We report bulk superconductivity induced by an isovalent doping of phosphorus in BaFe(2)(As(1-x)P(x))(2). The P-for-As substitution results in shrinkage of the lattice, especially for the FeAs block layers. The resistivity anomaly associated with the spin-density-wave (SDW) transition in the undoped compound is gradually suppressed by the P doping. Superconductivity with a maximum T(c) of 30 K emerges at x = 0.32, coinciding with a magnetic quantum critical point (QCP) which is shown by the disappearance of SDW order and the linear temperature-dependent resistivity in the normal state. The T(c) values were found to decrease with further P doping and no superconductivity was observed down to 2 K for x≥0.77. The appearance of superconductivity in the vicinity of QCP hints at the superconductivity mechanism in iron-based arsenides.

Tl,K)FexSe2 single crystals were first successfully synthesized with the Bridgeman method. The physical properties are characterized by electrical resistivity, magnetic susceptibility and Hall coefficient. We found that the (Tl,K)FexSe2 (1.30 x 1.65) compounds show an antiferromagetic (AFM) insulator behavior, which may be associated with the Fe-vacancy ordering in the crystals. While in the 1.70 x < 1.78 crystals, superconductivity (SC) coexists with an insulating phase. As Fe content further increases, the bulk SC with Tc = 31 K (and a T onset c as high as 40 K) appears in the 1.78 x 1.88 crystals. Our discovery represents the first Fe-based high-temperature superconductivity (HTSC) at the verge of an AFM insulator.

We have studied EuFe2(As0.7P0.3)2 by the measurements of x-ray diffraction, electrical resistivity, thermopower, magnetic susceptibility, magnetoresistance and specific heat. Partial substitution of As with P results in the shrinkage of lattice, which generates chemical pressure to the system. It is found that EuFe2(As0.7P0.3)2 undergoes a superconducting transition at 26 K, followed by ferromagnetic ordering of Eu 2+ moments at 20 K. This finding is the first observation of superconductivity stabilized by internal chemical pressure, and supplies a rare example showing coexistence of superconductivity and ferromagnetism in the ferro-arsenide family.

Ternary iron arsenide EuFe2As2 with ThCr2Si2-type structure has been studied by magnetic susceptibility, resistivity, thermopower, Hall and specific heat measurements. The compound undergoes two magnetic phase transitions at about 200 K and 20 K, respectively. The former was found to be accompanied with a slight drop in magnetic susceptibility (after subtracting the Curie-Weiss paramagnetic contribution), a rapid decrease in resistivity, a large jump in thermopower and a sharp peak in specific heat with decreasing temperature, all of which point to a spin-density-wave-like antiferromagnetic transition. The latter was proposed to be associated with an A-type antiferromagnetic ordering of Eu 2+ moments. Comparing with the physical properties of the iso-structural compounds BaFe2As2 and SrFe2As2, we expect that superconductivity could be induced in EuFe2As2 through appropriate doping. [18,19,20] EuFe 2 As 2 is another member of the ternary iron arsenide family, [21] however, only few work was performed on this material. Mössbauer and magnetic susceptibility studies [22] indicated that EuFe 2 As 2 experienced two magnetic transitions. The first one around 200 K was due to the AFM transition in the iron sublattice. The second one at 19 K arose from the AFM ordering of Eu 2+ magnetic moments. No other physical properties of EuFe 2 As 2 have been reported. In order to assess the potential of inducing superconductivity in this compound, we have carried out a systematic study of the physical properties of EuFe 2 As 2 . We found that the transition at about 200 K was accompanied by a rapid decrease in resistivity, a large jump in thermopower and a sharp peak in specific heat. In addition, a slight drop in magnetic susceptibility was observed after subtracting the Curie-Weiss paramagnetic contribution of Eu 2+ magnetic moments. These properties are quite similar with those of BaFe 2 As 2 and SrFe 2 As 2 , suggesting that EuFe 2 As 2 is another possible parent compound in which superconductivity may be found by proper doping. Polycrystalline samples of EuFe 2 As 2 were synthesized from stoichiometric amounts of the elements as reported previously [21]. Fresh Eu grains, Fe powders and As grains were mixed in a ratio of 1:2:2, sealed in an evacuated quartz tube and sintered at 773 K for 12 hours then 1073 K for another 12 hours. After cooling, the reaction product was thoroughly ground in an agate mortar and pressed into pellets under a pressure of 2000 kg/cm 2 in an argon-filled glove-box. The pellets were annealed in an evacuated quartz tube at 1123 K for 12 hours and furnace-cooled to room temperature. The EuFe 2 As 2 samples were obtained as black powders, which is stable in air.