The large piezoelectric response of TMCMCdCl 3 cannot be explained by intrinsic effects as calculated from DFT. Furthermore, it has been shown experimentally that the piezoelectric response is improved at the morphotrobic phase boundary between the P6 3 mc and Cc phases for its solid solutions. As mentioned above, there has been a discussion in the literature about the origin of the large piezoelectric response, which is beyond the standard displacive regime. Both effects of ferroelasticity 9 and metastable phases have been suggested. Based on the easy accessibility of xy-disorder observed in our MC model, we instead suggest that the origin may be related to defects and intergrowths of differently ordered domains in an ordered Cc-symmetry matrix.