Instability of oxide perovskite surfaces induced by vacancy formation

Abstract: This work presents a first principles study of the (001) surface energetics of nine oxide perovskites, with a focus on the role of surface vacancies in determining termination stability. Additionally, investigation into the behaviour of vacancies as a function of depth from the surface in these perovskites, ABO$_{3}$ (A=Ca, Sr, Ba; B=Ti, Zr, Sn), is carried out, and results are compared to formation of the vacancies in bulk. Combining results from these investigations reveals a general trend for all nine perovskites - the undefected AO surface is more energetically favourable to form than the BO2 surface. This dominance of the AO over the BO2 surface is further enforced by the phase diagrams of perovskite surfaces. However, A-site vacancies at the AO surface are far more favourable (1-2 eV lower in energy) than B-site vacancies at the BO2 surface. Charged vacancies only drive this further under oxygen-rich conditions, showing a smaller range of stability for the AO than the BO2 surface. These results indicates that, whilst the AO surface is easier to form, the BO2 surface will display better long term stability, making it more suitable for use in potential applications. This study furthers the understanding of oxide perovskite (001)-terminated surface stability, which will aid in surface growth and manufacturing.