DFT+U Analysis on Stability of Low-Index Facets in Hexagonal LaCoO3 Perovskite: Effect of Co3+ Spin States

Abstract

By the first-principles calculations, most studies indicated that the (1102)-CoO2 termination of LaCoO3 cannot be stabilized, which disagrees with the experimental observation. Besides the crystal structure, we found that the spin states of Co3+ ions could affect surface stability, which previously were not well considered. By examining the different states of Co3+ ions in hexagonal-phase LaCoO3, including low spin, intermediate spin, and high spin states, the surface grand potentials of these facets are calculated and compared. The results show that the spin states of Co3+ ions have an important influence on stability of the LaCoO3 facets. Different from the previous results, the stability diagrams demonstrate that the (1102)-CoO2 termination can stably exist under O-rich condition, which can get an agreement with the experimental ones. Furthermore, the surface oxygen vacancy formation energies (EOv) of stable facets are computed in different spin states. The EOv of these possible exposed terminations strongly depend on the spin state of Co3+ ions: in particular, the EOv of the HS states is lower than that of other spin states. This indicates that one can tune the properties of LaCoO3 by directly tuning the spin states of Co3+ ions.