Superconductivity at the surprisingly high temperature of 55 K has been recently reported in fluorine-doped rare-earth iron oxyarsenides, REFeAsO1-xFx (RE = rare earth) [1]. The magnitude of Tc and the apparent similarities with the high-Tc cuprates – layered conducting FeAs slabs and proximity to antiferromagnetic and structural instabilities – have made these systems an intensely studied research field. REFeAsO possesses a tetragonal crystal structure comprising layers of edge-sharing FeAs4 tetrahedra interleaved with REO layers. On cooling, a structural phase transition to orthorhombic crystal symmetry, accompanied by the development of long range antiferromagnetic order occurs. The magnetic instability is suppressed upon fluorine-doping before the onset of superconductivity, while orthorhombic symmetry survives well within the superconducting dome [2]. The mechanism of superconductivity in these materials is still under debate. Inelastic X-ray measurements on NdFeAsO1–xFx on ID28 have shown that electron-phonon coupling could play a role in determining the superconducting state properties [3]. The family of Fe-based superconductors has now further expanded to include, among others, the binary FeSe1–x phases [4].

While electron-doping in REFeAsO1–xFx has been intensively studied, very little is known about the structural and electronic properties of hole-doped phases (e.g. partial replacement of the RE ion by alkaline earths). Confirming the existence of superconducting phases upon hole doping, probing how Tc varies with composition, and revealing the influence of doping on the structural and magnetic transitions are of fundamental importance for the understanding of the pairing mechanism.

Recently, we have investigated the new family of hole-doped systems, Nd1–xSrxFeAsO (x = 0.05, 0.1, 0.2). Inspection of the room temperature diffraction profiles collected at beamline ID31 reveals the tetragonal unit cell (P4/nmm) established before for REFeAsO (Figure 39). Rietveld analysis shows that the a-axis increases monotonically with x as expected considering the larger ionic radius of Sr2+, while the c-axis initially contracts at small x before undergoing a considerable expansion at x > 0.1. The c-axis dimensions are influenced by the geometry of the AsFe4 pyramidal units with the Fe-As-Fe angle, , first increasing for 0 < x < 0.1 and then decreasing for x > 0.1 (Figure 39). The electronic structure of the REFeAsO systems depends on small changes in the geometry of the AsFe4 pyramidal units which controls both the Fe near- and next-near-neighbour exchange interactions and the width of the electronic conduction band. The sudden increase in the compression of the AsFe4 pyramidal units coupled with the larger lattice dimensions for the x = 0.2 composition may lead to a much smaller bandwidth and changes the electronic behaviour as compared to that at small x. Low temperature diffraction profiles of all compositions showed the presence of a structural phase transition from tetragonal to orthorhombic (space group Cmma) in analogy with the undoped and electron-doped REFeAsO systems. However, increasing the Sr2+ content does not suppress the structural transition which occurs at almost the same temperature for all compositions.

Fig. 39: Synchrotron X-ray ( = 0.40301 Å) powder diffraction profile of Nd0.8Sr0.2FeAsO at room temperature. Inset: evolution of the room temperature tetragonal lattice constants as a function of Sr2+ doping and schematic diagram of the AsFe4 pyramidal unit.

In agreement with the diffraction experiments, resistivity () measurements show considerable differences with increasing x. While the parent compound displays the typical behaviour of REFeAsO systems, the resistivity curve for x = 0.05 reveals a drastic change with r increasing on cooling. It thus appears that at low x, the electron charge carriers in NdFeAsO are initially depleted pushing the system through a transition from bad metal to semiconductor. Further hole doping increases the amount of hole-type carriers, and metallic conductivity is restored. Indeed, the x = 0.2 system shows metallic-type behaviour with r decreasing with temperature and a superconducting transition occurs at Tc =13.5 K (Figure 40).

Fig. 40: Electronic phase diagram of electron/hole doped NdFeAsO. Blue (red) squares mark the superconducting transition temperatures for electron (hole) doping. Inset: schematic diagram of the tetragonal structure of Sr2+ and F– doped NdFeAsO and temperature dependence of the resistivity for Nd1–xSrxFeAsO [x = 0 (black), x = 0.05 (green), x = 0.2 (red)].

Our experimental observations show that hole-doping of NdFeAsO via partial replacement of Nd3+ by Sr2+ is a successful route to induce superconductivity. However, the structural and electronic response with doping is different from and non-symmetric to that in the electron-doped side of the phase diagram (Figure 40). For NdFeAsO, higher levels of hole doping are necessary to observe a similar phenomenology to the electron-doped systems as an increased number of carriers is necessary to overcome the initial semiconducting behaviour.

 

Principal publication and authors

K. Kasperkiewicz (a), J.W.G. Bos (a), A.N. Fitch (b), K. Prassides (c), S. Margadonna (a), Chem. Commun., DOI:10.1039/B815830D (2009).
(a) Department of Chemistry, University of Edinburgh (UK)
(b) ESRF
(c) Department of Chemistry, University of Durham (UK)

References

[1] Y. Kamihara, T. Watanabe, M Hirano and H. Hosono, JACS 130, 3296 (2008).
[2] S. Margadonna, Y. Takabayashi, M.T. McDonald, M. Brunelli, G. Wu, R.H. Liu, X.H. Chen and K. Prassides, Phys. Rev. B 79, 014503 (2009).
[3] M. Le Tacon, M. Krisch, A. Bosak, J.-W.G. Bos, and S. Margadonna, Phys. Rev. B 78, 140505 (2008).
[4] S. Margadonna, Y. Takabayashi, M.T. McDonald, K. Kasperkiewicz, Y. Mizuguchi, Y. Takano, A.N. Fitch, E. Suard and K. Prassides, Chem. Commun., 5607 (2008).