Many factors influence the ease of molecular rotation. First, the crystal packing density affects the steric hindrance of rotation. The shapes of the constituent molecules also affect the molecule's ability to rotate as it changes the moment of inertia, small globular molecules rotate more easily than large elongated molecules, in accordance with Timmermans characterization of the plastic crystal material class. In this study, the plastic ionic crystals containing globular organic cations and tetrahedral inorganic anions, the dabco and the quinuclidinium compounds, have both the largest anisotropies and shear piezoelectric response. Finally, strong intermolecular bonds can hinder rotations of globular molecules. This effect explains the lower piezoelectric response of [F -Q]ReO 4 compared to HQReO 4 .