For the intergrowth model, the formation energy is 26 (39) meV with (without) unit cell relaxation. As this is an extended defect, it is not directly comparable to the thermal energy, but by normalising it by the area of the intergrowth plane, we can compare it to other calculated stacking fault energies. For our case we get 43 (62) meV/nm 2 , which, for comparison, is slightly lower than the formation energy of hexagonal stacking faults in cubic metal halide perovskites, which are experimentally observed to be present. In the real solids, a complex mixture of defects and intergrowths will be present, and thus we shall not make an attempt to calculate the piezoelectric coefficient arising from these. However, it is very likely that applied stress can lead to reorientations of defects, movement of intergrowths etc., which will lead to a change in polarisation and therefore a piezoelectric response beyond the small-displacement limit. Furthermore, these defects are of significantly lower energy than the metastable phase suggested by Ghosh et al. , and therefore more likely to be involved in the piezoelectric mechanism.