L4Fe2As2Te1−xO4−yFy (L = Pr, Sm, Gd): A layered oxypnictide superconductor with Tc up to 45 K
The synthesis and structural and physical properties of iron lanthanide oxypnictide superconductors, L4Fe2As2Te1−xO4 (L = Pr, Sm, Gd), with transition temperature at ∼25 K are reported. Single crystals have been grown at high pressure using the cubic anvil technique. The crystal structure consists of layers of L2O2 tetrahedra separated by alternating layers of chains of Te and of Fe2As2 tetrahedra: -L2O2-Te-L2O2-Fe2As2-L2O2-Te-L2O2- (space group: I4/mmm, a ∼ 4.0, c ∼ 29.6 Å). Substitution of oxygen by fluorine increases the critical temperature, e.g., in Gd4Fe2As2Te1−xO4−yFy up to 45 K. Magnetic torque measurements reveal an anisotropy of the penetration depths of ∼31.