X-ray Absorption Spectroscopy Study of La$_{1-y}$Sr$_{y}$Co$_{1-x}$Nb$_x$O$_3$

TL;DR

This study uses x-ray absorption spectroscopy to analyze the local structure and electronic states of La$_{1-y}$Sr$_{y}$Co$_{1-x}$Nb$_x$O$_3$, revealing valence states, Jahn-Teller distortions, and structural transformations induced by Sr and Nb substitutions.

Contribution

It provides detailed insights into the valence states and local structural changes in La$_{1-y}$Sr$_{y}$Co$_{1-x}$Nb$_x$O$_3$ using advanced XAS techniques, highlighting the effects of Sr and Nb doping.

Findings

Co in LSCNO is in the 3+ valence state.

Jahn-Teller distortion present in LSCNO, absent in LCNO for x>0.025.

La ions remain trivalent with decreasing local disorder upon substitution.

Abstract

We use x-ray absorption spectroscopy to investigate the local structure and electronic properties of bulk La$_{1-y}$Sr$_{y}$Co$_{1-x}$Nb$_x$O$_3$ ($y=$ 2$x$ as LSCNO and $y=$ 0 as LCNO) samples. The x-ray absorption near-edge spectra (XANES) of LSCNO at Co K-edge affirm the valence state of Co in 3+. However, in the case of the LCNO, a subtle variation in the valence state of Co ions from 3+ to 2+ is evident with Nb substitution. The detailed analysis of the Fourier transform (FT) of the extended x-ray absorption fine structure (EXAFS) for the LSCNO samples exhibit the two groups of the bond-lengths owing to the Jahn-Teller (JT) distortion in the CoO$_6$ octahedra, which manifest that the Co$^{3+}$ ions exist in the intermediate spin-state (t$^5_{2g}$e$^1_g$) at room temperature. However, we find that the JT distortion is not present in LCNO samples for $x>$0.025 due to an increase in the high-spin Co$^{2+}$/Co$^{3+}$ ions accompanied by the Nb--induced structural transformation. Intriguingly, the La L$_3$-edge spectra for the LSCNO samples exhibit that the La ions exist in the trivalent state and the local disorder around La atoms decreases with Sr and Nb substitution. Interestingly, the simulated FT of the EXAFS spectra at the La L$_3$-edge demonstrates the three groups of La--O bond lengths, which exhibit a monotonous change with Sr and Nb substitution. Moreover, the XANES measured at Sr and Nb K-edges confirm their oxidation state to be in 2+ and 5+, respectively.