FA_xCs_1–xPbI₃ Nanocrystals: Tuning Crystal Symmetry by A-Site Cation Composition

Nanoscale semiconductors show remarkably tunable properties. For metal halide perovskite (MHP) nanocrystals (NCs), surface energy and lattice strain stabilize desirable MHP compositions and crystallographic phases that are unstable in the bulk. We report an X-ray scattering study of the average room-temperature crystal structure of ∼15 nm FAxCs1–xPbI3 (FA = formamidinium) NCs. All compositions crystallize in the perovskite structure; however, the average structure lowers in symmetry from the α (cubic) to β (tetragonal) to γ (orthorhombic) perovskite phases with decreasing x (Cs addition). The corresponding α- to β- and β- to γ-phase transitions occur between x = 0.75–0.5 and x = 0.25–0.1, respectively. Structural refinements also indicate large octahedral tilt angles (10–30°) in the β- and γ-phases and an increase in (pseudo)cubic unit cell volume upon FA addition. This work establishes the composition–structure relationship for FAxCs1–xPbI3 NCs and demonstrates the ability to target average crystal symmetry with facile synthetic control.