Ca Solubility in a BiFeO3‑Based System with a Secondary Bi2O3Phase on a Nanoscale

In BiFeO3 (BFO), Bi2O3 (BO) is a known secondaryphase, which can appear under certain growth conditions. However,BO is not just an unwanted parasitic phase but can be used to createthe super-tetragonal BFO phase in films on substrates, which wouldotherwise grow in the regular rhombohedral phase (R-phase). Thesuper-tetragonal BFO phase has the advantage of a much largerferroelectric polarization of 130−150 μC/cm2, which is around 1.5times the value of the rhombohedral phase with 80−100 μC/cm2.Here, we report that the solubility of Ca, which is a common dopantof bismuth ferrite materials to tune their properties, is significantlylower in the secondary BO phase than in the observed R-phase BFO.Starting from the film growth, this leads to completely different Caconcentrations in the two phases. We show this with advancedanalytical 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 isexpected in the BFO phase than in the secondary one. Depending on the cooling rate after fabrication, this can further increase sincea larger Ca concentration difference is expected at lower temperatures. When fabricating functional devices using Ca doping and thesecondary BO phase, the difference in solubility must be considered because, depending on the ratio of the BO phase, the Caconcentration in the BFO phase can become much higher than intended. This can be critical for the intended device functionalitybecause the Ca concentration strongly influences and modifies the BFO properties.