Surface Stabilization of the Cubic Phase in Lithium Alloyed Double Perovskite Nanocrystals

The high surface-to-volume ratio in nanocrystals (NCs) enables surface energy effects that stabilize phases that are otherwise unstable in a bulk state. Double perovskites (DP) containing lithium show exactly this effect. Bulk Cs₂LiInCl₆ adopts a triclinic structure; however, we show here that the cubic phase can be stabilized in a nanocrystalline form at a wide range of lithium-sodium alloyed compositions. Density functional theory (DFT) calculations support this finding. Although bulk formation energy favors the triclinic structure for Li-rich compositions, the cubic phase becomes stable when the surface energy of the {100} facet is significant due to the large surface-to-volume ratio in nanocrystals. The importance of this Li-Na alloying is seen in the change in the physical properties, apparent in the controllable blue shift of the emission with increased Li content. This advantageous effect, which is also observed for Li-cation exchange in presynthesized colloidal nanocrystals, is overshadowed by a competing phase that we identify as an orthorhombic hydrate phase, Cs₂InCl₅⋅H₂O. We characterize its emergence and propose strategies to mitigate its impact.