**4. Conclusion**

38 Superconductors – Materials, Properties and Applications

may determine superconducting behavior.

other, revealing a distinct site occupation and local in-homogeneity. A detailed polarization study of the Se K-edge demonstrates changes in the A2 and B2 peaks with the xy and z characteristic of the *p* states (Joseph et al., 2010). Owing to the natural characteristic of the *p* orbitals, the multiple scattering in the XAS results that are associated with *p* orbital symmetry reveals their different orbital orientations, thus reflecting *p*xy and *pz* character. This feature shifts to a lower energy owing to local inhomogeneity caused by doping. This finding is supported by the work of Joseph *et al.,* (2010) and implies distortion of the tetrahedral symmetry at the Se sites. Te K-edge spectra in Fig. 9 (b) include one edge feature that is associated with 1*s* to 5*p* transitions in the coordination sphere, which reflect the local envelopment of the Te ions. The inset in Fig. 9 (b) describes details of the absorption edge. The photon energy associated with the chemical shifts increases in the order *y=*0.5, 0.7 and 0.3, i.e. a trend which contradicts the Se K-edge observations. The *y=*0.5 substitution exhibits the lowest valence state, a finding which contrasts with the Se K-edge results, possibly owing to that the charge is gained in the 5*p* orbital at the Te site. The critical and corresponding energy shift in the Se and Te K-edge features upon Te substitution is consequently indicative of an increase in the 4*p* holes and a decrease in the 5*p* holes at *y*=0.5. The tetragonal phase of FeSe has a planar sub-lattice layered structure with Se ions at the tips of the pyramid chain and an Fe plane between Se ions. The substituted Te has an ionic radius which exceeds that of Se, subsequently increasing the hybridization of Fe 3*d*-Se 4*p*/Te 5*p*. Comparing Se and Te K-edges reveals an expected charge transfer between Se and Te: as Te is doped into tetragonal FeSe crystals, the number of 4*p* holes is increased by Fe 3*d*–Se 4p/Te 5*p* hybridization. These results are consistent with an earlier study of the structural distortion that is associated with variation in the angle γ and electron-transport properties (Yeh et al., 2008). Importantly, the change in the number of *p* holes between Fe-Se and Fe-Te

Te doping causes structural distortions in FeSe, as revealed by detailed x-ray refinement (Yeh et al., 2008). Doping expands the lattice because the ionic radius of Te exceeds that of Se. As the doping concentration increases, angle γ varies, subsequently increasing the bond length along the *c*-axis and altering the density of states at the Fermi level (Yeh et al., 2008), which corresponds to density-functional calculations (Subede et al., 2008). *T*c and angle γ display a similar trend: both reach their maxima at *y*=0.5. Figure 6 plots *T*c against Te doping (*T*c is denoted by a red star); a simple sketch of the varying γ angle is also shown. Since the electronic structure around the Fe site in FeSe1-*y*Te*y* does not significantly change, exactly how Se 4*p* holes affect superconductivity should be examined. Either the energy shift or the area under the absorption feature of XAS yields the hole concentration. Therefore, in this study, the energy shift with respect to pure FeSe is determined from the Se K-edge of FeSe1 *<sup>y</sup>*Te*y* and is presented in Fig. 10 as a black circle. Evolution of the edge shift reasonably estimates the hole concentration. Variation in the number of Se 4*p* holes is closely related to the change in transition temperature. The correlation between the Se 4*p* hole concentration and *T*c suggests that *T*c depends more heavily on the variation in the number of 4*p* holes than on the Fe-Fe interaction in the Fe plane, a claim which is consistent with the absence of This study elucidates the electronic properties related to the electron correlation and superconductivity of FeSe*x* and FeSe1-*y*Te*y*, with reference to measurements of XAS and RIXS. Spectroscopic data exhibit the signature of Fe 3d localization and different hybridization effects from those of "1111" and "122" systems. The charge balance considerations from *p*hole also result in itinerant electrons. Fluctuation in the number of ligand 4p holes may arise from the charge transfer between Se and Te in the FeSe1-*y*Te*y* crystals. Analysis results indicate that the superconductivity in Fe-based compounds of this class is strongly associated with the ligand 4p hole state. Additionally, the variation of Tc correlates well with the structural deformation and the change in the Se 4p holes. Moreover, the symmetry of Fe in the ab plane changes from the 4p orbital to modulating (varying) coordination geometry. XRD measurements indicate that this lattice distortion that increases with Se deficiency and the Te doped. Tetragonal FeSe with a PbO structure not only has the same planar sub-lattice as layered Fe-based quaternary oxypnictides, but also exhibits a structural stability upon Te substitution; it is a promising candidate for determining the origin of Tc in Fe-based superconductors. A fundamental question concerning the role of Fe magnetism in these superconductors is yet to be answered. The importance of charge transfer and the ligand 4p hole state should be considered as well.

## **Author details**

Chi Liang Chen and Chung-Li Dong *Institute of Physics, Academia Sinica , Nankang, Taipei, National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan* 

### **Acknowledgement**

The authors would like to thank the National Science Council of the Republic of China, Taiwan (Contract Nos. NSC-98-2112-M-213-006-MY3 and NSC-099-2112-M-001-036-MY3) for financially supporting this research. M. K. Wu, Y. Y. Chen, S. M. D. Rao, and K. W. Yeh at Academia Sinica are appreciated for providing study samples and their valuable discussions. J.-F. Lee, T. S. Chan, C. W. Pao, J. M. Chen, and J. M. Lee of NSRRC are commended for their valuable discussions and experimental support. J.-H. Guo and W. L. Yang of Advanced Light Source are gratefully acknowledged for their experimental support.

X-Ray Spectroscopy Studies of Iron Chalcogenides 41

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**Chapter 3** 

© 2012 Machado et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Machado et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Defect Structure Versus Superconductivity in** 

**MeB2 Compounds (Me = Refractory Metals)** 

A.J.S. Machado, S.T. Renosto, C.A.M. dos Santos, L.M.S. Alves and Z. Fisk

More than 24,000 inorganic phases are known. Of these phases approximately 16,000 are binary or pseudobinary while about 8,000 are ternary or pseudo-ternary. However, it is surprising to note that the observation of superconductivity in these alloys is a rare phenomenon. Superconductivity is ubiquitous but sparsely distributed and can be considered a rare phenomenon among the known alloys. BCS theory has been enormously successful in explaining the superconducting phenomena from the microscopic view point. The fundamental idea of this theory is the formation of Cooper pairs of electrons, mediated by phonons, the quantum of vibration of the crystal lattice [1]. Thus maximizing the critical temperature is involved with maximizing the electron-phonons coupling. Among the intermetallic materials, the binary cubic (A3B) so-called A15 compounds displayed the highest Tc, until the discovery of superconducting cuprates. Among these materials in particular, Nb3Sn and V3Si with critical temperatures of 18.0 K and 17.1 K respectively have lattice instabilities of martensitic-type occurring at temperatures Tm very close to the maximum Tc. In the phase diagram of Tm and Tc versus Pressure (P) of V3Si, the martensitic phase line intersects and stops exactly at the superconducting phase boundary. A qualitative example of this kind of the behavior can be observed in the Figure 1. Data exists beyond the extrapolated intersection shown and finds that there is no martensitic distortion occurring below Tc in this pressure regime. One way to think about this behavior is in terms of a lattice softening arising from strong electron-phonon coupling. Both the martensitic distortion and superconductivity arise from this coupling, and when the superconductivity Tc occurs at higher temperature than that of the lattice distortion, the energy gap that opens in the superconducting state gaps out at the same time phonon fluctuations that give rise to the lattice distortion [2-3]. One has, then, two phases that are competing for the same resource.

**and One-Dimensional Superconductors** 

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/48625

**1. Introduction** 

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