Probing the electronic and local structure of Sr$_{2-x}$La$_x$CoNbO$_6$ using near-edge and extended x-ray absorption fine structures

TL;DR

This study investigates the local and electronic structures of Sr$_{2-x}$La$_x$CoNbO$_6$ double perovskites using X-ray absorption techniques, revealing how La substitution affects octahedral distortions, magnetic interactions, and electronic states.

Contribution

It provides detailed insights into the local structural distortions and electronic changes in Sr$_{2-x}$La$_x$CoNbO$_6$, highlighting the correlation between octahedral distortions and magnetic and electronic properties.

Findings

Identification of Jahn-Teller distortions in CoO$_6$ octahedra.

Observation of a change in NbO$_6$ octahedral distortion around x=0.4.

Detection of K$_{N1}$ double electron excitation in Sr K-edge EXAFS.

Abstract

We report the electronic and local structural investigation of double pervoskites Sr$_{2-x}$La$_x$CoNbO$_6$ ($x=$ 0--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 1$s$ electrons to the mixed $p-d$ hybridized states. We reveal a $z$-out Jahn-Teller (JT) distortion in the CoO$_6$ octahedra, which decreases monotonically due to an enhancement in the JT inactive Co$^{2+}$ ions with $x$ . On the other hand, the $z$-in distortion in 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 $x \geqslant$ 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 reason for this are discussed. More interestingly, we observe the signature of 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 ($\Delta_{ct}$) with the La substitution.