Hydrothermal synthesis of pure and Sb-doped BiFeO3 with the typical hysteresis loops of ideal ferroelectrics

BiFeO3 (BFO) samples with nearly perfect ferroelectric hysteresis loops were synthesized from chemical solution via hydrothermal route at 200 °C. However, for many applications, ceramic samples of reasonable bulk density (>80%) have to be sintered at temperature over 700 °C, which in this case results in a significant reduction in resistivity due to increased amounts of Fe2+. Interestingly, doping of a few percent Sb minimized such a problem and the sintered Sb:BFO ceramics retained a similarly high resistivity as samples cold-pressed from the chemical-solution synthesized powders. However, for cold-pressed samples, Sb:BFO actually had higher conductivity than undoped BFO. Temperature-dependent conductivity showed that cold-pressed samples of both undoped and Sb doped BFO had the similar activation energy of 1.0 eV, typical for electrons trapped in oxygen vacancies. After sintering, the activation energy of Sb:BFO remained almost unchanged, but the activation energy of undoped BFO changed to 0.4 eV, which is associated to electron hopping between Fe2+/Fe3+. X-ray photoelectron spectroscopy (XPS) showed a significant increase in Fe2+/Fe3+ ratio from 6.6/93.4 to 25.7/74.3 in undoped BFO after sintering, while for 1% Sb doped BFO the increase was much milder from 10.9/89.1 to 14.1/85.9. XPS also showed that Sb had single +3 oxidation state before sintering, but after sintering a fairly large portion of Sb5+ occurred. So, charge compensation for oxygen vacancies in undoped BFO was achieved dominantly by reduction of Fe3+ to Fe2+, while in Sb:BFO it was achieved more by cation vacancies.