Ionic plastic crystals are crystalline materials where a solid-solid phase transition occurs at elevated temperatures due to the introduction of molecular reorientational disorder. Plastic crystals thus have the potential of functioning as solid-solid phase change materials for thermal energy storage. Our study concerns solid solution engineering of plastic crystals for the tuning and engineering of phase transition properties. We aim to investigate the structural details of such solid solutions, in particular the role of intramolecular disorder. Our research will fill a knowledge gap in the theory of solid solutions for plastic crystals, and provide fundamental knowledge into the physical mechanisms allowing for the tuning of solid-solid phase transition properties.