Superconductivity in anti-PbO-type iron chalcogenides Fe1-xTe1-ySey (x = 0, 0.1, y = 0.1 0.4) depends on the amount (x) of interstitial iron atoms located between the FeTe1-ySey layers. Non-superconducting samples of nominal Fe1.1Te1-ySey convert to
superconductors with critical temperatures up to 14 K after annealing at 300{deg}C in an oxygen atmosphere. The process is irreversible upon subsequent hydrogen annealing. Magnetic measurements are consistent with the formation of iron oxides suggesting that oxygen annealing preferably extracts interstitial iron from Fe1-xTe1-ySey which interfere with superconductivity.
[(Li0.8Fe0.2)OH]FeS and the series [(Li0.8Fe0.2)OH]Fe(S1-xSex) (0<x<1) were synthesized by hydrothermal methods and characterized by X-ray single crystal and powder diffraction, EDX and chemical analysis. Selenium-rich compounds show the coexistence
of magnetic ordering with superconductivity known from the pure selenium compound. Sulphur doping decreases the critical temperature through chemical pressure until superconductivity is completely absent in [(Li0.8Fe0.2)OH]FeS, while the ferromagnetism in the [(Li0.8Fe0.2)OH] layers persists. The Li:Fe ratio in the hydroxide layer, and thus the charge transfer of 0.2 electrons from the hydroxide to the iron chalcogenide layers remains unchanged in [(Li0.8Fe0.2)OH]Fe(S1-xSex), which indicates that the chemical pressure effect of the smaller sulphide ions impedes superconductivity in [(Li0.8Fe0.2)OH]FeS
We report superconductivity in polycrystalline samples of the 1038-type compounds (Ca$_{1-x}$RE$_x$)$_{10}$(FeAs)$_{10}$(Pt$_3$As$_8$) up to T$_c$ = 35 K with RE = Y, La-Nd, Sm, Gd-Lu. The critical temperatures are independent of the trivalent rare e
arth element used, yielding an universal T$_c$($x$) phase diagram for electron doping in all these systems. The absence of superconductivity in Eu$^{2+}$ doped samples, as well as the close resemblance of (Ca$_{1-x}$RE$_x$)$_{10}$(FeAs)$_{10}$(Pt$_3$As$_8$) to the 1048 compound substantiate that the electron doping scenario in the RE-1038 and 1048 phases is completely analogous to other iron-based superconductors with simpler crystal structures.
We present a detailed investigation of the electronic phase diagram of effectively charge compensated Ba1-xKx(Fe1-yCoy)2As2 with x/2 = y. Our experimental study by means of x-ray diffraction, Mossbauer spectroscopy, muon spin relaxation and ac suscep
tibility measurements on polycrystalline samples is complemented by density functional electronic structure calculations. For low substitution levels of x/2 = y < 0.13, the system displays an orthorhombically distorted and antiferromagnetically ordered ground state. The low temperature structural and magnetic order parameters are successively reduced with increasing substitution level. We observe a linear relationship between the structural and the magnetic order parameter as a function of temperature and substitution level for x/2 = y < 0.13. At intermediate substitution levels in the range between 0.13 and 0.19, we find superconductivity with a maximum Tc of 15 K coexisting with static magnetic order on a microscopic length scale. For higher substitution levels x/2 = y > 0.25 a tetragonal non-magnetic ground state is observed. Our DFT calculations yield a signifcant reduction of the Fe 3d density of states at the Fermi energy and a strong suppression of the ordered magnetic moment in excellent agreement with experimental results. The appearance of superconductivity within the antiferromagnetic state can by explained by the introduction of disorder due to non-magnetic impurities to a system with a constant charge carrier density. Our experimental study by means of x-ray diffraction, Mossbauer spectroscopy, muon spin relaxation and ac susceptibility measurements on polycrystalline samples is complemented by density functional electronic structure calculations.
The solid solution of antimonide-oxides Ba1-xKxTi2Sb2O (0 < x < 1) has been synthesized by solid-state reactions and characterized by X-ray powder diffraction (CeCr2Si2C-type structure; P4/mmm, Z = 1). The crystal structure consists of Ti2Sb2O-layers
that are stacked with layers of barium atoms along the c-axis. BaTi2Sb2O is a known superconductor with a critical temperature (Tc) of 1.2 K. Substitution of barium through potassium raises Tc up to 6.1 K at 12 % potassium, while no superconductivity emerges with concentrations higher than 20 %. Anomalies in electrical transport and magnetic susceptibility indicate charge density wave (CDW) instabilities. The CDW transition temperatures (Ta) decrease from 50 K in the parent compound to 28 K at 10 % potassium substitution. No CDW transition was detected at higher concentrations, and no evidence for a reduction of the lattice symmetry below Ta was found. The lattice parameters vary linearly while the unit cell volume increases with higher potassium concentrations. The phase diagrams Tc(x) and Ta(x) of Ba1-xKxTi2Sb2O are remarkably similar to the known series Ba1-xNaxTi2Sb2O (0 < x < 0.33) in spite of the reverse volume effect. From this we conclude that the charge and not the volume determines the phase diagrams of these superconducting antimony oxides.
