Magnetization, Mössbauer and isothermal dilatometric behavior of oxidized YBa(Co,Fe)4O7+δ

Abstract

Mössbauer spectroscopy and magnetization studies of YBaCo4-xFexO7+δ (x = 0–0.8) oxidized at 0.21 and 100 atm O2, indicate an increasing role of penta-coordinated Co3+ states when the oxygen content approaches 8–8.5 atoms per formula unit. Strong magnetic correlations are observed in YBaCo4-xFexO8.5 from 2 K up to 55–70 K, whilst the average magnetic moment of Co3+ is lower than that for δ ≤ 0.2, in correlation with the lower 57Fe3+ isomer shifts determined from Mössbauer spectra. The hypothesis on dominant five-fold coordination of cobalt cations was validated by molecular dynamics modeling of YBaCo4O8.5. The iron solubility limit in YBaCo4-xFexO7+δ corresponds to approximately x ≈ 0.7. The oxygen intercalation processes in YBaCo4O7+δ at 470–700 K, analyzed by X-ray diffraction, thermogravimetry and controlled-atmosphere dilatometry, lead to unusual volume expansion opposing to the cobalt cation radius variations. This behavior is associated with increasing cobalt coordination numbers and with rising local distortions and disorder in the crystal lattice on oxidation, predicted by the computer simulations. When the oxygen partial pressure increases from 4 × 10−5 to 1 atm, the linear strain in YBaCo4O7+δ ceramics at 598 K is as high as 0.33%. © Royal Society of Chemistry 2021