  Mechanical strain can modify the structural and electronic properties of
methylammonium lead iodide MAPbI3. The consequences of ferroelastic hysteresis,
which involves the retention of structural memory upon cycles of deformation,
in polycrystalline thin films of MAPbI3 are reported. Repeatedly bent films of
MAPbI3 on flexible polyimide substrates were examined using Grazing Incidence
Wide-Angle X-ray Scattering (GIWAXS) to quantitatively characterize the strain
state, populations, and minimum sizes of twin domains. Approximate locations
for the coercive stress and saturation on the ferroelastic stress-strain curve
are identified, and domains from differently-strained twin sets in the films
are found to interact with each other. The presence of specific twin domains is
found to correlate to reports of the heterogeneity of strain states with defect
content. Long-term stability testing reveals that domain walls are highly
immobile over extended periods. Nucleation of new domain walls occurs for
specific mechanical strains and correlates closely with degradation of the
films. These results help to explain the behavior of ion migration, degradation
rate, and photoluminescence in thin films under compressive and tensile strain.
