Extended Magnetic Dome Induced by Low Pressures in Superconducting FeSe_{1-x}S_{x}.

We report muon spin rotation and magnetization measurements under pressure on Fe_{1+δ}Se_{1-x}S_{x} with x≈0.11. Above p≈0.6 GPa we find a microscopic coexistence of superconductivity with an extended dome of long range magnetic order that spans a pressure range between previously reported separated magnetic phases. The magnetism initially competes on an atomic scale with the coexisting superconductivity leading to a local maximum and minimum of the superconducting T_{c}(p). The maximum of T_{c} corresponds to the onset of magnetism while the minimum coincides with the pressure of strongest competition. A shift of the maximum of T_{c}(p) for a series of single crystals with x up to 0.14 roughly extrapolates to a putative magnetic and superconducting state at ambient pressure for x≥0.2.

Systematic dimensional reduction of the layered ß-FeSe structure by solvothermal synthesis.

Dimensional reduction of superconducting anti PbO-type iron selenide has been achieved by terminating the tetragonal square layers of FeSe4/4 tetrahedra by ethylenediamine (en) ligands. We obtained three new structures in the Fe-Se-en system. Fe3Se4(en)3 contains FeSe2 single chains bridged via Fe(en)3 complexes. Fe10Se12(en)7 has Fe2Se3 double strands separated by Fe(en)3 complexes and free en molecules. Fe0.85Se(en)0.3 conserves the tetragonal layers of bulk FeSe which are now widely separated by en molecules. Through systematic dilution of the solvent we were able to introduce an additional parameter in solvothermal synthesis and thus have control over the connectivity of the tetrahedra. Additionally, a phase diagram of the Fe-Se-en system is generated by variation of the reaction temperature. The magnetic properties of the FeSe derivatives range from superconductivity and antiferromagnetism to paramagnetism.

Structural transition and superconductivity in hydrothermally synthesized FeX (X = S, Se).

Iron selenide obtained by mild hydrothermal reaction is not superconducting and exhibits a triclinic crystal structure below 60 K unlike superconducting FeSe from conventional solid state synthesis which is orthorhombic. In contrast, tetragonal iron sulphide FeS from hydrothermal synthesis is superconducting but undergoes no structural change on cooling.

[(Li(0.8)Fe(0.2))OH]FeS and the ferromagnetic superconductors [(Li(0.8)Fe(0.2))OH]Fe(S(1-x)Se(x)) (0 < x ≤ 1).

Superconductivity up to 43 K and magnetic ordering coexist in the iron chalcogenides [(Li(0.8)Fe(0.2))OH]Fe(S(1-x)Se(x)) (0 < x ≤ 1). Substitution of sulphur for selenium gradually suppresses superconductivity while the ferromagnetic signature persists up to non-superconducting [(Li(0.8)Fe(0.2))OH]FeS.

The specific heat of the electron-doped La-1038 compound (Ca0.85La0.15)10(FeAs)10(Pt3As8).

The specific heat of polycrystalline (Ca0.85La0.15)10(FeAs)10(Pt3As8), an electron-doped iron-based superconductor (T(c)(onset) = 34.6 K) with Ca/La ions and Pt3As8 separating the FeAs layers, was measured between 0.4 and 48 K.This compound has been recently reported to represent an electron-doped variant of the non-superconducting 10-3-8 phase, featuring a superconducting transition in the range of that of the 10-4-8 phase. This family of compounds is unique among the iron pnictide superconductors discovered to date due to the second metal pnictide layer, Pt3As8, present in the structure competing with the familiar FeAs layer for the electron from the Ca/La. This superconductor is further unusual in that it has a rather low crystalline symmetry (triclinic) for such a high superconducting transition temperature. The specific heat γ is found to be approximately 26 mJ/(Ca/La mol)K(2), comparable to 122 iron-based superconductors electron-doped on the Fe sites and a factor of two smaller than 122 compounds hole-doped on the cation site, e.g., Ba(1-x)K(x)Fe2As2. The present work also investigates the discontinuity in the specific heat at T(c), ΔC, to compare with the global trend, established by Bud'ko, Ni and Canfield (BNC), of ΔC/T(c) versus T(c) found for essentially all iron-based superconductors. The result is a value lower than the BNC trend by a factor of ten, consistent with a severely broadened superconducting transition.

Local structural studies of Ba(1-x)K(x)Fe2As2 using atomic pair distribution function analysis.

Systematic local structural studies of the Ba(1-x)K(x)Fe(2)As(2) system are undertaken at room temperature using atomic pair distribution function analysis. The local structure of the Ba(1-x)K(x)Fe(2)As(2) is found to be well described by the long-range structure extracted from diffraction experiments, but with anisotropic atomic vibrations of the constituent atoms (U11 = U22 not equal U33). The crystal unit cell parameters, the FeAs(4) tetrahedral angle and the pnictogen height above the Fe-plane are seen to show systematic evolution with K doping, underlining the importance of the structural changes, in addition to the charge doping, in determining the properties of Ba(1-x)K(x)Fe(2)As(2).

Transition from Mott insulator to superconductor in GaNb4Se8 and GaTa4Se8 under high pressure.

Electronic conduction in GaM4Se8 (M=Nb,Ta) compounds with the fcc GaMo4S8-type structure originates from hopping of localized unpaired electrons (S=1 / 2) among widely separated tetrahedral M4 metal clusters. We show that under pressure these systems transform from Mott insulators to a metallic and superconducting state with T(C)=2.9 and 5.8 K at 13 and 11.5 GPa for GaNb4Se8 and GaTa4Se8, respectively. The occurrence of superconductivity is shown to be connected with a pressure-induced decrease of the MSe6 octahedral distortion and simultaneous softening of the phonon associated with M-Se bonds.

Strontium selenogermanate(III) and barium selenogermanate(II,IV): synthesis, crystal structures, and chemical bonding

The new selenogermanates Sr2Ge2Se5 and Ba2Ge2Se5 were synthesized by heating stoichiometric mixtures of binary selenides and the corresponding elements to 750 degrees C. The crystal structures were determined by single-crystal X-ray methods. Both compounds adopt previously unknown structure types. Sr2Ge2Se5 (P2(1)/n, a = 8.445(2) A, b = 12.302 A, c = 9.179 A, beta = 93.75(3) degrees, Z = 4) contains [Ge4Se10]8- ions with homonuclear Ge-Ge bonds (dGe-Ge = 2.432 A), which may be described as two ethane-like Se3Ge-GeSeSe2/2 fragments sharing two selenium atoms. Ba2Ge2Se5 (Pnma, a = 12.594(3) A, b = 9.174(2) A, c = 9.160(2) A, Z = 4) contains [Ge2Se5]4- anions built up by two edge-sharing GeSe4 tetrahedra, in which one terminal Se atom is replaced by a lone pair from the divalent germanium atom. The alkaline earth cations are arranged between the complex anions, each coordinated by eight or nine selenium atoms. Ba2Ge2Se5 is a mixed-valence compound with GeII and GeIV coexisting within the same anion. Sr2Ge2Se5 contains exclusively GeIII. These compounds possess electronic formulations that correspond to (Sr2+)2(Ge3+)2(Se2-)5 and (Ba2+)2- Ge2+Ge4+(Se2-)5. Calculations of the electron localization function (ELF) reveal clearly both the lone pair on GeII in Ba2Ge2Se5 and the covalent Ge-Ge bond in Sr2Ge2Se5. Analysis of the ELF topologies shows that the GeIII-Se and GeIV-Se covalent bonds are almost identical, whereas the GeII-Se interactions are weaker and more ionic in character.