.3 GPa, the superconducting transition temperatures (T(c)) of samples of LiFeAs are lowered approximately monotonically at approximately -2 K GPa(-1). Measurements of the X-ray powder diffraction pattern at hydrostatic pressures of up to 17 GPa applied by a He gas pressure medium in a diamond anvil cell reveal a bulk modulus for LiFeAs of 57.3(6) GPa which is much smaller than that of other layered arsenide and oxyarsenide superconductors. LiFeAs also exhibits much more isotropic compression than other layered iron arsenide superconductors. The higher and more isotropic compressibility is presumably a consequence of the small size of the lithium ion. At ambient pressure the FeAs(4) tetrahedra are the most compressed in the basal plane of those in any of the superconducting iron arsenides. On increasing the pressure the Fe-Fe distance contracts more rapidly than the Fe-As distance so that the FeAs(4) tetrahedra become even more distorted from the ideal tetrahedral shape. The decrease in T(c) with applied pressure is therefore consistent with the observations that in the iron arsenides and related materials investigated thus far, T(c) is maximized for a particular electron count when the FeAs(4) tetrahedra are close to regular.

Structure, antiferromagnetism and superconductivity of the layered iron arsenide NaFeAs

A new layered iron arsenide NaFeAs isostructural with the superconducting lithium analogue displays evidence for the coexistence of superconductivity and magnetic ordering.