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

Abstract

Mössbauer spectroscopy and magnetization studies of YBaCo_4-xFe_xO_7+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 YBaCo_4-xFe_xO_8.5 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 YBaCo₄O_8.5. The iron solubility limit in YBaCo_4-xFe_xO_7+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%.