The ternary iron arsenide ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at ${T}_{c}=38\text{ }\text{ }\mathrm{K}$ in $({\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ with $x\ensuremath{\approx}0.4$. The parent compound ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ crystallizes in the tetragonal ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure, which consists of $(\mathrm{FeAs}{)}^{\ensuremath{\delta}\ensuremath{-}}$ iron arsenide layers separated by ${\mathrm{Ba}}^{2+}$ ions. ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ is a poor metal and exhibits a spin density wave anomaly at 140 K. By substituting ${\mathrm{Ba}}^{2+}$ for ${\mathrm{K}}^{+}$ ions we have introduced holes in the $(\mathrm{FeAs}{)}^{\ensuremath{-}}$ layers, which suppress the anomaly and induce superconductivity. The ${T}_{c}$ of 38 K in $({\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ is the highest in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure.

Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2.

Here we report bulk superconductivity in ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_{2}$ single crystals below ${T}_{c}=22\text{ }\text{ }\mathrm{K}$, as demonstrated by resistivity, magnetic susceptibility, and specific heat data. Hall data indicate that the dominant carriers are electrons, as expected from simple chemical reasoning. This is the first example of superconductivity induced by electron doping in this family of materials. In contrast with cuprates, the ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ system appears to tolerate considerable disorder in the FeAs planes. First principles calculations for ${\mathrm{BaFe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_{2}$ indicate the interband scattering due to Co is weak.

Superconductivity at 22 K in Co-doped BaFe2As2 crystals.

The synthesis and properties of LiFeAs, a high-${T}_{c}$ Fe-based superconducting stoichiometric compound, are reported. Single crystal x-ray studies reveal that it crystallizes in the tetragonal PbFCl type (P4/nmm) with $a=3.7914(7)\text{ }\text{\AA{}}$ and $c=6.364(2)\text{ }\text{\AA{}}$. Unlike the known isoelectronic undoped intrinsic FeAs compounds, LiFeAs does not show any spin-density wave behavior but exhibits superconductivity at ambient pressures without chemical doping. It exhibits a respectable transition temperature of ${T}_{c}=18\text{ }\text{K}$ with electronlike carriers and a very high critical field, ${\text{H}}_{c2}(0)g80\text{ }\text{T}$. LiFeAs appears to be the chemical equivalent of the infinite layered compound of the high-${T}_{c}$ cuprates.

/pdf/lifeas-an-intrinsic-feas-based-superconductor-with-tc-18-k-1etpmnyy2d.pdf

LiFeAs: An intrinsic FeAs-based superconductor with Tc=18 K

New high-${T}_{c}$ Fe-based superconducting compounds, $A{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ with $A=\mathrm{K}$, Cs, $\mathrm{K}/\mathrm{Sr}$, and $\mathrm{Cs}/\mathrm{Sr}$, were synthesized. The ${T}_{c}$ of ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{CsFe}}_{2}{\mathrm{As}}_{2}$ is 3.8 and 2.6 K, respectively, which rises with partial substitution of Sr for K and Cs and peaks at 37 K for 50%--60% Sr substitution, and the compounds enter a spin-density-wave state with increasing electron number (Sr content). The compounds represent $p$-type analogs of the $n$-doped rare-earth oxypnictide superconductors. Their electronic and structural behavior demonstrate the crucial role of the (${\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$) layers in the superconductivity of the Fe-based layered systems, and the special feature of having elemental $A$ layers provides new avenues to superconductivity at higher ${T}_{c}$.

/pdf/superconducting-fe-based-compounds-a1-xsrx-fe2as2-with-a-k-2gfma5clvv.pdf

Superconducting Fe-based compounds (A1-xSrx)Fe2As2 with A=K and Cs with transition temperatures up to 37 K.

http://absimage.aps.org/image/MAR09/MWS_MAR09-2008-003725.pdf

Superconducting Fe-Based Compounds (A$_{1-x}$Sr$_{x })$Fe$_{2}$As$_{2}$ with A = K and Cs with Transition Temperatures up to 37 K

Zusammenfassung Neue ternare und quaternare farbige Phasen der Zusammensetzungen MgMX und LiMgMX mit (M ≡ Pd, Pt; X ≡ Sn, Sb) konnten dargestellt und rontgenographisch sowie reflektionsspektroskopisch untersucht werden. Strukturuntersuchungen an Einkristallen ergaben eine “aufgefullte” Blendestruktur (Raumgruppe, F 43 m ) fur diese Phasen. Die optischen Eigenschaften der Phasen wurden nach einer Methode von Avery im Bereich von 0.5 bis 4.0 eV bestimmt. Samtliche untersuchten Phasen zeigen im niederenergetischen Bereich hohe Reflektivitat und starke Interbandubergange im sichtbaren Bereich des Spektrums.

Optische Untersuchungen an farbigen Intermetallischen phasen

Abstract Ternary and quaternary, mostly coloured compounds of Au, Pd and Pt, resp., with Li, Mg, Sc and Sn were prepared and structurally investigated. They crystallize in a \"filled\" Zinkblende type structure (space group F?3m) and can be characterized as ternary or quaternary Zintl-phases. Colours seem to depend on unfilled d-orbitals of Au, Pd or Pt.

Zur Kenntnis farbiger ternärer und quaternärer Zintl-Phasen / Coloured Ternary and Quaternary Zintl-Phases

Systematische Untersuchungen in den Systemen Barium–Indium/Zink/Cadmium–Germanium fuhrten zur Darstellung einer Reihe neuer ternarer, elektrovalent zusammengesetzter Varianten des A8X46-Clathrat-I-Typs. Sie kristallisieren kubisch in der Raumgruppe Pm3n. Fur Ba8In16Ge30 (a = 1 075,8 pm), Ba8Zn8Ge38 (a = 1 082,0 pm) und Ba8Cd8Ge38 (a = 1 096,0 pm) wurden die Strukturen rontgenographisch aus Einkristalldaten bestimmt. Die Verbindungen lassen sich im Rahmen des Zintl-Konzepts beschreiben und sind Halbleiter, wie Messungen der Temperaturabhangigkeit der elektrischen Leitfahigkeit belegen. Die Phasenanalysen zeigten, das sich ein Teil des 2B- bzw. 3B-Metalls durch Germanium substituieren last, wobei die Ladungsbilanz durch „Einbau von Lucken” in die Strukturen ausgeglichen wird. Es entstehen Defektvarianten des A8X46-Typs. Die Grenzzusammensetzungen liegen bei Ba8In4Ge9[]3Ge30 (a = 1 084,9 pm), Ba8Zn4Ge10[]2Ge30 (a = 1 073,6 pm) und Ba8Cd4Ge10[]2Ge30 (a = 1 082,0 pm).



New Ternary Clathrate Compounds in the Systems Barium–Indium/Zinc/Cadmium–Germanium: Zintl Compounds with Phase Width?



By systematic investigations in the systems barium–indium/zinc/cadmium–germanium we found a couple of new electrovalent ternary compounds with A8X46 clathrate (I) type structures. They crystallize cubically in space-group Pm3n. For Ba8In16Ge30 (a = 1 075.8 pm), Ba8Zn8Ge38 (a = 1 082.0 pm) and Ba8Cd8Ge38 (a = 1 096.0 pm) the structures were verified by X-ray single crystal diffraction data. According to valence and bounding distances the new clathrates should be Zintl compounds. Measurements of the temperature dependence of the electrical resistivity proved, that they are indeed semiconductors. A part of the 2B/3B metal atoms can be substituted by germanium. Charge balance will be retained by creation of vacancies in the A8X46 type structures. By phase analysis the limits of the composition range were determined as Ba8In4Ge9[]3Ge30 (a = 1 084.9 pm), Ba8Zn4Ge10[]2Ge30 (a = 1 073.6 pm) and Ba8Cd4Ge10[]2Ge30 (a = 1 082.0 pm).

Neue ternäre Käfigverbindungen aus den Systemen Barium–Indium/Zink/Cadmium–Germanium: Zintl-Verbindungen mit Phasenbreite?

Neutronenbeugungsexperimente fuhrten zur Bestimmung antiferromagnetischer Momentanordnungen der Verbindungen NaMnP, NaMnAs, NaMnSb, NaMnBi, LiMnAs und KMnAs. Fur die Natriumverbindungen wurde eine Anordnung gefunden, die sich in der kristallographischen Elementarzelle beschreiben last, fur LiMnAs und KMnAs ergibt sich eine Verdoppelung der c-Achse. Die Shubnikov-Gruppen sind P4/n′m′m′ bzw. P4/n′ce. Die gefundenen Werte fur die magnetischen Momente der Manganatome lassen sich mit Kristallfeldgrosen der AMnX-Strukturen korrelieren.



The Magnetic Properties of NaMnP, NaMnAs, NaMnSb, NaMnBi, LiMnAs, and KMnAs, Characterized by Neutron Diffraction Experiments



Neutron diffraction experiments led to the determination of the antiferromagnetic spin structures of NaMnP, NaMnAs, NaMnSb, NaMnBi, LiMnAs and KMnAs. For the sodium compounds an arrangement was found that can be described in the crystallographic cell; in the case of LiMnAs and KMnAs the c-axes have to be doubled. The Shubnikov-groups are P4/n′m′m′ and P4/n′cc, respectively. The values determined for the magnetic moments can be correlated with the crystal field splittings in the AMnX-structures.

Zur Charakterisierung der magnetischen Eigenschaften von NaMnP, NaMnAs, NaMnSb, NaMnBi, LiMnAs und KMnAs über Neutronenbeugungsexperimente

Four new ternary compounds KB₂X₂ (B = Fe, Co, Rh; X = P, As) were prepared. They are isotypic and crystallize in a ThCr ₂Si₂-type structure (I4/mmm) with the following lattice constants:KFe₂As₂: a = 384.2 pm c = 1386.1 pm KCO₂AS₂: a = 379.4 pm c = 1357.0 pmKRh₂As₂ : a = 398.7 pm c = 1326.7 pmKRh₂P₂ : a = 388.4 pm c = 1298.6 pm

Hans-Uwe Schuster

Papers

Optische Untersuchungen an farbigen Intermetallischen phasen

Zur Kenntnis farbiger ternärer und quaternärer Zintl-Phasen / Coloured Ternary and Quaternary Zintl-Phases

Neue ternäre Käfigverbindungen aus den Systemen Barium–Indium/Zink/Cadmium–Germanium: Zintl-Verbindungen mit Phasenbreite?

Zur Charakterisierung der magnetischen Eigenschaften von NaMnP, NaMnAs, NaMnSb, NaMnBi, LiMnAs und KMnAs über Neutronenbeugungsexperimente

Zur Struktur von KFe2As2, KCo2As2, KRh2As / Crystal Structure of KFe2As2, KCo2As2, KRh2As 2 and KRh2P2