Determine the basal oxygen-vacancy distribution in NdBaCo2O5.65

Determine whether oxygen vacancies in NdBaCo2O5.65 occur at basal oxygen positions in addition to the apical positions, and quantify the amount and type of vacancy ordering using high-resolution neutron diffraction so that the basal and apical oxygen Wyckoff-site occupancies can be individually refined.

Background

The Co K-edge multiple-scattering calculations reproduce the experimental spectrum most closely when the tetragonal 222 structure associated with NdBaCo2O5.75 is used, but the measured sample is more oxygen deficient, with an estimated composition near NdBaCo2O5.65. The relatively low experimental white-line intensity, compared with calculations for the fully occupied basal oxygen sites, suggests that some basal oxygen sites may also contain vacancies in addition to the customary apical vacancies in the Nd layer.

The authors explicitly leave the basal-vacancy interpretation unresolved and identify high-resolution neutron diffraction as the method needed to determine the vacancy amount and type. Such measurements would refine the occupancies of the basal and apical oxygen Wyckoff positions separately and test whether the proposed vacancy distribution accounts for the magnetic disorder, Griffiths-like behavior, and variable-range-hopping transport discussed in the paper.

References

It is interesting to note here that the intense white line observed in the calculated spectrum is mainly due to the inclusion of basal O7 and O8 atoms in the cluster. However, the intensity of the white line of the experimental spectrum is quite low. Such low intensity could be due to the vacancy-generated discrepancy in the oxygen ions from some of the O7 and O8 sites in the basal plane, in addition to the vacancy in the oxygen at the apical site, as the prepared sample is more oxygen deficient ($\delta$ = 0.65) as compared to the calculated spectrum ($\delta$ = 0.75). To the best of our knowledge, among the possible superstructures available in literature for non-stoichiometry $\delta <$ 0.75, $\delta$ = 0.50 is well studied, where the extra oxygen goes away from the apical positions in the Nd layer, giving rise to 122 superstructure investigated using neutron and synchrotron diffraction studies. Other than that, the common consensus for RBaCo$2$O${5.5}$ (R other than Nd) systems also shows 122 superstructure. As previously discussed, in our multiple scattering calculations, it can also be seen that the $\delta$ = 0.50 system has a higher intensity of the white line in comparison to $\delta$ = 0.75, which is not in line with the experimental observation, as shown in Fig. \ref{XANES1}(a). However, to ascertain the amount and type of oxygen vacancies, high-resolution neutron diffraction experiments will be helpful, where the Wyckoff positions corresponding to basal and apical oxygens can be individually refined to get the \% vacancies on different sites.

— X-ray absorption spectroscopic studies of oxygen-deficient double perovskite oxide NdBaCo$_2$O$_{5+δ}$ ($δ\sim 0.65$)  (2610.13155 - Pant et al., 9 Oct 2026) in Section ‘Results and discussion,’ subsection ‘Co K-edge’