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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.