Bond length

Organic chemistry

Formamidinium

Crystallography

Crystal structure

Perovskite (structure)

Chemical physics

Halide

Inorganic chemistry

Chemistry

Materials science

Octahedron

Bromide

Iodide

Phase (matter)

Photoluminescence

Optoelectronics

Caesium

Chemistry of Materials

We report the cooling-induced crystallization of layered two-dimensional (2D) lead halide perovskites with controllable inorganic quantum-well thicknesses (n = 1, 2, 3, and 4), organic-spacer chain lengths (butyl-, pentyl-, and hexylammonium), A-site cations (methylammonium and formamidinium), and halide anions (iodide and bromide). Using single-crystal X-ray diffraction, we refined crystal structures for the iodide family as a function of these compositional parameters and across their temperature-dependent phase transitions. In general, lower-symmetry crystal structures, increasing extents of organic-spacer interdigitation, and increasing organic-spacer corrugation tilts are observed at low temperature. Moreover, greater structural distortions are observed in lead halide octahedra closest to the organic-spacer layer, and higher-n structures exhibit periodic variation in Pb–I bond lengths. These structural trends are used to explain corresponding temperature-dependent changes in the photoluminescence spectra. We also provide detailed guidance regarding the combination of synthetic parameters needed to achieve phase-pure crystals of each composition and discuss difficulties encountered when trying to synthesize particular members of the 2D perovskite family containing formamidinium or cesium as the A-site cation. These results provide a foundation for understanding structural trends in 2D lead halide perovskites and the effects these trends have on their thermal, electronic, and optical properties.

Synthetic Variation and Structural Trends in Layered Two-Dimensional Alkylammonium Lead Halide Perovskites