X-ray absorption spectroscopic studies of oxygen-deficient double perovskite oxide NdBaCo$_2$O$_{5+δ}$ ($δ\sim 0.65$)

Abstract

The degree of oxygen vacancy in A-site-doped layered double perovskite oxides can lead to anion vacancy ordering, which subsequently results in distinct superstructures. We have studied oxygen deficient double perovskite oxide, NdBaCo$2$O$_{5+δ}$ ($δ\sim$ 0.65), utilising X-ray absorption near edge structure (XANES) spectroscopy complemented by multiple scattering and first-principles calculations based on density functional theory (DFT+U). The present results establish a direct correlation between oxygen-vacancy distribution, magnetic inhomogeneity, and charge transport in layered cobaltates. Our investigations on the Co K, Ba, and Nd $L_{\mathrm{III}}$ edges suggest that the near-edge spectra, based on the number and shape of the spectral features, indicate the value of $δ$ for the compound under study lies closer to 0.75. The analysis of the Ba $L_{\mathrm{III}}$ edge spectrum confirms the +2 oxidation state for Ba ions. The Co K-edge near-edge spectra suggest the possibility of oxygen vacancies arising at the basal sites in addition to the apical sites. This is counterintuitive to the common understanding that oxygen vacancies usually occur at the apical sites, i.e., in the lanthanide (Nd) layer in these compounds. Such a distribution of oxygen vacancies naturally explains the Griffith phase and the conduction mechanism of variable range hopping observed in the compound. The vacancy distribution, when at the optimum level, can also bring about easier oxygen ion mobility and a more sensitive and faster oxygen sensor. We believe that our results will help in tailoring oxygen-ion mobility and enhancing oxygen-sensing performance.