By tuning the size and shape of the anions and cations, one can manipulate the diffusion pathways and barriers for cationic diffusion. This leads to the formation of novel anions, such as M 2 B n H n , where (M = Li, Na, and n = 10, 12), that have wider interstitial spaces and thus facilitate cation diffusion. These anions also exhibit remarkable stability due to the strong B-B bonding within the cages . These materials also exhibit the phase transition from an O to a D structure, accompanied by a dramatic increase in ionic conductivity (Fig. ). This transition is triggered by the thermal motion of the anions, which causes them to adopt different orientations and positions in the lattice. For example, Na 2 B 12 H 12 undergoes a transition from an O monoclinic structure to a fully D bcc structure at 545K, passing through an intermediate pseudo-bcc structure at 529K . This results in three orders of magnitude enhancement in Na + ion conductivity, reaching a high value of 0.1 S cm -1 at 540K . A similar but more pronounced transition occurs in Na 2 B 10 H 10 at a much lower temperature of 373 K, leading to a remarkable conductivity above 0.01 Scm -1 at 383K . Unlike the sodium compounds, both Li 2 B 10 H 10 and Li 2 B 12 H 12 require much higher temperatures to change their phase. In the case of Li 2 B 12 H 12 , the anions are arranged in a face-centered position. A possible factor that influences the conductivity of (Li/Na) 2 B 12 H 12 is the size of the anions, which creates a bottleneck effect for the cations. The larger Na ions can pass through the anions more easily than the smaller Li ions, resulting in a higher Na + ion conductivity than Li + . Another factor that affects the T tran of (Li/Na)B 12 H 12 and (Li/Na)B 10 H 10 is the geometrical shape of the anions: