This structural feature facilitates facile anion rotation, associated with large cell volume change (Fig. )), making these materials unique. The ordered (O)-disordered (D) phase transition in these materials can be qualitatively elucidated by considering the Gibbs free energy difference between the high-temperature (high-T) D and low-temperature (low-T) O phases, denoted as ∆G ≡ G D -G O , as depicted in Fig. (c) .