Hard x-ray spectroscopy of the itinerant magnets $R{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ $(R=\mathrm{Na}, \mathrm{K}, \mathrm{Ca}, \mathrm{Sr}, \mathrm{Ba})$

Hard x-ray spectroscopy of the itinerant magnets $R {Fe}_{4} {Sb}_{12}$ $(R = Na, K, Ca, Sr, Ba)$

Bassim Mounssef, Jr., Marli R. Cantarino, Eduardo M. Bittar, Tarsis M. Germano, Andreas Leithe-Jasper, and Fernando A. Garcia

Phys. Rev. B 99, 035152 – Published 28 January 2019

Abstract

Ordered states in itinerant magnets may be related to magnetic moments displaying some weak local moment characteristics, as in intermetallic compounds hosting transition metal coordination complexes. In this paper, we report on the Fe $K$ -edge x-ray absorption spectroscopy (XAS) of the itinerant magnets $R {Fe}_{4} {Sb}_{12}$ $(R = Na, K, Ca, Sr, Ba)$ , aiming at exploring the electronic and structural properties of the octahedral building block formed by Fe and the Sb ligands. We find evidence for strong hybridization between the Fe $3 d$ and Sb $5 p$ states at the Fermi level, giving experimental support to previous electronic structure calculations of the $R {Fe}_{4} {Sb}_{12}$ skutterudites. The electronic states derived from Fe $3 d$ –Sb $5 p$ mixing are shown to be more occupied and/or less localized in the cases of the magnetically ordered systems, for which $R = Na$ or K, connecting the local Fe electronic structure to the itinerant magnetic properties. Moreover, the analysis of the extended region of the XAS spectra (EXAFS) suggests that bond disorder may be a more relevant parameter to explain the suppression of the ferromagnetically ordered state in ${CaFe}_{4} {Sb}_{12}$ than the decrease of the density of states.

Received 19 September 2018
Revised 28 December 2018

DOI:https://doi.org/10.1103/PhysRevB.99.035152

Physics Subject Headings (PhySH)

Electronic structure Magnetism

Skutterudites

Density functional calculations X-ray absorption fine structure spectroscopy X-ray absorption spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Bassim Mounssef, Jr.¹, Marli R. Cantarino², Eduardo M. Bittar³, Tarsis M. Germano², Andreas Leithe-Jasper⁴, and Fernando A. Garcia²

¹IQUSP, Universidade de São Paulo, São Paulo, São Paulo 05508-090, Brazil
²IFUSP, Universidade de São Paulo, São Paulo, São Paulo 05508-090, Brazil
³Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, Rio de Janeiro 22290-180, Brazil
⁴Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany

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Vol. 99, Iss. 3 — 15 January 2019

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Figure 1
Left: Ternary filled skutterudite structure (space group $I m \bar{3}$ ) displaying the rectangular Sb-Sb rings and the ${FeSb}_{3}$ distorted octahedra [28]. Right: Detail of the Fe-Sb octahedral coordination. Below: The lattice parameters $a$ (red squares) and the ${Fe − Sb − Fe}^{'}$ bond angles $δ$ (blue line) along the series. Is is noteworthy that in spite of the large variation of the cationic sizes, $a$ changes only slightly along the series, evidencing that the ${Fe}_{4} {Sb}_{12}$ framework is particularly stiff.
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Figure 2
(a) Fe $K$ -edge XANES spectra for ${NaFe}_{4} {Sb}_{12}$ , the reference Fe foil, along with FEFF calculations for ${NaFe}_{4} {Sb}_{12}$ , and (b) the derivative of the same spectra. Two prominent features $A$ and $B$ are marked in reference to the maximum of the energy derivatives of the spectrum. In the inset, the ${NaFe}_{4} {Sb}_{12}$ and ${CaFe}_{4} {Sb}_{12}$ spectra (offset for better visualization) are compared. It is shown that a second maximum (black arrows) of the energy derivative can be observed for ${CaFe}_{4} {Sb}_{12}$ but not for ${NaFe}_{4} {Sb}_{12}$ . The overall shape of the spectrum is reminiscent of the Fe $K$ -edge spectra obtained for ${Fe}^{2 +}$ oxides in octahedral coordination complexes.
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Figure 3
(a) FEFF calculations of the XANES spectrum and (b)–(d) site projected $l$ -DOS of Na, Fe, and Sb for ${NaFe}_{4} {Sb}_{12}$ . The most relevant contributions are displayed. The main features of the XANES spectrum correlate well with the Fe $4 p$ (general), Sb $5 p$ ( $A$ feature), Sb $d$ ( $B$ feature), and Na $p$ ( $B$ feature) site projected density of states. The thick line is a guide to the eyes highlighting the contribution from Sb $5 p$ and Fe $4 p$ states.
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Figure 4
(a) Fe K-edge XANES spectra for all $R {Fe}_{4} {Sb}_{12}$ ( $R = Na$ , K, Ca, Sr, Ba) skutterudites. Upper inset: Fitting of a representative spectrum, wherein we highlight a Gaussian curve describing the pre-edge. Lower inset: Detail of the energy derivative of the pre-edge transition for all samples. A second maximum can be observed for the $R = Ca$ and likely for the $R = Sr$ and Ba skutterudites. (b) Pre-edge transition intensity (peak area $A$ , left axis) at $T = 300$ K (red squares) and $T = 10$ K (blue squares) and the $γ$ coefficient (black line) of the electronic contribution to heat capacity (right axis, from Ref. [24]), both as a function of $R$ .
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Figure 5
(a) Representative Fe $K$ -edge EXAFS spectra measured at $T = 10$ K. In the inset, the Fourier transformed spectra in a broad interval are presented. (b)–(f) Fourier-transformed spectra (squares) up to $4 \overset{˚}{A}$ from the absorber and the theoretical model (solid red line) used to investigate the Fe coordination structure along the series ( $R = Na, K, Ca, Sr, Ba$ ).
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Hard x-ray spectroscopy of the itinerant magnets $R {Fe}_{4} {Sb}_{12}$ $(R = Na, K, Ca, Sr, Ba)$

Bassim Mounssef, Jr., Marli R. Cantarino, Eduardo M. Bittar, Tarsis M. Germano, Andreas Leithe-Jasper, and Fernando A. Garcia

Phys. Rev. B 99, 035152 – Published 28 January 2019

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Hard x-ray spectroscopy of the itinerant magnets RFe4Sb12 (R=Na, K, Ca, Sr, Ba)

Bassim Mounssef, Jr., Marli R. Cantarino, Eduardo M. Bittar, Tarsis M. Germano, Andreas Leithe-Jasper, and Fernando A. Garcia

Phys. Rev. B 99, 035152 – Published 28 January 2019

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Hard x-ray spectroscopy of the itinerant magnets $R {Fe}_{4} {Sb}_{12}$ $(R = Na, K, Ca, Sr, Ba)$