  The interaction of high-energy electrons and X-ray photons with soft
semiconductors such as halide perovskites is essential for the characterisation
and understanding of these optoelectronic materials. Using nano-probe
diffraction techniques, which can investigate physical properties on the
nanoscale, we perform studies of the interaction of electron and X-ray
radiation with state-of-the-art
(FA$_{0.79}$MA$_{0.16}$Cs$_{0.05}$)Pb(I$_{0.83}$Br$_{0.17}$)$_3$ hybrid halide
perovskite films (FA, formamidinium; MA, methylammonium). We track the changes
in the local crystal structure as a function of fluence using scanning electron
diffraction and synchrotron nano X-ray diffraction techniques. We identify
perovskite grains from which additional reflections, corresponding to PbBr$_2$,
appear as a crystalline degradation phase after fluences of ~200
e$^-${\AA}$^{-2}$. These changes are concomitant with the formation of small
PbI$_2$ crystallites at the adjacent high-angle grain boundaries, with the
formation of pinholes, and with a phase transition from tetragonal to cubic. A
similar degradation pathway is caused by photon irradiation in nano-X-ray
diffraction, suggesting common underlying mechanisms. Our approach explores the
radiation limits of these materials and provides a description of the
degradation pathways on the nanoscale. Addressing high-angle grain boundaries
will be critical for the further improvement of halide polycrystalline film
stability, especially for applications vulnerable to high-energy radiation such
as space photovoltaics.
