Low-Temperature Solid-State Synthesis of High-Purity BiFeO₃ Ceramic for Ferroic Thin-Film Deposition

The synthesis of high-purity BiFeO₃ (BFO) ceramic by solid-state reaction is known to be very difficult due to inevitable formation of the secondary phases, mostly mullite-type Bi₂Fe₄O₉ and sillenite-type Bi₂₅FeO₃₉. In particular, it is very difficult to completely remove the Bi-deficient Bi₂Fe₄O₉ phase from sintered ceramic BFO targets. This problem consequently leads to the difficulty of fabricating high-quality BFO thin films using these sintered targets. Herein, we introduce a simple but effective low-temperature processing scheme for removing impurity phases in which optimized processing conditions are obtained by chemically correlating the first calcination step with the subsequent leaching and sintering steps. More specifically, we suitably avoid the formation of the high-temperature-stable Bi₂Fe₄O₉ phase by performing the calcination at significantly low temperatures (between 650 and 675 °C) with Bi-excess starting powders. We have then fabricated epitaxially grown BFO thin films using these phase-pure ceramic targets and consequently achieved high-quality ferroelectricity and switchable photovoltaic responses. On the basis of the present experimental observations, we suggest that a low impurity concentration in the sintered BFO ceramic target, even with a low relative density, is advantageous for high-quality thin-film fabrication.