  Using nanoprobe X-ray diffraction microscopy, we investigate the relationship
between residual strains from crystal growth in CsPbBr$_3$ thin film crystals,
their stability, and local bandgap. We find that out-of-plane compressive
strain that arises from cooldown from crystallization is detrimental to
material stability under X-ray irradiation. We also find that the optical
photoluminescence red shifts as a result of the out-of-plane compressive
strain. The sensitivity of bandgap to strain suggests possible applications
such as stress-sensitive sensors. Mosaicity, the formation of small
misorientations in neighboring crystalline domains we observe in some
CsPbBr$_3$ single crystals, indicates the significant variations in crystal
quality that can occur even in single-crystal halide perovskites. The
nano-diffraction results suggest that reducing local strains is a necessary
path to enhance the stability of perovskite optoelectronic materials and
devices from light-emitting diodes to high-energy detectors.
