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The role of yttrium doping on the structural and superconducting properties of SmFeAsO $$_{1-x}$$ F $$_x$$ (Sm1111) is studied in detail and is compared with other yttrium-doped RE1111 (RE-rare earth) iron oxypnictide superconductors on the basis of the existing reports. It is found that the partial substitution of lanthanides such as La, Ce, and Nd with yttrium systematically enhances $$T_\mathrm{C}$$ of the system to different levels depending on their crystal ionic radii difference with yttrium. The isovalent substitution of these lanthanides at relatively smaller yttrium ion site causes inner chemical pressure and enhances the $$T_\mathrm{C}$$ . However, yttrium doping does not seem to be beneficial for Sm1111 because of its larger ionic size. [ABSTRACT FROM AUTHOR]

A carrier doping by a hydrogen substitution in LaFeAsO 1- x H x is known to cause two superconducting (SC) domes with the magnetic order at both end sides of the doping. In contrast, SmFeAsO 1- x H x has a similar phase diagram but shows single SC dome. Here, we investigated the electronic and crystal structures for iron oxynitride Ln FeAsO 1- x H x ( Ln = La, Sm) with the range of x = 0-0.5 by using x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction. For both compounds, we observed that the pre-edge peaks of x-ray absorption spectra near the Fe- K edge were reduced in intensity on doping. The character arises from the weaker As-Fe hybridization with the longer As-Fe distance in the higher doped region. We can reproduce the spectra near the Fe- K edge according to the Anderson impurity model with realistic valence structures using the local-density approximation (LDA) plus dynamical mean-field theory (DMFT). For Ln = Sm, the integrated-absolute difference (IAD) analysis from x-ray Fe- Kβ emission spectra increases significantly. This is attributed to the enhancement of magnetic moment of Fe 3 d electrons stemming from the localized picture in the higher doped region. A theoretical simulation implementing the self-consistent vertex-correction method reveals that the single dome superconducting phase for Ln = Sm arises from a better nesting condition in comparison with Ln = La., (© 2021 IOP Publishing Ltd.)

We examine electronic and crystal structures of iron-based superconductors Ln FeAsO 1- x H x ( Ln = La, Sm) under pressure by means of x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES), and x-ray diffraction. In LaFeAsO the pre-edge peak on high-resolution XAS at the Fe- K absorption edge gains in intensity on the application of pressure up to 5.7 GPa and it saturates in the higher pressure region. We found integrated-absolute difference values on XES for Ln = La, corresponding to a spin state, decline on the application of pressure, and then it is minimized when the T c approaches the maximum at around 5 GPa. In contrast, such the optimum value was not detected for Ln = Sm. We reveal that the superconductivity is closely related to the lower spin state for Ln = La unlike Sm case. We observed that As height from the Fe basal plane and As-Fe-As angle on the FeAs 4 tetrahedron for Ln = La deviate from the optimum values of the regular tetrahedron in superconducting (SC) phase, which has been widely accepted structural guide to SC thus far. In contrast, the structural parameters were held near the optimum values up to ∼15 GPa for Ln = Sm., (© 2021 IOP Publishing Ltd.)