Thermal Phase Transitions in Superlattice Assemblies of Cuboidal CH₃NH₃PbI₃ Nanocrystals Followed by Grazing Incidence X‑ray Scattering

Colloidal nanocrystals of CH₃NH₃PbI₃ (MAPI) with the tetragonal crystal structure and cuboidal shape terminated by {110} and {002} facets were assembled into superlattices with cubatic structure and heated under nitrogen while collecting in situ grazing incidence small- and wide-angle X-ray scattering (GISAXS and GIWAXS). The nanocrystals have completed a tetragonal-to-cubic phase transition by 60 °C similar to bulk films. GISAXS shows that the superlattice remains stable until 90 °C. At this temperature, GIWAXS reveals the evolution of PbI₂. In situ photoluminescence shows that PbI₂ begins to form at 75 °C, which is about 10–25 °C lower than in bulk MAPI films. Another difference compared to bulk MAPI is that both hexagonal and rhombohedral phases of PbI₂ are observed as thermal degradation products, when the nanocrystals are heated up to 150 °C.