Homologous series of iron pnictide oxide superconductors (Fe2As2)[Can+1(Sc,Ti)nOy] (n=3,4,5) with extremely thick blocking layers

Abstract

2We have discovered first homologous series of iron pnictide oxide superconductors (Fe 2 As 2 )(Ca n+1 (Sc,Ti) n O y ) [n = 3,4,5]. These compounds have extremely thick blocking layers up to quintuple perovskite oxide layers sandwiched by the Fe 2 As 2 layers. These samples exhibited bulk superconductivity with relatively high T c up to 42 K. The relationship between T c and the iron-plane interlayer distance suggested that superconductivity due to the mono Fe 2 As 2 layer is substantially 40 K-class.In a recently discovered layered iron pnictide family 1 , the superconducting transition temperature T c increased under ambient pressure in the order of FeSe (T c = 8-13 K, d = 5.49 Å) 2, (Ba,K)Fe 2 As 2 (38 K, 6.65 Å) 3 and SmFeAs(O,F) (55 K, 8.44 Å) 4 , where d represents the distance between the iron-planes. This suggested that emphasis of two-dimensionality by an increase of the interlayer distance of superconducting planes would be a promising method to develop new high-T c superconductors. On the other hand, a new family of layered iron pnictides with perovskite-type oxide blocking layers has recently been discovered 5-12 . The thickness of the perovskite-type layer can be controlled according to the composition and the synthesis conditions; therefore, an attempt to develop new layered iron pnictides with thicker oxide layers may result in a new series of homologous superconductors. Novel compounds with double or triple perovskite layers were discovered in our recent study, such as (Fe 2 As 2 )(Ba 3 Sc 2 O 5 ) and (Fe 2 As 2 )(Sr 4 (Sc,Ti) 3 O 8 ) 12 . These new compounds indicated that a perovskite layer, which can maintain electroneutrality with the (Fe 2 As 2 ) -2 layer and have an appropriate lattice size a~4 Å, can form layered iron pnictide oxides. According to these 3 empirical observations, three new compounds with different crystal structures, (Fe 2 As 2 )(Ca n+1 (Sc,Ti) n O y ) [n = 3,4,5 and y ~3n-1] were discovered.Samples were synthesized by solid-state reaction from starting materials of FeAs (3N), Ca (2N), CaO (2N), Ti (3N), TiO 2 (3N) and Sc 2 O 3 (3N). The starting reagents were moisture sensitive, so that manipulation was carried out in a glove box filled with argon gas. Powder mixtures were pelletized, sealed in evacuated quartz ampoules and heated at 1000-1200 °C for 60-100 h followed by slow cooling to room temperature.Phase identification was carried out using powder XRD (Rigaku Ultima-IV) and intensity data were collected in the 2θ range of 3-80° at a step of 0.02° using Cu-K α radiation. High resolution TEM images were taken using JEM-2010F and JEM-4010 (JEOL). Magnetic susceptibility was measured using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-XL5s). Electrical resistivity was evaluated by the AC four-point-probe method (Quantum Design PPMS). (Fe 2 As 2 )(Ca 4 (Sc 0.67 Ti 0.33 ) 3 O y ), (Fe 2 As 2 )(Ca 5 (Sc 0.5 Ti 0.5 ) 4 O y ) and (Fe 2 As 2 )(Ca 6 (Sc 0.4 Ti 0.6 ) 5 O y ) samples were successfully obtained by sintering for 72 h at 1050,...

Superconductivity in Ca1-xLaxFeAs2: A Novel 112-Type Iron Pnictide with Arsenic Zigzag Bonds

“…[2][3][4][5][6][7][8][9][10][11][12][13][14] All of the iron pnictide superconductors identified so far consist of a common structural motif, i.e., Fe 2 As 2 layers that are alternately stacked with various kinds of spacer layers. Therefore, the central goal for realizing a higher T c has been finding a novel spacer layer that can suitably tune the electronic states of Fe 2 As 2 layers.…”

Fabrication of binary FeSe superconducting wires by diffusion process

“…Since the discovery of the superconductivity in LaFeAsO 1-x F x 1 , several types of iron-based superconductors with layered structure have been discovered [2][3][4][5] . Among these iron-based superconductors, tetragonal FeSe with transition temperature of T c zero ~8 K and T c onset ~12 K has the simplest structure (PbO-type) and binary composition, consist of a stack of FeSe layers along the c-axis 4,6 .…”

NMR investigation of the iron-based superconductors Ca4(Mg,Ti)3Fe2AsTomita1, Kotegawa2, Tao3 et al. 2012Phys. Rev. B

“…[1,8] The MgTi:3 exhibits T onset c = 47 K in a resistivity measurement, which is the highest value among perovskite systems. [1] The presence of a clear diamagnetism is confirmed below 43 K. The T onset c of ScTi:4 is 41 K as per a resistivity measurement, and diamagnetism is observed below 36 K. [8] The nominal composition of each compounds is given as Ca 4 (Mg 0.25 Ti 0.75 ) 3 Fe 2 As 2 O 8−y and Ca 5 (Sc 0.5 Ti 0.5 ) 4 Fe 2 As 2 O 11−y , however the doping level is unclear due to a difficulty of the quantitative analysis of the stoichiometric ratio. The samples were crushed into powder for NMR measurements.…”

“…[1,[4][5][6][7][8][9][10] Figure 1 shows the crystal structures of the latter type, Ca n+1 M n Fe 2 As 2 O 3n−1 (n = 3 and 4). The thick blocking layer composed of Ca n+1 M n O 3n−1 expands the distance between FeAs layers along the c axis, and this layer distance ranges from ∼ 15to25 A.…”