resumo
Mossbauer spectroscopy and magnetization studies of YBaCo4-xFexO7+delta (x = 0-0.8) oxidized at 0.21 and 100 atm O-2, 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-x)Fe(x)O8.5 from 2 K up to 55-70 K, whilst the average magnetic moment of Co3+ is lower than that for delta <= 0.2, in correlation with the lower Fe-57(3+) isomer shifts determined from Mossbauer 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+delta corresponds to approximately x approximate to 0.7. The oxygen intercalation processes in YBaCo4O7+delta 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+delta ceramics at 598 K is as high as 0.33%.
palavras-chave
ION MIGRATION; YBACO4O7+DELTA; MECHANISMS; TRANSITION; SIMULATION; CAPABILITY; EXPANSION; PROGRAM; GULP
categoria
Chemistry
autores
Waerenborgh, JC; Tsipis, EV; Pereira, LCJ; Avdeev, M; Naumovich, EN; Kharton, VV
nossos autores
Maria de Lourdes Gomes Pereira
Professor Associado com Agregação
Vladislav Kharton
Investigador Principal CovidadoProjectos
agradecimentos
Financial support from the FCT, Portugal (projects PTDC/CTM/64357/2006, PTDC/CTM-CER/114561/2009 and grant SFRH/BPD/28629/2006) is acknowledged.