Ca Solubility in a BiFeO₃-Based System with a Secondary Bi₂O₃ Phase on a Nanoscale

Abstract

In BiFeO 3 (BFO), Bi 2 O 3 (BO) is a known secondary phase, which can appear under certain growth conditions. However, BO is not just an unwanted parasitic phase but can be used to create the super-tetragonal BFO phase in films on substrates, which would otherwise grow in the regular rhombohedral phase (R-phase). The super-tetragonal BFO phase has the advantage of a much larger ferroelectric polarization of 130–150 μC/cm 2 , which is around 1.5 times the value of the rhombohedral phase with 80–100 μC/cm 2 . Here, we report that the solubility of Ca, which is a common dopant of bismuth ferrite materials to tune their properties, is significantly lower in the secondary BO phase than in the observed R-phase BFO. Starting from the film growth, this leads to completely different Ca concentrations in the two phases. We show this with advanced analytical transmission electron microscopy techniques and confirm the experimental results with density functional theory (DFT) calculations. At the film’s fabrication temperature, caused by different solubilities, about 50 times higher Ca concentration is expected in the BFO phase than in the secondary one. Depending on the cooling rate after fabrication, this can further increase since a larger Ca concentration difference is expected at lower temperatures. When fabricating functional devices using Ca doping and the secondary BO phase, the difference in solubility must be considered because, depending on the ratio of the BO phase, the Ca concentration in the BFO phase can become much higher than intended. This can be critical for the intended device functionality because the Ca concentration strongly influences and modifies the BFO properties.

Structural and Chemical Insights into Designer Defects in Tetragonal-like Epitaxial BiFeO3 Thin Films

“…For instance, T' BFO can be stabilized on STO substrates, which induce a compressive strain of only 1.5%, by the coexistence of a unit-cell-thick BO layers or plates as a secondary phase. 11,12 Previous studies, while providing some structural and chemical information, were not able to pinpoint the full structural details of these BO inclusions. In particular, it was not clear if they were the β-BO phase 5,7,11,13 (previously reported as a secondary phase in the thin films) or the δ-BO phase 14−16 (previously stabilized as the major phase).…”

“…Note that the stabilization of T’ BFO by BO phases is not restricted to growth on LAO substrates alone. For instance, T’ BFO can be stabilized on STO substrates, which induce a compressive strain of only 1.5%, by the coexistence of a unit-cell-thick BO layers or plates as a secondary phase. , …”