Sensitivity of As K-edge absorption to rare earth (RE) doping in Ca1−xRExFeAs2: A first principles study

doi:10.1016/j.jpcs.2021.109993

Journal of Physics and Chemistry of Solids

Volume 153, June 2021, 109993

https://doi.org/10.1016/j.jpcs.2021.109993 Get rights and content

Highlights

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As K-edge absorption spectra of rare-earth (RE) doped Ca_1-xRE_xFeAs2 compounds.
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Chain As atom absorption spectra quantitatively differ from those of plane As atoms.
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As K-edge absorption method may be useful to check stability of 112 RE-doped materials.
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Indirect heavy electron doping by RE-elements is responsible for red shift of the white line feature with decreasing RE size.
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Origin of the spectral features are described using atom projected partial density of states.

Abstract

Systematic density functional theory-based first principles studies on As K-edge electron energy loss near edge structures are presented for rare earth (RE)-doped iron-based superconducting materials ${Ca}_{1 - x} {RE}_{x} {FeAs}_{2}$ (known as 112 compounds). The As K-edge electron energy losses near edge differ for two different types of As atoms (belonging to Fe–As plane and ${As}_{2}$ chains between the planes). The As K-edge spectra (of both As atoms in chain and plane) are very sensitive to the nature of RE doping (RE = La, Ce, Pr, Nd, Sm, Gd). For example, the As K-edge shifts to higher energies successively with decreasing RE size (La $\to$ Gd), indicating larger binding energies and hence stability of the corresponding compound. In contrast, the white-line peak successively shifts to lower energies for La $\to$ Gd doping, resulting in complete suppression of another low energy absorption peak for Gd. These are in essence an artifact of heavy electron doping caused by RE atoms. Interrelationship between the electron doping and corresponding shift in the white-line feature for various RE-doped samples is established through Bader charge analysis. Each absorption spectra consists of several peaked features of particular significance; they are thoroughly analyzed in terms of the unoccupied partial density of states. To emphasize the influence of the “core-hole” effect, absorption spectra in the presence and absence of core-holes are presented separately. Relevance of our results with respect to experimental scenarios are also discussed.

Introduction

Iron-based superconducting materials (FeSC) were discovered in 2008 [1] and form a sister high- $T_{c}$ superconductor (HTS) family after cuprates. The discovery of superconducting phenomena in ${LaFeAsO}_{1 - x} F_{x}$ at 26 K surprised the condensed matter community because the presence of Fe, which has large magnetic moment, is known to be harmful for emergence of superconductivity [2]. Due to such surprising features, extensive research was carried out and a large number of new FeSCs were discovered and nick-named according their structural composition, including 1111 type (e.g. ${LaFeAsO}_{1 - x} F_{x}$ and ${SmFeAsO}_{1 - x}$ [3]), 122 type (e.g. ${Ba}_{1 - x} K_{x} {Fe}_{2} {As}_{2}$ [4] and ${Sr}_{1 - x} K_{x} {Fe}_{2} {As}_{2}$ [5]), 11 type (e.g. FeSe [6], ${FeSe}_{1 - x} {Te}_{x}$ [7]), 111 type (e.g. LiFeAs [8], NaFeAs [9]), and 123 ladder type (e.g. ${BaFe}_{2} S_{3}$ [10]). All the FeSCs contain a structural unit of FePn/FeCh (Pn, pnictogen; Ch, chalcogen) layer formed by a square net of Fe atoms with tetrahedrally co-ordinated Pn/Ch atoms.

The rich chemistry of As atoms allowed the development of various new types of FeSCs. Depending on the number of valence electrons, As exhibits a wide variety of chemical networks. Due to these striking properties of As, a new family of iron-arsenide superconducting materials ${Ca}_{1 - x} {RE}_{x} {FeAs}_{2}$ was discovered in 2013 with $T_{c} = 34$ K [11]. These materials have an additional As-chain in between the FePn layers. This new family was nicknamed 112 and later it was established that substitution of a rare-earth (RE) element on the Ca site is necessary to stabilize these compounds [11]. Other members of this family are ${Ca}_{1 - x} {Pr}_{x} {FeAs}_{2}$ with $T_{c} = 20$ K [12], ${Ca}_{1 - x} {Nd}_{x} {FeAs}_{2}$ with $T_{c} = 11.9$ K, ${Ca}_{1 - x} {Sm}_{x} {FeAs}_{2}$ with $T_{c} = 11.6$ K, ${Ca}_{1 - x} {Eu}_{x} {FeAs}_{2}$ with $T_{c} = 9.3$ K, and ${Ca}_{1 - x} {Gd}_{x} {FeAs}_{2}$ with $T_{c} = 12.6$ K [13] for different values of x. The 112-type compounds have monoclinic crystal structure with space group $P 2_{1}$ (No. 4) or $P 2_{1 / m}$ (No. 11). The structural unit consists of alternatively stacked Fe–As layers in between Ca–La layers, with an As zigzag chain (Fig. 1). The measured As–As bond length in the As₂ chain is 2.42 Å [12] with formal valence state ${As}^{- 1}$ ( $4 p^{4}$ configuration). In contrast, As in the Fe–As plane layer has ${As}^{- 3}$ oxidation state with filled p-orbital ( $4 p^{6}$ configuration). Although doping at the Ca site by RE restricts $T_{c}$ up to 34 K, simultaneous doping of RE at the Ca site and P/Sb at the chain As site, with chemical formula ${Ca}_{1 - x} {La}_{x} Fe {({As}_{1 - y} {Sb}_{y})}_{2}$ , increases transition temperature up to 47 K [14]. In the case of Pr and Nd-doped samples $T_{c}$ increases up to 43 K. Density functional theory (DFT)-based electronic structure calculations also show that Sb doping enhances electronic density of states at the Fermi level more when Sb is substituted in chain-As than in plane-As [15], which was also supported by later experimental observations [16]. Thus doping site identification, whether at plane or chain As, is crucial in regard to raising $T_{c}$ . This is achievable through element site-specific energy loss near edge structure (ELNES)/X-ray absorption near edge structure (XANES) study.

General aspects of 112 Fe-based superconductors are also very encouraging, e.g., these materials are thought to be candidates for achieving topological superconductivity [[17], [18], [19], [20]]. Very high resolution inelastic neutron scattering measurements evidenced a two-dimensional spin resonance mode at around E $≃$ 11 meV. The resonance energy linearly scales with $T_{c}$ , independent of temperature and indicates $s^{\pm}$ -type superconductivity [21]. Systematic detailed electronic structure calculations of 112 materials are available in Refs. [15,22], which show Lifshitz transition and orbital selective correlation effects in band width renormalization.

X-ray absorption spectroscopy (XAS) is a well-established tool to investigate structural and electronic properties in condensed matter and material science. Analysing the near-edge part of the absorption spectrum, the so-called XANES, one can get information regarding (i) oxidation state of the absorber, (ii) local structural environment of the absorbing atom, and (iii) conduction band electronic density of states [23]. Theoretically, XANES can be described as excitation of the core electron to the empty conduction band state using a suitable amount of incident photon energy. These transitions are observed at specific absorption energies, corresponding to the binding energy of the core electron, which forms the ionization edges. Typically XANES spectra consist of three regions “pre-edge”, “edge”, and “near-edge”, which have their own physical significances. The higher energy features are not well defined due to broadness of the spectrum. The edge energy is directly related to the oxidation state and binding energy of the absorber. Materials with higher binding energy form the most stable compounds. Since stability is one of the concerns in 112 materials and RE doping can stabilize them, we argue and show that the ELNES/XANES technique can be used to study the stability condition.

Experimental and theoretical spectroscopic studies of several FeSCs are available in the literature, for ${FeSe}_{1 - x} {Te}_{x}$ [24,25], NaFeAs [26], ${Ba}_{1 - x} K_{x} {Fe}_{2} {As}_{2}$ [27], ${Sm}_{0.95} {La}_{0.05} {FeAsO}_{0.85} F_{0.15}$ [28], and ${CaFeAsO}_{1 - x} F_{x}$ [29]. Incidentally, As K-edge experimental XANES of 112 compounds for the monoclinic phase are not available in the literature; experimentally it would be difficult to probe the chain and plane As atoms separately. In this article, we present theoretically calculated As K-edge absorption spectra of ${Ca}_{0.9} {RE}_{0.1} {FeAs}_{2}$ (for different RE = La, Ce, Pr, Nd, Sm, Gd) individually for As atoms belonging to chain and plane. Here we mainly emphasize the effect of RE doping in the absorption spectra and comment on the stability of the compound on that basis. The core-hole effects on absorption spectra are also presented separately for two distinct As atoms.

The rest of the paper is organized as follows, in section-2 we discuss the basic theory of electron energy loss structure simulation and details of computational methodologies. In section-3, detailed results of As K-edge absorption spectra with different RE doping (both in presence and absence of core-hole) and the corresponding analysis of different distinct absorption features are described in terms of atom projected partial density of states (PDOS) of the constituent atoms. This section also includes a comparative study of absorption edge/white-line features from chain as well as plane atoms along with the effect of RE doping. Here we also include Bader charge analysis to explain the modifications of the white-line feature upon different RE doping. Section-4 summarizes the important discussions and conclusions. In our extended study, we show that the results are independent of supercell sizes, hence free from any core hole-core hole interaction effects. Possible effects of disorder due to RE doping on absorption spectra are also discussed in the supplementary material.

Section snippets

Theory of near edge structure simulation

Recent developments in computational methodologies allow us to simulate XANES/ELNES using ab-initio DFT. Experimentally measured absorption cross-section can be simulated theoretically using the transition rule of quantum mechanics [30]: $σ_{a b s} (ω) = 4 π^{2} α_{0} ω \sum_{f} | M_{i f} |^{2} δ (E_{f} - E_{i} - ℏ ω)$ where $| M_{i f} |$ is the transition matrix element from initial state $| Φ_{i} ⟩$ of energy $E_{i}$ to a final state $| Φ_{n, K}^{f} ⟩$ of energy $E_{n, f}$ . Electron-photon interaction operator can be written as follows: ${\hat{O}}_{e l - p h} = \hat{ε} . \vec{p} + i (\hat{ε} . \vec{p}) (\vec{k} . \vec{r}) + \dots$ where the

Results and discussions

This section contains 12 figures, Fig. 2 to Fig. 13. Figs. 2 and 3 demonstrate that the As K-edge absorption from plane and chain atoms of 112 compounds differ irrespective of the nature of RE doping and in presence or absence of the core-hole effect. Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9 are used to explain possible origins of various features in Figs. 2 and 3. Effects of various RE-atom doping on core-electron spectroscopic features are analyzed in Figs. 12 and 13 with the help of

Conclusions

Systematic DFT-based calculations of As K-edge absorption for ${Ca}_{0.9} {RE}_{0.1} {FeAs}_{2}$ , 112 Fe-based superconductors are presented for six different RE elements. The studied 112 Fe-based superconductors with distinguishing features of zig-zag As chains between the Fe–As planes and As K-edge absorption for both types of As atoms from chain as well as plane are evaluated. We demonstrate that the nature of chain As atom absorption spectra quantitatively differs from that of the plane As atom. This may have

Credit authorship contribution statement

Soumyadeep Ghosh performed numerical calculations, drew figures, and wrote the first draft of the manuscript as conceptualized by Haranath Ghosh. Methodology, visulization, and re-writing of the manuscript were performed by Haranath Ghosh. Formal and critical analysis of the results was performed by both the authors. Both authors contributed equally.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank H. Luo and A. Ghosh for discussions. The authors acknowledge the Computer Division, RRCAT, for providing computational facilities. SG also acknowledges the HBNI-RRCAT, for financial support.

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Sensitivity of As K-edge absorption to rare earth (RE) doping in Ca1−xRExFeAs2: A first principles study

Highlights

Abstract

Introduction

Section snippets

Theory of near edge structure simulation

Results and discussions

Conclusions

Credit authorship contribution statement

Declaration of competing interest

Acknowledgements

Solid State Commun.

Intermetallics

Comput. Mater. Sci.

J. Phys. Chem. Solid.

J. Am. Chem. Soc.

Sci. Technol. Adv. Mater.

Europhys. Lett.

Phys. Rev. Lett.

Phys. Rev. Lett.

Proc. Natl. Acad. Sci. Unit. States Am.

Europhys. Lett.

Chem. Commun.

Nat. Mater.

J. Phys. Soc. Japan

J. Am. Chem. Soc.

Appl. Phys. Express

J. Phys. Soc. Japan

J. Phys. Soc. Japan

Appl. Phys. Lett.

Appl. Phys. Lett.

J. Appl. Phys.

Appl. Phys. Lett. Mater.

Phys. Rev. Lett.

J. Anal. At. Spectrom.

Journal of Superconductivity and Novel Magnetism

Core electron spectroscopic studies for 11-Fe-based superconductors

Sensitivity of As K-edge absorption to rare earth (RE) doping in ${Ca}_{1 - x} {RE}_{x} {FeAs}_{2}$ : A first principles study