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

Abstract

We report the electronic and local structural investigation of double pervoskites Sr2−xLaxCoNbO6 (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 1s electrons to the mixed p−d hybridized states. We reveal a z-out Jahn-Teller (JT) distortion in the CoO6 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 NbO6 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 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 KN1 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 L2,3 edge shows the reduction in the crystal field strength and hence an increase in the charge transfer energy (∆ct) with the La substitution.

Observation of Anisotropic Thermal Expansion and the Jahn–Teller Effect in Double Perovskites Sr2–xLaxCoNbO6 Using Neutron Diffraction

“…The reduction in the octahedral distortion with the La substitution is also observed in our recent local structural investigations of the Sr 2−x La x CoNbO 6 (x = 0−1) samples using the extended Xray absorption fine structure (EXAFS) measurements. 35 Further, the average distortion around oxygen atoms increases with the La substitution due to reduction in the lattice symmetry (see Figure 5c). The local distortion around all the atoms show the nonmonotonic behavior with the temperature analogous to the x = 0.4 sample, as presented in Figure 5a−c.…”