Strikingly dissimilar effect of Mn and Zn dopants imposed on local structural distortion of Ba_0.5K_0.5Fe₂As₂ superconductor

To clarify the contrasting impurity effects of Mn and Zn dopants on the critical temperature of optimally doped Ba_0.5K_0.5Fe₂As₂ superconductors, extended X-ray absorption fine-structure spectroscopy was implemented at the Fe and As K-edge. In Mn-doped compounds a gradual deviation of the symmetric FeAs₄ tetrahedron and weakening of the Fe—As bond was observed. Conversely, in Zn-doped compounds the perfect FeAs₄ tetrahedron is maintained and the Fe—As bond is rigid. The local structural details are consistent with the development of superconductivity in these two systems, suggesting a significant role played by the topology of the FeAs₄ tetrahedron and rigidness of the Fe—As bond in Mn/Zn-doped Ba_0.5K_0.5Fe₂As₂ superconductors.

1. Introduction

The recently discovered Fe-based superconductor (FeSC) in 2008 (Kamihara et al., 2008; Mizuguchi et al., 2009; Rotter et al., 2008) has spurred renewed interest in the mysterious superconducting mechanisms of high-T_c superconductors (Chiao, 2008; Grant, 2008). The parent phases of FeSC are `bad metals' accompanied by antiferromagnetism, and the superconductivity occurs upon chemical doping or pressure. For instance, the superconductivity of the first reported FeSC, LaFeAsO_1–xF_x, is attained by the replacement of O atoms by F (electron-doping) (Kamihara et al., 2008); with K-doping the Ba_1–xK_xFe₂As₂ system can exhibit its maximum critical temperature (T_c) of T_c = 38 K (hole-doping) (Rotter et al., 2008); meanwhile, the T_c of FeSe_1–x is extremely sensitive to applied external pressure, rising from 13.5 K at ambient pressure to 27 K at 14.8 GPa (Margadonna et al., 2008). Here, strikingly different from the cuprates in which doping at the Cu site would suppress superconductivity, the substitution of Co/Ni or rather the isovalent Ru for the Fe-site doped FeSC induces superconductivity (Kim et al., 2011; Sefat et al., 2008; Wang et al., 2010), while to date the superconducting phase has not been observed by Cr/Mn/Cu doping (Canfield et al., 2009; Kim et al., 2010; Sefat et al., 2009). In analogy to cuprates, the phase diagram of FeSC displays the competition between antiferromagnetism and superconductivity, and the onset of the T_c always displays a dome-like shape (Ahilan et al., 2009; Chen et al., 2009; Gooch et al., 2008; Kamihara et al., 2008). That is, in the lightly doped region T_c increases rapidly until it reaches a critical doping concentration (i.e. the so-called optimal doping level); however, the reduction of T_c emerges in heavily doped regimes. In the case of the Ba_1–xK_xFe₂As₂ system, the K optimal doping concentration is x ≃ 0.5 (Chen et al., 2009).

To better understand the superconducting properties of FeSC, researchers have also resorted to co-doped systems (Canfield et al., 2009; Luo et al., 2011; Suzuki et al., 2010). According to Anderson's theorem (Anderson, 1959), non-magnetic impurities are regarded as important probes for the pairing symmetry of high-T_c superconductors. As a consequence, several studies regarding the Zn impurity effect on the superconductivity of FeSC have been reported (Guo et al., 2010; Li et al., 2011, 2010a,b; Kitagawa et al., 2011). Cheng et al. (2010a) found that in optimally doped Ba_0.5K_0.5Fe₂As₂ superconductor the superconducting temperature T_c is drastically reduced by a Mn impurity whereas it is not sensitive to non-magnetic Zn impurities, which was interpreted as a consequence of the different magnetic scattering strengths of Mn and Zn dopants. Until now a satisfactory understanding of the superconducting mechanism has far from been achieved. Considering the intimate relationship between superconductivity and the atomic structure of high-T_c superconductors (Lee et al., 2008; Mizuguchi et al., 2010), it is desirable to identify local lattice structural distortions induced by impurities, thus illustrating the mechanism of different impurity effects between Mn and Zn dopants on the optimally doped Ba_0.5K_0.5Fe₂As₂ superconductor (Cheng et al., 2010a).

Conventional techniques such as X-ray diffraction can only provide the average structural information, and is not sensitive to the local structural distortions induced by different dopants. Synchrotron-radiation-based extended X-ray absorption fine structure (EXAFS), however, is a powerful element-selective local structural probe and has been effective in investigating the local lattice structure of FeSC (Cheng et al., 2010b; Granado et al., 2011) and other related high-T_c superconductors such as cuprates (Zhang & Oyanagi, 2009). Meanwhile, by applying EXAFS in a temperature-dependent experiment, useful lattice dynamic information, such as the force constant of the atomic bond (Cheng et al., 2010b; Joseph et al., 2011), can also be quantitatively estimated. In this contribution we report a quantitative estimation of local structural distortions induced by Mn and Zn impurities in Ba_0.5K_0.5Fe₂As₂ compounds, through a comprehensive analysis of temperature-dependent EXAFS at the Fe and As K-edge.

2. Experiments

The Mn-doped and Zn-doped polycrystalline compounds Ba_0.5K_0.5(Fe_1–xTM_x)₂As₂ (TM = Mn, x = 0, 0.02, 0.04, 0.08; TM = Zn, x = 0, 0.08) were synthesized by conventional solid-state reaction method (Cheng et al., 2010a). Samples were ground into fine powders and then brushed onto tapes that were stacked to give approximately one X-ray-absorption length at their corresponding absorption edges. Fe and As K-edge EXAFS spectra of Ba_0.5K_0.5(Fe_1–xTM_x)₂As₂ were recorded at room temperature in transmission mode at the 1W2B beamline of the Beijing Synchrotron Radiation Facility (BSRF), running at an electron energy of 2.5 GeV with a maximum current of ∼250 mA. The flux on the sample of beamline 1W2B is 10¹² photons s⁻¹ at 9 keV, and the energy resolution is better than 4 × 10⁻⁴ at 9 keV. Moreover, the temperature-dependent Fe and As K-edge EXAFS experiments on Ba_0.5K_0.5Fe₂As₂ and Ba_0.5K_0.5(Fe_1–xTM_x)₂As₂ (TM = Mn and Zn, x = 0.08) were carried out to an accuracy of ±1 K using a Si(111) double-crystal monochromator at the 1W1B beamline at BSRF. The EXAFS spectra were analyzed using IFEFFIT (Newville, 2001); for the data fitting the backscattering amplitudes and total phase shifts were calculated using the BaFe₂As₂ model cluster of type ThCr₂Si₂ with the FEFF8.2 code (Ankudinov et al., 1998).

3. Results and discussions

Fourier transforms (FTs) of the EXAFS spectra of the Mn-doped and Zn-doped Ba_0.5K_0.5Fe₂As₂ samples are plotted in Fig. 1. It should be pointed out that the k-range for the Fourier transform was 2.4–12.1 Å⁻¹ for the As-edge and 2.5–12.2 Å⁻¹ for the Fe-edge. For As-edge EXAFS, the first peak in the radial distribution function (RDF) results from the contribution of the four nearest Fe atoms. For Fe-edge EXAFS the first peak of the RDF is a superposition of two subshells including Fe—As and Fe—Fe bonds. It can be found that, despite the lower intensity, the overall RDF of Mn/Zn-doped compounds is similar to that of Ba_0.5K_0.5Fe₂As₂, indicating that Mn/Zn doping finely modulates the local structure of the FeAs superconducting layer, which was believed to be intimately related to the superconductivity of FeSC (Cheng et al., 2010b).

Figure 1 Comparison of the Fourier transforms of the EXAFS signals for Ba0.5K0.5(Fe1−xTMx)2As2 compounds at the As K-edge [panels (a) and (c)] and Fe K-edge [panels (b) and (d)].

To estimate the Mn/Zn doping effect quantitatively, we performed structural fittings on the first peak of both As and Fe K-edge EXAFS spectra, which were isolated from the FTs by a rectangular window. The range in k-space was 2.4–12.1 Å⁻¹ and that in R-space was 1.4–2.6 Å for the As K-edge; for the Fe K-edge the k-range was 2.5–12.2 Å⁻¹ and the R-range was 1.4–2.9 Å. During fitting, the Mn/Zn—As and Mn/Zn—Fe contributions are not considered, owing to the low doping level of Mn/Zn. The number of independent parameters which could be determined by EXAFS is limited by the number of independent data points N_ind ≃ (2ΔkΔR)/π, where Δk and ΔR are, respectively, the ranges of the fit in the k- and R-space. Thus N_ind is 7 (Δk = 9.7 Å⁻¹, ΔR = 1.2 Å) for the As—Fe shell at the As-edge, and N_ind is 9 (Δk = 9.7 Å⁻¹, ΔR = 1.5 Å) for the Fe—As and the Fe—Fe shells at the Fe-edge, sufficient to obtain all parameters. At first we fit the As-edge, fixing the As—Fe coordination number and obtaining the As—Fe distance and the Debye–Waller factor (DWF) . For the Fe-edge we applied two-shells fitting. To reduce fit variables, the coordination numbers of Fe—As and Fe—Fe, the distance of Fe—As and the DWF (the same as obtained from the As-edge fitting) were fixed, leaving the other parameters free. The fitting results are summarized in Figs. 2 and 3. Owing to the lattice mismatch, Mn doping induces a gradual increase of the Fe—As distance and a reduction in the Fe—Fe bond length. Both Fe—As and Fe—Fe distances are almost independent of the Zn impurity, confirming the structural stability of the FeAs layers.

Figure 2 Fe—As distance (a), Fe—Fe distance (b), Fe—As—Fe angle (c) and As height (d) for Ba0.5K0.5(Fe1–xTMx)2As2 compounds as a function of Mn/Zn doping concentration. For the sake of simplicity, the Ba0.5K0.5Fe2As2 sample is labeled as BaK122, while Mn2, Mn4, Mn8 and Zn8 are, respectively, the abbreviations of Ba0.5K0.5(Fe1–xTMx)2As2 compounds with TM = Mn, x = 0.02, 0.04, 0.08 and TM = Zn, x = 0.08.

The superconductivity in FeSC is known to be extremely sensitive to the structural topology of the FeAs₄ tetrahedron and As height from the Fe layer (Lee et al., 2008; Mizuguchi et al., 2010). Many statistical investigations claim that the highest T_c is attained as the FeAs₄ coordination approaches a regular tetrahedron with an Fe—As—Fe angle of about 109.47° (Lee et al., 2008). Meanwhile, the optimal value of the As height is about 1.38 Å for the best superconducting property (Mizuguchi et al., 2010). According to the above findings, the Fe—As—Fe angle and the As height are important structural parameters; therefore we evaluated the Fe—As—Fe bond angle and As height as a function of Mn/Zn doping. The result is shown in Figs. 2(c) and 2(d). With the doping concentration of Mn increasing, the Fe—As—Fe bond angle gradually deviates from a perfect tetrahedron, accompanied by the elevating distance of As from the Fe layer. It was found that small structural changes in the FeAs₄ tetrahedron and As height can affect the band-filling/Fermi-surface nesting and thus the superconducting gap (Kuroki et al., 2009). Accordingly, the suppression of superconductivity in Mn-doped samples can be attributed to local structural modulations in FeAs₄ tetrahedra. In contrast, the FeAs₄ tetrahedra are maintained regularly in the Zn-doped samples with the optimal As height. It is once more proved that structural modification of FeAs layers, especially the topology of the FeAs₄ tetrahedron, plays a crucial role in the superconductivity of FeSC.

The doping mechanism may also be investigated by studying the behavior of the DWF, σ², measuring the mean square relative displacement (MSRD) of the photon absorber–backscatterer pairs. In Fig. 3 we plot the MSRDs of Fe—As and Fe—Fe pairs as a function of Mn/Zn doping concentration. The data show that in the presence of Mn/Zn impurity the DWFs of both Fe—As and Fe—Fe pairs exhibit a clear increase.

Figure 3 Mean square relative displacement (MSRD) of the Fe—As and Fe—Fe pairs for Ba0.5K0.5(Fe1–xTMx)2As2 compounds as a function of Mn/Zn doping concentration.

The DWF of each atom pair is the sum of two contributions, i.e. = . The term is temperature independent and associated with the static disorder. originates from thermal vibrations and can be described by the Einstein model (Knapp et al., 1985) as = . In this equation is the Plank constant, k_B is the Boltzmann constant, is the reduced mass of the atom pair, and T is the temperature. The Einstein temperature T_E = is proportional to the square root of the force constant for a bond; therefore this value can be utilized to estimate and compare the stiffness of different bonds.

In order to recognize whether the increased disorders of the Fe—As and Fe—Fe pairs versus Mn/Zn doping are static or dynamic, temperature-dependent EXAFS was performed. Fig. 4 displays the Fe—As and Fe—Fe DWFs as a function of temperature for the Ba_0.5K_0.5Fe₂As₂ and 8% Mn/Zn-doped compounds, and the simulations are implemented by the classical Einstein model (Cheng et al., 2010b). It can be seen that all the Fe—As and Fe—Fe DWFs show almost identical temperature dependence for these samples. Although the temperature dependence looks similar, there seems to be a clear discrepancy in the Einstein temperature T_E. For the Fe—Fe bond, the T_E values for Ba_0.5K_0.5Fe₂As₂, 8% Mn-doped and 8% Zn-doped Ba_0.5K_0.5Fe₂As₂ compounds are 228.7 K, 220.2 K and 205.8 K, respectively. Therefore the Fe—Fe bond seems to become softer in the Mn/Zn-doped system, accompanied by a weakening of the Fe—Fe interactions. On the other hand, Mn/Zn dopants also affect the DWFs of the Fe—As pair significantly. The T_E value of the Fe—As bond for Ba_0.5K_0.5Fe₂As₂ is about 352.3 K; it becomes 349.7 K for the 8% Mn-doped compound, while Zn doping yields a higher T_E of 362.7 K, clearly indicating the contrastingly different doping effects on the Fe—As bond between Mn and Zn impurities.

Figure 4 Temperature-dependent MSRD of the Fe—As and Fe—Fe pairs (symbols) for Ba0.5K0.5(Fe1–xTMx)2As2 compounds as a function of Mn/Zn doping concentrations and their simulations using the classical Einstein model (dotted lines).

A systematic variation of the T_E values of the Fe—As bond for the ReFeAsO (Re = La, Sm, Nd) compounds was reported by Joseph et al. (2010), revealing that a harder Fe—As bond is favorable for the higher T_c in oxypnictide systems. Similarly, here for the Ba_0.5K_0.5Fe₂As₂-based system, an enhanced Fe—As bond can be beneficial for Zn doping, while the weakening of that bond may be intimately associated with a gradual suppression of the superconductivity in Mn-doped compounds.

4. Conclusion

We compared the local structure of the optimally doped Ba_0.5K_0.5Fe₂As₂ superconductor and Mn/Zn-doped Ba_0.5K_0.5Fe₂As₂ compounds by employing temperature-dependent EXAFS at the As and Fe K-edge. For Mn-doped samples in which superconductivity is remarkably suppressed, a noticeable distortion of the regular FeAs₄ tetrahedron is observed. By contrast, in Zn-doped samples the FeAs₄ tetrahedron remains rigid and regular. On the other hand, with doping the Fe—Fe bond becomes softer in both cases while the Fe—As bond is more rigid in the Zn-doped system than in the Mn-doped system, as suggested by the Einstein temperature of the bonds.

The superconductivity is known to be closely related to the regular configuration of the FeAs₄ tetrahedron which can directly modulate the band-filling/Fermi-surface nesting and also the superconducting gap for FeSC. Therefore, we propose that the strikingly dissimilar impurity effects on T_c originate primarily on the local structural distortions which are more pronounced in the Mn-doped system than in the Zn-doped system. This new scenario may compensate for the previous magnetic scattering description, thus offering new insights on the interplay of atomic spin and charge in iron-based superconductors.