Probing the electronic and local structure of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Sr</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi>La</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi>CoNbO</mml:mi><mml:mn>6</mml:mn></mml:msub></mml:mrow></mml:math> using near-edge and extended x-ray absorption fine structures
Ajay Kumar, Rishabh Shukla, Ravi Kumar, R. J. Choudhary, S. N. Jha, R. S. Dhaka
Abstract
We report the electronic and local structural investigation of double pervoskites ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{CoNbO}}_{6}$ ($x=0\ensuremath{-}1$) using x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structures (EXAFS) at the Nb, Co, and Sr $K$ edges. The ab initio simulations and detailed analysis of the Nb and Co $K$-edge XANES spectra demonstrate that the observed pre-edge features arise from the transition of $1s$ electrons to the $p\text{\ensuremath{-}}d$ hybridized states. We reveal a $z$-out Jahn-Teller (JT) distortion in the ${\mathrm{CoO}}_{6}$ octahedra, which decreases monotonically due to an enhancement in the JT inactive ${\mathrm{Co}}^{2+}$ ions with $x$. On the other hand, the $z$-in distortion in ${\mathrm{NbO}}_{6}$ octahedra remains unaltered up to $x=0.4$ and then decreases with further increase in $x$. This sudden change in the local coordination around Nb atoms is found to be responsible for the evolution of the antiferromagnetic interactions in $x\ensuremath{\geqslant}$ 0.6 samples. Also, we establish a correlation between the degree of octahedral distortion and intensity of the white-line feature in the XANES spectra and possible reasons for this are discussed. More interestingly, we observe the signature of ${\mathrm{KN}}_{1}$ double-electron excitation in the Sr $K$-edge EXAFS spectra for all the samples, which is found to be in good agreement with the $Z+1$ approximation. Further, the Co ${L}_{2,3}$ edge shows the reduction in the crystal-field strength and hence an increase in the charge-transfer energy (${\mathrm{\ensuremath{\Delta}}}_{\text{ct}}$) with the La substitution.