This free energy difference comprises three key terms: internal energy (∆E), entropy (-T ∆S), and volume (p∆V ). Notably, the internal energy remains relatively constant throughout the phase transition, while the volume term opposes the stabilization of the D phase due to ∆V > 0. Consequently, within the domain of cluster anion-type materials, the O-D phase transition appears to be predominantly governed by the change in entropy, denoted as ∆S. Importantly, this change in entropy assumes heightened significance in the context of ion conductivity and molecular reorientational frequencies, particularly above the transition temperature (T tran ), where it is expected to be relatively substantial.