The analysis of the molecular rotations in HQReO 4 showed that the primary cause of the large shear piezoelectric response of this material is the rotation of the non-polar inorganic tetrahedra, rather than the polar HQ molecule. This finding highlights the importance of intermolecular charge transfer. In fact, the importance of the rotation of rigid molecules in plastic crystals is reminiscent of tilting of the corner-sharing oxygen octahedra in the perovskite piezoelectrics. The rotation mechanism in the inorganic piezoelectrics is often linked to a morphotropic phase boundary (MPB), i.e, a compositionally engineered phase boundary where the free energy barrier between two structural symmetries is low. The polarization vector is allowed to rotate between its position in each phase at this boundary due to small energy difference from one phase to the other . In metal oxide perovskite ferroelectrics, engineering solid solutions with compositions close to a MPB is the state-of-the-art approach for enhancing piezoelectric coefficients. The fact that rotation of nonpolar molecules in both cases are critical for the piezoelectric shear response, suggest that an alloy-like design approach can also be suitable for plastic ionic crystals, leveraging greater ability to enhance molecular rotations. Combined with the immense possibilities offered by the different combinations of cation and anions, these design tools brings vast opportunities for designing novel plastic ionic molecular piezoelectrics with record-breaking properties.