Zero-dimensional mixed-cation hybrid lead halides with broadband emissions

Abstract

Zero-dimensional (0D) metal halides have received significant attention in recent years due to their attractive light-emitting properties derived from the presence of isolated building units. Here we report the synthesis, crystal structures, and linear optical and phonon properties of three newcomers to the family of 0D lead halides – Cs₂MHy₂PbBr₆, Cs₂MHy₂PbI₆ and Cs₂MHy₂PbBr₃I₃ (MHy⁺ = methylhydrazinium) which are the first examples of 0D lead halides with mixed cations. These compounds crystallize in the orthorhombic Cmce structure with isolated PbX₆⁴⁻ octahedral units and statistically disordered MHy⁺ cations. X-ray diffraction revealed the selective substitution of halide ions in Cs₂MHy₂PbI₃Br₃ with Br⁻ occupying the positions of ions acting as hydrogen-bond (HB) acceptors and I⁻ occupying non-interacting sites. This preferential occupation leads to a giant increase of the octahedral distortion (27- and 249-fold, compared to the bromide and iodide, respectively). Raman spectra confirm the static disorder of MHy⁺ and preferential occupation of halide sites. With the use of linear optical measurements, we demonstrate that all compounds exhibit broadband orange-yellow emission attributed to self-trapped excitons (STEs). The observed Stokes shifts of Cs₂MHy₂PbBr₆ and Cs₂MHy₂PbBr₃I₃ are record large among 0D lead halides. A large increase of the octahedral distortion due to the preferential occupation of halide sites in Cs₂MHy₂PbBr₃I₃ is reflected in the spectra by broadening and red-shift of its emission. This study paves the way for developing a new class of light-emitting 0D lead halides by synthesizing mixed-cation analogues.