Novel Low-Temperature Phase of CsSnI3 Nanocrystals

In the halide perovskites, the B-site metal lone-pair s electrons are strongly suspected to underpin many of the interesting properties of the inorganic halide perovskites. Recently, a stable low-temperature monoclinic polar phase was predicted for CsSnI3, opening the possibility of direct investigation of a both these lone pair electrons and a ferroelectric distorted structure. To date there are no reports of such a structure in CsSnI3, and a known low-temperature monoclinic structure in CsSnBr3 remains unexplored. We have found optical evidence of a transformation occurring around 240 K in CsSnI3 nanocrystals, with several changes in optical behavior below this transition point, including novel high-energy photoluminescene and new states in the transient absorption spectrum. We have successfully characterized the optical properties of this low-temperature phase and are working to acquire structural characterization to confirm its polar nature. Discovery of a stable monoclinic polar structure in the halide perovskites opens many new potential directions for further research and electronics applications.