Orbital localization and the role of the Fe and As <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>4</mml:mn><mml:mi>p</mml:mi></mml:mrow></mml:math> orbitals in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">BaFe</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">As</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math> probed by XANES
Alvaro G. Figueiredo, Marli R. Cantarino, Wagner R. da Silva Neto, K. R. Pakuszewski, R. Grossi, Denise S. Christovam, J. C. Souza, M. M. Piva, G. S. Freitas, P. G. Pagliuso, C. Adriano, F. A. Garcia
Abstract
The polarization dependence of the near edge x-ray absorption spectroscopy (XANES) is an element specific probe to the real-space distribution of the density of unoccupied states in solid-state materials. In this paper, we present Fe and As $K$-edge experiments of $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{M}_{x})}_{2}{\mathrm{As}}_{2}$ ($M=\mathrm{Mn}$, Co, and $x=0.0$ and 0.08). The experiments reveal a strong polarization dependence of the probed XANES spectra, which concerns mainly an increase in the intensity of electronic transitions when the beam polarization is set out of the sample's $ab$ crystallographic plane. The results show that states with ${p}_{z}$-orbital character dominate the density of unoccupied states close to the Fermi level. Partial substitution of Fe by Co is shown to decrease the intensity anisotropy, suggesting that Co promotes electronic transfer preferentially to states with ${p}_{z}$-orbital character. On the other hand, Mn substitution causes the increase in the spectra ${p}_{z}$-orbital anisotropy, which is proposed to take place by means of an enhanced local $\mathrm{Fe}\phantom{\rule{0.28em}{0ex}}3d4p$ mixing, unveiling the role of $\mathrm{Fe}\phantom{\rule{0.28em}{0ex}}4p$ states in the localization of the $\mathrm{Fe}\phantom{\rule{0.28em}{0ex}}3d$ orbitals. Moreover, by comparing our results to previous experiments, we identify the relative mixing between Fe and pnictide $4{p}_{x,y,z}$ orbitals as a clear divide between the electronic properties of iron arsenides and selenides. Our conclusions are supported by multiple-scattering theory calculations of the XANES spectra and by quantum chemistry calculations of the Fe coordination electronic structure.