Physical vapor deposition of the halide perovskite CsBi₂Br₇

In cesium bismuth bromides comprising [BiBr₆]³⁻ octahedra, the octahedra behave as quantum dots and their interactions can be manipulated by tailoring their connectedness (e.g., corner-sharing, edge-sharing, or unconnected). Of the four compounds reported, CsBi₂Br₇, CsBiBr₄, Cs₃Bi₂Br₉, and Cs₃BiBr₆, there is only one publication each on CsBi₂Br₇ and CsBiBr₄. Here, we synthesize CsBi₂Br₇ and attempt to synthesize CsBiBr₄ using co-evaporation of CsBr and BiBr₃ where the precursor fluxes are controlled precisely. The structure, composition, morphology, and optical properties of the films are characterized using x-ray diffraction (XRD), scanning electron microscopy, energy dispersive x-ray spectroscopy, Raman scattering, and optical absorption as a function of time from hours to several months. When the CsBr:BiBr₃ flux ratio is 1:2, CsBi₂Br₇ forms but its XRD, Raman spectrum, and morphology change with time. CsBi₂Br₇ is ultimately unstable with respect to dissociation into Cs₃Bi₂Br₉ and BiBr₃ over a time period of weeks. Its optical absorption shows a peak at 407 nm, between that of Cs₃Bi₂Br₉ at 435 nm and Cs₃BiBr₆ at 386 nm, indicating that the interactions between the [BiBr₆]³⁻ octahedra in CsBi₂Br₇ is between those in Cs₃Bi₂Br₉, where the octahedra share corners, and Cs₃BiBr₆, where the octahedra are not connected. When the CsBr:BiBr₃ flux ratio is maintained at 1:1 to form CsBiBr₄, the XRD of the resulting film is consistent with a mixture of Cs₃Bi₂Br₉ and CsBi₂Br₇ suggesting that CsBiBr₄, if it exists and forms, is also unstable. We see remarkable fluidity and mobility of matter in the film with tens of micrometer size crystals growing or disappearing in thermodynamically frustrated films even at room temperature over a period of days to weeks.